Part 618 – Soil Properties and Qualities

Subpart A  – General Information

618.0  Definition and Purpose

1. Soil properties are measured or inferred from direct observations in the field or laboratory. Examples of soil properties are particle-size distribution, cation-exchange capacity, and salinity.
    
2. Soil qualities are behavior and performance attributes that are not directly measured. They are inferred from observations of dynamic conditions and from soil properties. Examples of soil qualities are corrosivity, natural drainage, frost action, and wind erodibility.
    
3. Soil properties and soil qualities are the criteria used in soil interpretations, as predictors of soil behavior, and for classification and mapping of soils. The soil properties entered in the National Soil Information System (NASIS) must be representative of the soil and the dominant land use for which the interpretations are based.
    

618.1  Policy and Responsibilities

1. Soil property data are collected, tested, and correlated as part of soil survey operations. These data are reviewed, supplemented, and revised as necessary.
    
2. The Major Land Resource Area (MLRA) soil survey office is responsible for collecting, testing, and correlating soil property data and interpretive criteria.
    
3. The MLRA soil survey regional office (MO) is responsible for the development, maintenance, quality assurance, correlation, and coordination of the collection of soil property data that are used as interpretive criteria. This includes all the data elements listed below.
    
4. The National Soil Survey Center (NSSC) is responsible for the training, review, and periodic update of soil interpretation technologies.
    
5. The state soil scientist is responsible for ensuring that the soil interpretations are adequate for the Field Office Technical Guide and that they meet the needs of Federal, State, and local programs.
    

618.2  Collecting, Testing, and Populating Soil Property Data

1. The collection and testing of soil property data is based on the needs described in the soil survey memorandum of understanding for individual soil survey areas. The collection and testing must conform to the procedures and guides established in this handbook.
    
2. As aggregated component data, soil properties and qualities that are populated in NASIS are not meant to be site-specific. They represent the component as it occurs throughout the extent of the map unit. Most data entries are developed by aggregating information from point data (pedons) to create low, high, and representative values for the component.
    

618.3  Soil Properties and Soil Qualities

1. The following sections list soil properties and qualities in alphabetical order and provide some grouping for climatic and engineering properties and classes. A definition, classes, significance, method, and guidance for NASIS database entry are given. The listing includes the soil properties and qualities in the NASIS database. For more details on the NASIS database, refer to part 639 of this handbook. For specifics on data structure, attributes, and choices in NASIS, refer to http://www.soils.usda.gov/technical/nasis/documents/metadata/index.html. Then follow the link to the “NASIS Version 6.x” index webpage.
    
2. Previous databases of soil survey information used metric or English units for soil properties and qualities. Values in English units, except for crop yields, were converted into metric units during transfer into the NASIS database. All future edits and entries in NASIS except yields and acreage will use metric units.
    
3. Ranges of soil properties and qualities that are posted in the NASIS database for map unit components may extend beyond the established limits of the taxon from which the component gets its name, but only to the extent that interpretations do not change. However, the representative value (rv) is within the range of the taxon.
    

618.4  Albedo, Dry

1. Definition.—“Albedo, dry” is the estimated ratio of the incident shortwave (solar) radiation that is reflected by the air-dry, less than 2 mm fraction of the soil surface to that received by it.
    
2. Significance
    
   1. Soil albedo, as a function of soil color and angle of incidence of the solar radiation, depends on the inherent color of the parent material, organic matter content, and weathering conditions.
       
   2. Estimates of the evapotranspiration rates and predictions of soil water balances require albedo values. Evapotranspiration and soil hydrology models that are part of water quality and resource assessment programs require this information.
       
3. Measurement.—There are instruments that measure albedo.
    
4. Estimation.—Approximate the values by use of the following formula:
    
   1. Soil Albedo = 0.069 × (Color Value) - 0.114.
       
   2. For albedo, dry, use dry color value. Surface roughness has a separate significant impact on the actual albedo. The equation above is the albedo of <2.0 mm smoothed soil condition; if the surface is rough because of tillage, the albedo differs.
       
5. Entries.—Enter the high, low, and representative values of the map unit component using the above formula. The range of valid entries is from 0 to 1, and hundredths (two decimal places) are allowed.
    

618.5  Available Water Capacity

1. Definition.—“Available water capacity” is the volume of water that should be available to plants if the soil, inclusive of fragments, were at field capacity. It is commonly estimated as the amount of water held between field capacity and wilting point, with corrections for salinity, fragments, and rooting depth.
    
2. Classes.—Classes of available water capacity are not normally used except as adjective ratings that reflect the sum of available water capacity in inches to some arbitrary depth. Class limits vary according to climate zones and the crops commonly grown in the areas. The depth of measurement also is variable.
    
3. Significance.—Available water capacity is an important soil property in developing water budgets, predicting droughtiness, designing and operating irrigation systems, designing drainage systems, protecting water resources, and predicting yields.
    
4. Estimates.—The most common estimates of available water capacity are made in the field or the laboratory as follows:
    
   1. Field capacity is determined by sampling the soil moisture content just after the soil has drained following a period of rain and humid weather, after a spring thaw, or after heavy irrigation. Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS, provides more information.
       
   2. The 15-bar moisture content of the samples is determined with pressure membrane apparatus.
       
   3. An approximation of soil moisture content at field capacity is commonly made in the laboratory using 1/3-bar moisture percentage for clayey and loamy soil materials and 1/10-bar for sandy materials. Recently, some soil physicists have been using 1/10-bar instead of 1/3-bar for clayey and loamy soil materials and 1/20-bar for sandy soil materials.
       
   4. Measure the bulk density of the moist soil. Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS, provides more information.
       
   5. Calculate available water capacity (AWC) using the following formula:

      AWC = (W_1/3 - W₁₅) x (Db_1/3) x Cm / 100

      Where

      AWC = volume of water retained in 1 cm³ of whole soil between 1/3-bar and 15-bar tension; reported as cm cm^-1 [numerically equivalent to inches of water per inch of soil (in in^-1)]

      W_1/3 = weight percentage of water retained at 1/3-bar tension

      W₁₅ = weight percentage of water retained at 15-bar tension

      Db_1/3 = bulk density of <2-mm fabric at 1/3-bar tension

      Cm = rock fragment conversion factor derived from: volume moist <2-mm fabric (cm³) / volume moist whole soil (cm³)

      Method 3A2b is used to determine volume moist <2-mm fabric (cm³).

      AWC (cm cm^-1 or in in^-1 horizon) = AWC (cm cm^-1 or in in^-1) x horizon thickness
       

   6. If data are available, estimates are based on available water capacity measurements. If data are not available, data from similar soils are used as a guide. The relationship between available water capacity and other soil properties has been studied by many researchers. Soil properties that influence available water capacity are particle size; size, shape, and distribution of pores; organic matter; type of clay mineral; and structure.
       
   7. If roots are excluded from a horizon such as a duripan, the amount of water available to plants is nearly zero. Available water capacity values are zero for layers that exclude roots. If roots are restricted but not excluded, estimates of available water capacity are reduced according to the amount of dense material in the layers and the space available for root penetration. Depending on the ability of roots to enter the soil mass and utilize the water, values for the soils with these dense layers may be significantly less than for soils of similar texture that do not have pans. Entries are made for all soil layers below dense layers only if roots are present.
       
   8. Depending on their abundance and porosity, rock and pararock fragments reduce available water capacity. Nonporous fragments reduce available water capacity in proportion to the volume they occupy. For example, 50-percent nonporous cobbles reduce available water capacity as much as 50 percent. Porous fragments, such as sandstone, may reduce available water capacity to a lesser extent.
       
   9. Several factors contribute to a lower amount of plant growth on saline soils. However, as a rough guide, available water capacity is reduced by about 25 percent per 4 mmhos cm^-1 electrolytic conductivity of the saturated extract.
       
   10. Soils high in gibbsite or kaolinite, such as Oxisols and Ultisols, may have available water capacity values that are about 20 percent lower than those with equal amounts of 2:1 layer-lattice clays.
        
   11. Soils high in organic matter have a higher available water capacity than soils low in organic matter if the other properties are the same.
        
5. Entries.—Enter high, low, and representative values for available water capacity in cm per cm for each horizon. Enter “0” for layers that exclude roots. The range of valid entries is from 0 to 0.7 cm per cm, and hundredths (two decimal places) are allowed.
    

618.6  Bulk Density, One-Third Bar

1. Definition.—“Bulk density, one-third bar” is the oven-dried weight of the less than 2 mm soil material per unit volume of soil at a water tension of 1/3 bar (33 kPa).
    
2. Significance.—Bulk density influences plant growth and engineering applications. It is used to convert measurements from a weight basis to a volume basis. Within a family level particle-size class, bulk density is an indicator of how well plant roots are able to extend into the soil. Bulk density is used to calculate porosity.
    
   1. Plant Growth.—Bulk density is an indicator of how well plant roots are able to extend into the soil. Root restriction initiation and root-limiting bulk densities are shown below for various particle-size classes.
       

      Particle-Size Class	Bulk Density (g cm-3)
      	Restriction-Initiation	Root-Limiting
      Sandy	1.69	>1.85
      Loamy
      coarse-loamy	1.63	>1.80
      fine-loamy	1.60	>1.78
      coarse-silty	1.60	>1.79
      fine-silty	1.54	>1.65
      Clayey*
      35-45% clay content	1.49	>1.58
      >45% clay content	1.39	>1.47

      *Soils with high iron oxide content (e.g., sesquic mineralogy) or with andic soil properties can initiate restriction at lower bulk densities.
       

   2. Engineering Applications.—Soil horizons with bulk densities less than those indicated below have low strength and would be subject to collapse if wetted to field capacity or above without loading. They may require special designs for certain foundations.
       

      Particle-Size Class	Bulk Density (g cm-3)
      Sandy	<1.60
      Loamy
      coarse-loamy	<1.40
      fine-loamy	<1.40
      coarse-silty	<1.30
      fine-silty	<1.40
      Clayey	<1.10



3. Estimates.—The weight applies to the oven-dry soil, and the volume applies to the soil at or near field capacity. Bulk density is a use-dependent property. The entry should represent the dominant use for the soil.
    
4. Entries.—Enter bulk density at one-third bar with the low, high, and representative values for each horizon. The range of valid entries is from 0.02 to 2.6 g cm^-3, and hundredths (two decimal places) are allowed. Values should be estimated to the nearest 0.05 g cm^-3.
    

618.7  Bulk Density, Oven Dry

1. Definition.—“Bulk density, oven dry” (pb_od) is the oven-dry weight of the less than 2 mm soil material per unit volume of oven-dry soil.
    
2. Estimation.—The value pb_od is derived by the following formula:

   pb_od = [(linear extensibility percent/100) + 1]³

   Where, linear extensibility percent is adjusted to a <2 mm basis.
    

3. Entries.—Enter the high, low, and representative values for each horizon. The range of valid entries is from 0.02 to 2.6 g cm^-3, and hundredths (two decimal places) are allowed.
    

618.8  Calcium Carbonate Equivalent

1. Definition.—“Calcium carbonate equivalent” is the quantity of carbonate (CO₃) in the soil expressed as CaCO₃ and as a weight percentage of the less than 2 mm size fraction.
    
2. Significance.—The availability of plant nutrients is influenced by the amount of carbonates in the soil. This is a result of the effect that carbonates have on soil pH and of the direct effect that carbonates have on nutrient availability. Nitrogen fertilizers should be incorporated into calcareous soils to prevent nitrite accumulation or ammonium-N volatilization. The availability of phosphorus and molybdenum is reduced by the high levels of calcium and magnesium which are associated with carbonates. In addition, iron, boron, zinc, and manganese deficiencies are common in soils that have a high calcium carbonate equivalent. In some climates, soils that have a high calcium carbonate equivalent in the surface layer are subject to wind erosion. This effect may occur in soils that have a calcium carbonate equivalent of more than 5 percent. A strongly or violently effervescent reaction to cold, dilute hydrochloric acid (HCL) defines calcareous in the wind erodibility groups because of the significance of finely divided carbonates.
    
3. Measurement.—Calcium carbonate equivalent is measured by a method that uses an aqueous solution of 3 normal hydrogen chloride. The method is outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. It also may be measured in the field using calcimeters.
    
4. Entries.—Enter the high, low, and representative values for each horizon listed. Round values to the nearest 5 percent for horizons that have more than 5 percent CaCO₃ and to the nearest 1 percent for those with less than 5 percent. Enter 0 if the horizon does not have free carbonates. The range of valid entries is from 0 to 110 percent, and only whole numbers (integers) are allowed.
    

618.9  Cation-Exchange Capacity NH₄OAc pH 7

1. Definition.—“Cation-exchange capacity” is the amount of exchangeable cations that a soil can adsorb at pH 7.0.
    
2. Significance.—Cation-exchange capacity is a measure of the ability of a soil to retain cations, some of which are plant nutrients. Soils that have a low cation-exchange capacity hold fewer cations and may require more frequent applications of fertilizer than soils that have a high cation-exchange capacity. Soils that have a high cation-exchange capacity have the potential to retain cations, which reduces the risk of the pollution of ground water.
    
3. Measurement.—Cation-exchange capacity is measured by the methods outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. The ammonium acetate method gives the cation-exchange capacity value (CEC-7) for soils that have pH >5.5 or contain soluble salts. This method uses a solution of one normal ammonium acetate buffered at pH 7.0 to provide the extracting index cation (NH₄⁺). Cation-exchange capacity is reported in centimoles per kilogram (cmol(+) kg^-1), which are equivalent to milliequivalents per 100 grams (meq 100 g^-1) of soil. If the pH is less than 5.5, use effective cation-exchange capacity (refer to Section 618.17).
    
4. Entries.—Enter the high, low, and representative values of the estimated range in cation-exchange capacity, in meq 100 g^-1, for each horizon with pH >5.5. The range of valid entries is from 0 to 400 meq 100 g^-1, and tenths (one decimal place) are allowed. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.99.
    

618.10  Climatic Setting

Climatic setting includes frost-free period, precipitation, temperature, and evaporation. These elements are useful in determining the types of natural vegetation or crops that grow or can grow in an area and in planning management systems for vegetation. Climatic data are observed nationally by the National Weather Service Cooperative Network, which consists of approximately 10,000 climate stations. The records are available from the Climatic Data Access Facility (CDAF) at Portland, Oregon. Climatic data are delivered to the field through the Climatic Data Access Network. The Climatic Data Access Network consists of climatic data liaisons established in each State and at National Headquarters. Climatic data that are input into NASIS are obtained from the respective climatic data liaison. Climatic data may also be obtained from project weather stations or from the State climatologist. NRCS has selected the current standard “normal” period of 1971 to 2000 for climate database entries. (Existing entries in the NASIS database may reflect the prior period of 1961 to 1990.) Always check with your State’s climatic data liaison before using a climate station that has less than 30 years of records or that is located outside a county. Footnote the source of the data, the station, and the starting and ending year of record. Means are given as a range to represent the change of the climate over the geographic extent of the assigned soil.

1. Frost-Free Period
    
   1. Definition.—“Frost-free period” is the expected number of days between the last freezing temperature (0 °C) in spring (January-July) and the first freezing temperature (0 °C) in fall (August-December). The number of days is based on the probability that the values for the standard “normal” period will be exceeded in 5 years out of 10.
       
   2. Entries.—Enter the high, low, and representative values for the map unit component. Enter 365 for each value for taxa that are frost-free all year and 0 for those that have no frost-free period. Entries are rounded to the nearest 5 days.
       
2. Precipitation, Mean Annual
    
   1. Definition.—“Mean annual precipitation” is the arithmetic average of the total annual precipitation taken over the standard “normal” period. Precipitation refers to all forms of water, liquid or solid, that fall from the atmosphere and reach the ground.
       
   2. Entries.—Enter the high, low, and representative values in millimeters of water to represent the spatial range for the map unit component. The range of valid entries is from 0 to 11,500 mm, and only whole numbers (integers) are allowed.
       
3. Air Temperature, Mean Annual
    
   1. Definition.—“Mean annual air temperature” is the arithmetic average of the daily maximum and minimum temperatures for a calendar year taken over the standard “normal” period.
       
   2. Entries.—Enter the high, low, and representative values for the map unit component to represent the spatial range in degrees Celsius (centigrade). The range of valid entries is from -50.0 to 50.0 degrees, and tenths (one decimal place) are allowed. Use a minus sign to indicate temperatures below zero.
       
4. Daily Average Precipitation
    
   1. Definition.—“Daily average precipitation” is the total precipitation for the month divided by the number of days in the month for the standard “normal” period.
       
   2. Entries.—Enter the high, low, and representative values, in millimeters. The range of valid entries is from 0 to 750 mm. Record values to the nearest whole number (integer).
       
5. Daily Average Potential Evapotranspiration
    
   1. Definition.—“Daily average potential evapotranspiration” is the total monthly potential evapotranspiration divided by the number of days in the month for the standard “normal” period.
       
   2. Entries.—Enter the high, low, and representative values for daily average potential evapotranspiration in millimeters. The range of valid entries is 0 to 300 mm. Record values to the nearest whole number (integer).
       

618.11  Corrosion

Various metals and other materials corrode when they are on or in the soil, and some metals and materials corrode more rapidly when in contact with specific soils than when in contact with others. Corrosivity ratings are given for two of the common structural materials, uncoated steel and concrete.

1. Uncoated steel
    
   1. Definition.—“Risk of corrosion for uncoated steel” is the susceptibility of uncoated steel to corrosion when in contact with the soil.
       
   2. Classes.—The classes for risk of corrosion to uncoated steel are low, moderate, and high.
       
   3. Significance.—Risk of corrosion to uncoated steel pertains to the potential soil-induced electrochemical or chemical action that converts iron into its ions, thereby dissolving or weakening uncoated steel.
       
   4. Guides.—Part 618, Subpart B, Exhibits, Section 618.80 gives the relationship of soil water, general texture group, acidity, and content of soluble salts (as indicated by either electrical resistivity at field capacity or electrolytic conductivity of the saturated extract of the soil) to corrosion classes.

      Soil reaction (pH) correlates poorly with corrosion potential; however, a pH of 4.0 or less almost always indicates a high corrosion potential.

      Ratings, which are based on a single soil property or quality, that place soils in relative classes for corrosion potential must be tempered by knowledge of other properties and qualities that affect corrosion. A study of soil properties in relation to local experiences with corrosion helps soil scientists and engineers to make soil interpretations. Special attention must be given to those soil properties that affect the access of oxygen and moisture to the metal, the electrolyte, the chemical reaction in the electrolyte, and the flow of current through the electrolyte. Special attention must be given to the presence of sulfides or of minerals, such as pyrite, that can be weathered readily and thus cause a high degree of corrosion in metals.

      The possibility of corrosion is greater for extensive installations that intersect soil boundaries or soil horizons than for installations that are in one kind of soil or in one soil horizon.

      Using interpretations for corrosion without considering the size of the metallic structure or the differential effects of using different metals may lead to wrong conclusions. Activities that alter the soil, such as construction, paving, fill and compaction, and surface additions, can increase the possibility of corrosion by creating an oxidation cell that accelerates corrosion. Mechanical agitation or excavation that results in aeration and in a discontinuous mixing of soil horizons may also increase the possibility of corrosion.
       

   5. Entries.—Enter the appropriate class of risk of corrosion for uncoated steel for the whole map unit component. The classes are low, moderate, and high.
       
2. Concrete
    
   1. Definition.—“Risk of corrosion for concrete” is the susceptibility of concrete to corrosion when in contact with the soil.
       
   2. Classes.—The classes for risk of corrosion to concrete are low, moderate, and high
       
   3. Significance.—Risk of corrosion to concrete pertains to the potential soil-induced chemical reaction between a base (the concrete) and a weak acid (the soil solution). Special cements and methods of manufacturing may be used to reduce the rate of deterioration in soils that have a high risk of corrosion. The rate of deterioration depends on soil texture and acidity; the amount of sodium or magnesium sulfate present in the soil, singly or in combination; and the amount of sodium chloride (NaCl) present in the soil. The presence of NaCl is evaluated because it is used to identify the presence of seawater, rather than because of its corrosive effects on concrete. Seawater contains sulfates, which are one of the principal corrosive agents. A soil that has gypsum or other sulfate minerals requires a special cement in the concrete mix. The calcium ions in gypsum react with the cement and weaken the concrete.
       
   4. Guides.—Part 618, Subpart B, Exhibits Section 618.81 gives the relationship of soil texture, soil acidity, sulfates, and NaCl to corrosion classes.
       
   5. Entries.—Enter the appropriate class of risk of corrosion for concrete for the whole map unit component. The classes are low, moderate, and high.
       

618.12  Crop Name and Yield

1. Definition.—“Crop name” is the common name for the crop. “Crop yield” is crop yield units per unit area for the specified crop.
    
2. Classes.—The crop names and the units of measure for yields that are allowable as data entries are listed in the NASIS data dictionary. See Part 618, Subpart B, Exhibits, Section 618.82 for the Web address of the current NASIS data dictionary.
    
3. Significance.—Crop names and units of measure are important as records of crop yield. Although the crops and yield often are specific to the time when the soil survey was completed, the ranking and comparison between soils within a soil survey are helpful. These crop and yield data are used to evaluate the soil productive capabilities, cash rent, and land values. Generally, only the most important crops are listed and only the best management is reflected.
    
4. Estimates
    
   1. Crop names and yields are specific to the soil survey area. Although the listing of crop names is not limited to any number, only the most important crops in the survey area should be used. The yields are derived in a number of ways but should represent a high level of management by leading commercial farmers, which tends to produce the highest economic return per acre. This level of management includes using the best varieties; balancing plant populations and added plant nutrients to the potential of the soil; controlling erosion, weeds, insects, and diseases; maintaining optimum soil tilth; providing adequate soil drainage; and ensuring timely operations.
       
   2. Generally only a representative value is used for each map unit component for non-MLRA soil survey areas. MLRA soil survey areas use the high and low representative values from map unit components of non-MLRA soil survey areas. High and low values represent the range of representative values for a high level of management across the survey area or across several survey areas.
       
5. Entries.—Enter the common crop name and units of measure. Enter the corresponding irrigated yields, nonirrigated yields, or both, as appropriate for the component. Yields can be posted as high, low, and representative values for the map unit component.
    

618.13  Diagnostic Horizon Feature — Depth to Bottom

1. Definition.—The diagnostic horizon feature “depth to bottom” is the distance from the top of the soil to the base of the identified diagnostic horizon or to the lower limit of the occurrence of the diagnostic feature.
    
2. Measurement.—Distance is measured from the top of the soil, which is defined as the top of the mineral soil, or, for soils with “O” horizons, the top of any organic layer that is at least slightly decomposed. For soils that are covered by 80 percent or more rock or pararock fragments, the top of the soil is the surface of the fragments. See pages 63-64 in the Soil Survey Manual for a complete discussion.
    
3. Entries.—The values for the diagnostic horizon feature depth to bottom used to populate component data in NASIS are not specific to any one point; they are a reflection of commonly observed values based on field observations and are intended to model the component as it occurs throughout the map unit. Enter the high, low, and representative values in whole centimeters. The high value represents either the greatest depth to which the base of the diagnostic horizon or feature extends or, for horizons for features extending beyond the limit of field observation, is the depth to which observation was made (usually no more than 200 cm). In the case of lithic contact, paralithic contact, and petroferric contact, the entries for depth to the bottom of the diagnostic feature will be the same as the entries for depth to the top of the feature since the contact has no thickness.
    

618.14  Diagnostic Horizon Feature — Depth to Top

1. Definition.—The diagnostic horizon feature “depth to top” is the distance from the top of the soil to the upper boundary of the identified diagnostic horizon or to the upper limit of the occurrence of the diagnostic feature.
    
2. Measurement.— Distance is measured from the top of the soil, which is defined as the top of the mineral soil, or, for soils with “O” horizons, the top of any organic layer that is at least slightly decomposed. For soils that are covered by 80 percent or more rock or pararock fragments, the top of the soil is the surface of the fragments. See pages 63-64 in the Soil Survey Manual for a complete discussion.
    
3. Entries.—The values for the diagnostic horizon feature depth to top used to populate component data in NASIS are not specific to any one point; they are a reflection of commonly observed values based on field observations and are intended to model the component as it occurs throughout the map unit. Enter the high, low, and representative values in whole centimeters.
    

618.15  Diagnostic Horizon Feature — Kind

1. Definition.—The diagnostic horizon feature “kind” is the kind of diagnostic horizon or diagnostic feature present in the soil.
    
2. Significance.—Diagnostic horizons and features are a particular set of observable or measurable soil properties, defined in Soil Taxonomy, that are used to classify a soil. They have been chosen because they are thought to be the marks left on the soil as a result of the dominant soil-forming processes. In many cases they are thought to occur in conjunction with other important accessory properties. The utilization of diagnostic horizons and features in the classification process allows the grouping of soils that have formed as a result of similar genetic processes. The grouping, however, is done on the basis of observable or measurable properties rather than by speculation about the genetic history of a particular soil.
    
3. Entries.—The diagnostic horizons and features are listed in the latest edition of the Keys to Soil Taxonomy. Allowable terms are given in the NASIS data dictionary.
    

618.16  Drainage Class

1. Definition.—“Drainage class” identifies the natural drainage condition of the soil. It refers to the frequency and duration of wet periods.
    
2. Classes.—The eight natural drainage classes are listed below. Chapter 3 of the Soil Survey Manual provides a description of each natural drainage class.
    
   1. Excessively drained
   2. Somewhat excessively drained
   3. Well drained
   4. Moderately well drained
   5. Somewhat poorly drained
   6. Poorly drained
   7. Very poorly drained
   8. Subaqueous
       
3. Significance.—Drainage classes provide a guide to the limitations and potentials of the soil for field crops, forestry, range, wildlife, and recreational uses. The class roughly indicates the degree, frequency, and duration of wetness, which are factors in rating soils for various uses.
    
4. Estimates.—Infer drainage classes from observations of landscape position and soil morphology. In many soils the depth and duration of wetness relate to the quantity, nature, and pattern of redoximorphic features. Correlate drainage classes and redoximorphic features through field observations of water tables, soil wetness, and landscape position. Record the drainage classes assigned to the series.
    
5. Entries.—Enter the drainage class name for each map unit component. Use separate map unit components for different drainage class phases or for drained versus undrained phases where needed.
    

618.17  Effective Cation-Exchange Capacity

1. Definition.—“Effective cation-exchange capacity” is the sum of ammonium acetate extractable bases plus potassium chloride extractable aluminum.
    
2. Significance.—Cation-exchange capacity (CEC) is a measure of the ability of a soil to retain cations, some of which are plant nutrients. Soils that have a low cation-exchange capacity hold fewer cations and may require more frequent applications of fertilizer and amendments than soils that have a high cation-exchange capacity. Soils that have a high cation-exchange capacity have the potential to retain cations. Effective CEC is a measure of CEC that is particularly useful in soils whose ion-exchange capacity is largely a result of variable charge components, such as allophane, kaolinite, hydrous iron and aluminum oxides, and organic matter. As a result, the CEC of these soils is not a fixed number but is a function of pH. Examples include some andic soils, Oxisols, and more weathered Ultisols with kaolinitic mineralogy.
    
3. Measurement.—Effective cation-exchange capacity is calculated from the results of two separate laboratory methods. One method measures the basic cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) extractable in a solution of one normal ammonium acetate buffered at pH 7.0. Another method measures the aluminum extractable in a solution of one normal potassium chloride. The methods are outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. The effective cation-exchange capacity (ECEC) value is reported for soil horizons that have pH <5.5 and that are low in soluble salts. For soils that have a pH of 5.5 or greater, the ECEC equals the sum of NH₄OAc extractable bases. Effective cation-exchange capacity is reported in centimoles per kilogram (cmol(+) kg^-1) of soil, which are equivalent to milliequivalents per 100 grams (meq 100 g^-1) of soil.
    
4. Entries.—Enter the high, low, and representative values of the estimated range in effective cation-exchange capacity at the field pH of the soil, in meq 100 g^-1, for the horizon. The range of valid entries is from 0 to 400 meq 100 g^-1, and tenths (one decimal place) are allowed. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.100.
    

618.18  Electrical Conductivity

1. Definition.—“Electrical conductivity” is the electrolytic conductivity of an extract from saturated soil paste.
    
2. Classes.—The classes of salinity are:
    

   Salinity Class	Electrical Conductivity (mmhos cm-1)
   Nonsaline	0 to <2
   Very slightly saline	2 to <4
   Slightly saline	4 to <8
   Moderately saline	8 to <16
   Strongly saline	≥16



3. Significance.—Electrical conductivity is a measure of the concentration of water-soluble salts in soils. It is used to indicate saline soils. High concentrations of neutral salts, such as sodium chloride and sodium sulfate, may interfere with the absorption of water by plants because the osmotic pressure in the soil solution is nearly as high as or higher than that in the plant cells. Salts may also interfere with the exchange capacity of nutrient ions, thereby resulting in nutritional deficiencies in plants.
    
4. Measurement.—The electrolytic conductivity of a saturated extract is the standard measure used to express salinity. Units of measure are decisiemens per meter (dS m^-1), which are equivalent to millimhos per centimeter (mmhos cm^-1), at 25 degrees C. The laboratory procedure used to measure electrical conductivity is described in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
    
5. Estimates.—Field estimates of salinity are made from observations of visible salts on faces of peds, throughout the horizon matrix, on the soil surface, or some combination of the three; from plant growth or productivity; from the presence of native plant indicator species; and from field salinity meters. The occurrences of bare spots, salt-tolerant plants, and uneven crop growth are used as indicators of salinity and high electrical conductivity. When keyed to measurements, these observations help to estimate the amount of salts.
    
6. Entries.—Enter the high, low, and representative values for the range of electrolytic conductivity of the saturation extract during the growing season for each horizon. Use the following ranges to represent the high and low values of the salinity classes: 0-2, 2-4, 4-8, 8-16, and 16-32 (or a reasonable high value for the strongly saline class) or use a combination of classes (for example, 2-8 for the high and low values). The range of valid entries is from 0 to 15000 mmhos cm^-1, and tenths (one decimal place) are allowed.
    

618.19  Elevation

1. Definition.—“Elevation” is the vertical distance from mean sea level to a point on the earth’s surface.
    
2. Significance.—Elevation, or local relief, influences the genesis of natural soil bodies. Elevation also may affect soil drainage within a landscape, salinity or sodicity within a climatic area, or soil temperature.
    
3. Estimates.—Elevation is normally obtained from U.S. Geological Survey topographic maps or measured using altimeters or global positioning systems.
    
4. Entries.—Enter the high, low, and representative values for elevation in meters for each map unit component. The range of valid entries is from -300 to 8550 meters, and tenths (one decimal place) are allowed.
    

618.20  Engineering Classification

1. AASHTO Group Classification
    
   1. Definition.—“AASHTO group classification” is a system that classifies soils specifically for geotechnical engineering purposes that are related to highway and airfield construction. It is based on particle-size distribution and Atterberg limits, such as liquid limit and plasticity index. This classification system is covered in Standard No. M 145-82, published by the American Association of State Highway and Transportation Officials (AASHTO), and consists of a symbol and a group index. The classification is based on that portion of the soil that is smaller than 3 inches in diameter.
       
   2. Classes.—The AASHTO classification system identifies two general classifications: granular materials having 35 percent or less, by weight, particles smaller than 0.074 mm in diameter and silt-clay materials having more than 35 percent, by weight, particles smaller than 0.074 mm in diameter. These two divisions are further subdivided into seven main group classifications. Part 618, Subpart B, Exhibits, Section 618.83 shows the criteria for classifying soil in the AASHTO classification system. The group and subgroup classifications are based on estimated or measured grain-size distribution and on liquid limit and plasticity index values.
       
   3. Significance.—The group and subgroup classifications of this system aid in the evaluation of soils for highway and airfield construction. The classifications can help to make general interpretations relating to performance of the soil for engineering uses, such as highways and local roads and streets.
       
   4. Measurements.—Measurements involve sieve analyses for the determination of grain-size distribution of that portion of the soil between a 3 inch and 0.074 mm particle size. ASTM Designations D 422, C 136, and C 117 have applicable procedures for the determination of grain-size distribution. The liquid limit and plasticity index values (ASTM Designation D 4318) are determined for that portion of the soil having particles smaller than 0.425 mm in diameter (no. 40 sieve). Measurements, such as laboratory tests, are made on most benchmark soils and on other representative soils in survey areas.
       
   5. Estimates.—During soil survey investigations and field mapping activities, the soil is classified by field methods. This classification involves making estimates of particle-size fractions by a percentage of the total soil, minus the greater-than-3-inch fraction. Estimates of liquid limit and plasticity index are based on clay content and mineralogy relationships. Estimates are expressed in ranges that include the estimating accuracy as well as the range of values for the taxon.
       
   6. Entries.—Enter classes and separate them by commas for each horizon, for example, A-7, A-6. The acceptable entries for AASHTO group are A-1, A-l-a, A-l-b, A-2, A-2-4, A-2-5, A-2-6, A-2-7, A-3, A-4, A-5, A-6, A-7, A-7-5, A-7-6, and A-8.
       
2. AASHTO Group Index
    
   1. Definition.—The AASHTO group and subgroup classifications may be further modified by the addition of a group index value. The empirical group index formula was devised for approximate within-group evaluation of the “clayey granular” materials and the “silty-clay” materials.
       
   2. Significance.—The group index (GI) aids in the evaluation of the soils for highway and airfield construction. The index can help to make general interpretations relating to performance of the soil for engineering uses, such as highways and local roads and streets.
       
   3. Measurement.—The group index is calculated from an empirical formula:

      GI = (F-35) [0.2 + 0.005 (LL-40)] + 0.01 (F-15) (PI-10)

      Where:
      F = percentage passing sieve No. 200 (75 micrometer), expressed as a whole number
      LL = liquid limit
      PI = plasticity index

      In calculating the group index of A-2-6 and A-2-7 subgroups, only the PI portion of the formula is used.
       

   4. Entries.—The group index is reported to the nearest integer. If the calculated group index is negative, the group index value is zero. The minimum group index value is 0 and the maximum is 120. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.98.
       
3. Unified Soil Classification
    
   1. Definition.—The “Unified soil classification system” is a system that classifies mineral and organic mineral soils for engineering purposes based on particle-size characteristics, liquid limit, and plasticity index.
       
   2. Classes
       
      1. The Unified soil classification system identifies three major soil divisions:
          
         • Coarse grained soils having less than 50 percent, by weight, particles smaller than 0.074 mm in diameter.
         • Fine grained soils having 50 percent or more, by weight, particles smaller than 0.074 mm in diameter.
         • Highly organic soils that demonstrate certain organic characteristics. These divisions are further subdivided into a total of 15 basic soil groups.
            
      2. The major soil divisions and basic soil groups are determined on the basis of estimated or measured values for grain-size distribution and Atterberg limits. ASTM Designation D 2487 shows the criteria chart used for classifying soil in the Unified system, the 15 basic soil groups of the system, and the plasticity chart for the system.
          
   3. Significance.—The various groupings of this classification have been devised to correlate in a general way with the engineering behavior of soils. This correlation provides a useful first step in any field or laboratory investigation for engineering purposes. It can be used to make some general interpretations relating to probable performance of the soil for engineering uses.
       
   4. Measurement.—The methods for measurement are provided in ASTM Designation D 2487. Measurements involve sieve analysis for the determination of grain-size distribution of that portion of the soil between 3 inches and 0.074 mm in diameter (no. 200 sieve). ASTM Designations D 422, C 136, and C 117 have applicable procedures that are used, where appropriate, for the determination of grain-size distribution. Values for the Atterberg limits (liquid limit and plasticity index) are also used. Specific tests are made for that portion of the soil having particles smaller than 0.425 mm in diameter (no. 40 sieve) according to ASTM Designations D 423 and D 424. Measurements, such as laboratory tests, are made on most benchmark soils and on other representative soils in survey areas.
       
   5. Entries for Measured Data.—For measured Unified data, enter up to four classes for each horizon. ASTM Designation D 2487 provides flow charts for classifying the soils. Separate the classes by commas, for example, CL-ML, ML. Acceptable entries are GW, GP, GM, GC, SW, SP, SM, SC, CL, ML, OL, CH, MH, OH, PT, CL-ML, GW-GM, GW-GC, GP-GM, GP-GC, GC-GM, SW-SM, SW-SC, SP-SM, SP-SC, and SC-SM.
       
   6. Estimates.—The methods for estimating are provided in ASTM Designation D 2488. During all soil survey investigations and field mapping activities, the soil is classified by field methods. The methods include making estimates of particle-size fractions by a percentage of the total soil. The Atterberg limits are also estimated based on the wet consistency, ribbon or thread toughness, and other simple field tests. These tests and procedures are explained in ASTM Designation D 2488. If samples are later tested in the laboratory, adjustments are made to field procedures as needed. Estimates are expressed in ranges that include the estimating accuracy as well as the range of values from one location to another within the map unit. If an identification is based on visual-manual procedures, it must be clearly stated so in reporting.
       
   7. Entries for Estimated Soils.—For estimated visual-manual Unified data, enter up to four classes for each horizon. ASTM Designation D 2488 provides flow charts for classifying the soils. Separate the classes by commas, for example, CL, ML, SC. Acceptable entries are GW, GP, GM, GC, SW, SP, SM, SC, CL, ML, CH, MH, OL/OH, PT, GW-GM, GW-GC, GP-GM, GP-GC, SW-SM, SW-SC, SP-SM, and SP-SC.
       

618.21  Erosion Accelerated, Kind

1. Definition.—“Erosion accelerated, kind” is the type of detachment and removal of surface soil particles that is largely affected by human activity.
    
2. Significance.—The type of accelerated erosion is important in assessing the current health of the soil and in assessing its potential for different uses. Erosion, whether natural or induced by humans, is an important process that affects soil formation and may remove all or parts of the soils formed in the natural landscape.
    
3. Classes.—There are five kinds of accelerated erosion:
    
   1. Water erosion, sheet
   2. Water erosion, rill
   3. Water erosion, gully
   4. Water erosion, tunnel
   5. Wind erosion


4. Entries.—Enter the appropriate class for each map unit component. Multiple entries are allowable, but a representative value should be indicated.
    

618.22  Erosion Class

1. Definition.—“Erosion class” is the class of accelerated erosion.
    
2. Significance
    
   1. The degree of erosion that has taken place is important in assessing the health of the soil and in assessing the soil’s potential for different uses. Erosion is an important process that affects soil formation and may remove all or parts of the soils formed in natural landscapes.
       
   2. Removal of increasing amounts of soil increasingly alters various properties and capabilities of the soil. Properties and qualities affected include bulk density, organic matter content, tilth, and water infiltration. Altering these properties affects the productivity of the soil.
       
3. Estimation.—During soil examinations, estimate the degree to which soils have been altered by accelerated erosion. The Soil Survey Manual describes the procedures involved.
    
4. Classes.—There are five erosion classes:
    
   1. None - deposition
   2. Class 1
   3. Class 2
   4. Class 3
   5. Class 4


5. Entries.—Enter the appropriate class for each map unit component.
    

618.23  Excavation Difficulty Classes

1. Definition.—“Excavation difficulty classes” are used for soil layers, horizons, pedons, or geologic layers and estimate the difficulty of making an excavation into them. Excavation difficulty, in most instances, is strongly controlled by water state, which should be specified.
    
2. Classes.—The excavation difficulty classes are defined in the following table.
    

   Class	Definition
   Low	Excavations can be made with a spade using arm-applied pressure only. Neither application of impact energy nor application of pressure with the foot to a spade is necessary.
   Moderate	Arm-applied pressure to a spade is insufficient. Excavation can be accomplished quite easily by application of impact energy with a spade or by foot pressure on a spade.
   High	Excavation with a spade can be accomplished with difficulty. Excavation is easily possible with a full length pick, using an over-the-head swing.
   Very High	Excavation with a full length pick, using an over-the-head swing, is moderately to markedly difficult. Excavation is possible in a reasonable period of time with a backhoe mounted on a 40 to 60 kW (50-80 hp) tractor.
   Extremely High	Excavation is nearly impossible with a full length pick using an over-the-head arm swing. Excavation cannot be accomplished in a reasonable time period with a backhoe mounted on a 40 to 60 kW (50-80 hp) tractor.



3. Significance.—Excavation difficulty classes are important for evaluating the cost and time needed to prepare shallow excavations.
    
4. Estimates.—Estimates of excavation difficulty classes are made from field observations.
    
5. Entries.—Enter the appropriate class for each horizon. The allowable entries are low, moderate, high, very high, and extremely high.
    

618.24  Extractable Acidity

1. Definition.—“Extractable acidity” is a measure of soil exchangeable hydrogen ions that may become active by cation exchange.
    
2. Significance.—Extractable acidity is important for soil classification and for certain evaluations of soil nutrient availability or of the effect of waste additions to the soil.
    
3. Measurement.—Extractable acidity is determined by a method using a solution of barium chloride-triethanolamine buffered at pH 8.2, as outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. Units of measure are centimoles per kilogram (cmol(+) kg^-1) of soil, which are equivalent to milliequivalents per 100 grams (meq 100 g^-1) of soil.
    
4. Entries.—Enter the range of extractable acidity in milliequivalents per 100 grams (meq 100 g^-1) of soil for the horizon. The range of valid entries is from 0 to 250 meq 100 g^-1, and tenths (one decimal place) are allowed. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.101.
    

618.25  Extractable Aluminum

1. Definition.—“Extractable aluminum” is the amount of aluminum that approximates the aluminum considered exchangeable. It is a measure of the “active” acidity present in soils with a 1:1 water pH <5.5.
    
2. Significance.—Extractable aluminum is important for soil classification and for certain evaluations of soil nutrient availability and of toxicities. An aluminum saturation of about 60 percent is usually regarded as toxic to most plants. It may be a useful measurement for assessing potential lime needs for acid soils.
    
3. Measurement.—Extractable aluminum is determined by a method using a solution of one normal potassium chloride, as outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. Units of measure are centimoles per kilogram (cmol(+) kg^-1) of soil, which are equivalent to milliequivalents per 100 grams (meq 100 g^-1) of soil.
    
4. Entries.—Enter the range of extractable aluminum as milliequivalents per 100 grams (meq 100 g^-1) of soil for the horizon. The range of valid entries is from 0 to 150 meq 100 g^-1, and hundredths (two decimal places) are allowed.
    

618.26  Flooding Frequency Class, Duration Class, and Month

1. Definition.—“Flooding” is the temporary covering of the soil surface by flowing water from any source, such as streams overflowing their banks, runoff from adjacent or surrounding slopes, inflow from high tides, or any combination of sources. Shallow water standing or flowing that is not concentrated as local runoff during or shortly after rain or snowmelt is excluded from the definition of flooding. Chapter 3 of the Soil Survey Manual provides additional information. Standing water (ponding) or water that forms a permanent covering is also excluded from the definition.
    
2. Classes.—Estimates of flooding class are based on the interpretation of soil properties and other evidence gathered during soil survey fieldwork. Flooding hazard is expressed by flooding frequency class, flooding duration class, and time of year that flooding occurs. Not considered here, but nevertheless important, are velocity and depth of floodwater. Frequencies used to define classes are generally estimated from evidence related to the soil and vegetation. They are expressed in wide ranges that do not indicate a high degree of accuracy. Flooding frequencies that are more precise can be calculated by performing complex analyses used by engineers. The class “very frequent” is used for areas subject to daily and monthly high tides.
    
   1. Flooding Frequency Class.—Flooding frequency class indicates the number of times flooding occurs over a period of time. The classes of flooding are defined as follows:
       

      Flooding Frequency Class	Definition
      None	No reasonable possibility of flooding; one chance out of 500 of flooding in any year or less than 1 time in 500 years.
      Very rare	Flooding is very unlikely but is possible under extremely unusual weather conditions; less than 1 percent chance of flooding in any year or less than 1 time in 100 years but more than 1 time in 500 years.
      Rare	Flooding is unlikely but is possible under unusual weather conditions; 1 to 5 percent chance of flooding in any year or nearly 1 to 5 times in 100 years
      Occasional	Flooding is expected infrequently under usual weather conditions; 5 to 50 percent chance of flooding in any year or 5 to 50 times in 100 years.
      Frequent	Flooding is likely to occur often under usual weather conditions; more than a 50 percent chance of flooding in any year (i.e., 50 times in 100 years), but less than a 50 percent chance of flooding in all months in any year.
      Very frequent	Flooding is likely to occur very often under usual weather conditions; more than a 50 percent chance of flooding in all months of any year.

   
   
   2. Flooding Duration Class.—The average duration of inundation per flood occurrence is given only for the occasional, frequent, and very frequent classes (defined above).
       

      Flooding Duration Class	Duration
      Extremely brief	0.1 to 4 hours
      Very brief	4 hours to < 2 days
      Brief	2 days to < 7 days
      Long	7 days to < 30 days
      Very long	> 30 days

   
   
   3. Assignment.—Yearly flooding frequency classes are assigned to months and indicate the months of occurrence and not the frequency of the flooding during the month, except for the very frequent class. The time period expressed includes two-thirds to three-fourths of the occurrences. Time period and duration of flooding are the most critical factors that determine the growth and survival of a given plant species. Flooding during the dormant season has few if any harmful effects on plant growth or mortality and may improve the growth of some species. If inundation from floodwater occurs for long periods during the growing season, the soil becomes oxygen deficient and plants may be damaged or killed.
       
3. Significance.—The susceptibility of soils to flooding is an important consideration for building sites, sanitary facilities, and other uses. Floods may be less costly per unit area of farmland as compared to that of urban land, but the loss of crops and livestock can be disastrous.
    
4. Estimates.—The most precise evaluation of flood-prone areas for stream systems is based on hydrologic studies. The area subject to inundation during a flood of a given frequency, such as one with a 1 percent or 2 percent chance of occurrence, is generally determined by one of two basic methods.
    
   1. The first method is used if stream flow data are available. In this method, the data are analyzed to determine the magnitude of floods of different frequencies. Engineering studies are made to determine existing channel capacities and flow on the flood plain by the use of valley cross sections and water-surface profiles.
       
   2. The second method is used if stream flow data are not available. In this method, hydrologists make an estimate of flood potential from recorded data on rainfall. They consider such factors as—
       
      1. Size, slope, and shape of the contributing watershed.
      2. Hydrologic characteristics of the soil.
      3. Land use and treatment.
      4. Hydraulic characteristics of the valley and channel system.
          
   3. With the use of either method, soil surveys can aid in the delineation of flood-prone areas. Possible sources of flooding information are—
       
      1. NRCS project-type studies, such as those arising from Public Law 556, flood protection, river basin, or resource conservation and development projects.
      2. Flood hazard analyses.
      3. Corps of Engineers (COE) flood plain information reports.
      4. Special flood reports.
      5. Local flood-protection and flood-control project reports.
      6. Department of Housing and Urban Development flood-insurance study reports.
      7. Maps by the U.S. Geological Survey (USGS), NRCS, Tennessee Valley Authority, COE, or National Oceanic and Atmospheric Administration.
      8. Studies by private firms and other units of Government.
      9. USGS quadrangle sheets and hydrologic atlases of flood-prone areas and stream gauge data.
          
   4. General estimates of flooding frequency and duration are made for each soil. However, in intensively used areas where construction has materially altered the natural water flow, flood studies are needed to adequately reflect present flooding characteristics.
       
   5. Soil scientists collect and record evidence of flood events during the course of the soil survey. The extent of flooded areas, flood debris in trees, damage to fences and bridges, and other signs of maximum water height are recorded. Information that is helpful in delineating soils that have a flood hazard is also obtained. Hydrologists may have flood stage predictions that can be related to kinds of soil or landscape features. Conservationists and engineers may have recorded elevations of high flood marks. Local residents may have recollections of floods that can help to relate the events to kinds of soil, topography, and geomorphology.
       
   6. Certain landscape features have developed as the result of past and present flooding and include former river channels, oxbows, point bars, alluvial fans, meander scrolls, sloughs, natural levees, backswamps, sand splays, and terraces. Most of these features are easily recognizable on aerial photographs when the photo image is compared to on-the-ground observations. Different kinds of vegetation and soils are normally associated with these geomorphic features.
       
   7. The vegetation that grows in flood areas may furnish clues to past flooding. In the central and southeastern United States, the survival of trees in flood-prone areas depends on the frequency, duration, depth, and time of flooding and on the age of the tree.
       
   8. Past flooding may sometimes leave clues in the soil, such as—
       
      1. Thin strata of material of contrasting color, texture, or both.
      2. An irregular decrease in organic matter content, not due to human-alteration by mixing or transportation of material, which is an indication of a buried genetic surface horizon.
      3. Soil layers that have abrupt boundaries to contrasting kinds of material, which indicate that the materials were laid down suddenly at different times and were from different sources or were deposited from stream flows of different velocities.
      4. Artifacts which are easily moved and deposited by flood waters (e.g., plastic bottles).
          
   9. Laboratory analyses of properly sampled layers are often helpful in verifying these observations. Organic carbon and particle-size analyses are particularly useful in verifying flood deposits. Microscopic observations may detect preferential horizontal orientation of plate-like particles; microlayering, which indicates water-laid deposits; or mineralogical differences between layers.
       
5. Entries.—Flooding and frequency are posted for each month of the year for each map unit component. Flooding entries reflect the current existing and mapped condition with consideration for dams, levees, and other human-induced changes affecting flooding frequency and duration.
    
   1. Enter the flooding frequency class name: none, very rare, rare, occasional, frequent, or very frequent.
       
   2. Enter the flooding duration class name that most nearly represents the soil: extremely brief, very brief, brief, long, or very long.
       

618.27  Fragments in the Soil

1. Definition.—“Fragments” are unattached, cemented pieces of bedrock, bedrock-like material, durinodes, concretions, nodules, or pedogenic horizons (e.g., petrocalcic fragments) 2 mm or larger in diameter and woody material 20 mm or larger in diameter in organic soils. Fragments are separated into three types: rock fragments, pararock fragments (which are distinguished by cementation class), and wood fragments. The words “rock” and “pararock” are used here in the broad sense and do not connote only natural fragments of geologic material.
    
   1. Rock fragments are unattached pieces of geologic or pedogenic material 2 mm in diameter or larger that are strongly cemented or more resistant to rupture. Rock fragments from 2 mm to 75 mm (3 inches) are considered when estimating the percent passing sieves, as discussed in section 618.43.
       
   2. Pararock fragments are unattached, cemented pieces of geologic or pedogenic material 2 mm in diameter or larger that are extremely weakly cemented to moderately cemented. These fragments are not retained on sieves because they are crushed by grinding during sample preparation.
       
   3. Wood fragments are woody materials that cannot be crushed between the fingers when moist or wet and are larger than 20 mm in size. Wood fragments are only used in organic soils. They are comparable to rock and pararock fragments in mineral soils.
       
2. Significance
    
   1. The fraction of the soil 2 mm or larger in diameter has an impact on the behavior of the whole soil. Soil properties, such as available water capacity, cation-exchange capacity, saturated hydraulic conductivity, structure, and porosity, are affected by the volume, composition, and size distribution of fragments in the soil. Fragments also affect the management of the soil and are used as interpretation criteria. Terms related to volume, size, and hardness of fragments are used as texture modifier terms.
       
   2. Generally, the fraction of soil greater than 75 mm (3 inches) in diameter is not included in the engineering classification systems. However, it can be added as a descriptive term to the group name, for example, poorly graded gravel with silt, sand, cobbles, and boulders. Estimates of the percent of cobbles and boulders are presented in the soil descriptions for a group name (see ASTM Designation D 2487, appendix X1.1). A small amount of these larger particles generally has little effect on soil properties. The particles may, however, have an effect on the use of a soil in certain types of construction. Often, the larger portions of a soil must be removed before the material can be spread in thin layers, graded, or compacted and graded to a smooth surface. As the quantity of this “oversized” fraction increases, the properties of the soil can be affected. If the larger particles are in contact with each other, the strength of the soil is very high and the compressibility very low. If voids exist between the larger particles, the soil will likely have high saturated hydraulic conductivity and may undergo some internal erosion as a result of the movement of water through the voids. Most of the smaller and more rapid construction equipment normally used in excavating and earthmoving cannot be used if the oversize fraction of a soil is significant.
       
3. Measurement.— The fraction from 2 to 75 mm in diameter may be measured in the field. However, 50 to 60 kg of sample material may be necessary if there is an appreciable amount of fragments near 75 mm. An alternative means of measuring is to visually estimate the volume of the 20- to 75-mm fraction, then sieve and weigh the 2- to 20-mm fraction. The fraction 75 mm (3 inches) or greater is usually not included in soil samples taken in the field for laboratory testing. Measurements can be made in the field by weighing the dry sample and the portion retained on a 3-inch screen. The quantity is expressed as a weight percentage of the total soil. A sample as large as 200 pounds to more than a ton may be needed to assure that the results are representative. Measurements of the fraction from 75 to 250 mm (3 to 10 inches) and the fraction greater than 250 mm (10 inches) in diameter are usually obtained from volume estimates.
    
4. Estimates
    
   1. Estimates are usually made by visual means and are on the basis of percent by volume. The percent by volume is converted to percent by weight by using the average bulk unit weights for soil and rock. These estimates are made during investigation and mapping activities in the field. They are expressed as ranges that include the estimating accuracy as well as the range of values for a component.
       
   2. Measurements or estimates of fragments less than strongly cemented are made prior to any rolling or crushing of the sample.
       
5. Rock Fragments Greater Than 10 Inches (250 mm)
    
   1. Definition.—“Rock fragments greater than 10 inches” is the percent by weight of the horizon occupied by rock fragments greater than 10 inches (250 mm) in diameter. Although the upper limit is undefined, for practical purposes it generally is no larger than a pedon, up to 10 meters square. For nonspherical material, the intermediate dimension is used for the 250 mm (10 inch) measurement. For example, a flat-shaped rock fragment that is 100 mm x 250 mm x 380 mm has an intermediate dimension of 250 mm and is not counted as greater than 250 mm. A flat-shaped rock fragment that is 100 mm x 275 mm x 380 mm has an intermediate dimension of 275 mm and is counted as greater than 250 mm.
       
   2. Entries.—Enter the high, low, and representative values in the Component Horizon Table in the NASIS database as whole number percentages for each horizon, as appropriate.
       
6. Rock Fragments 3 to 10 Inches (75 to 250 mm)
    
   1. Definition.—“Rock fragments 3 to 10 inches” is the percent by weight of the horizon occupied by rock fragments 3 to 10 inches (75 to 250 mm) in diameter.
       
   2. Entries.—Enter the high, low, and representative values in the Component Horizon Table in the NASIS database as whole number percentages for each horizon, as appropriate.
       
7. Fragment Kind
    
   1. Definition.—“Fragment kind” is the lithology or composition of the 2 mm or larger fraction of the soil (20 mm or larger for wood fragments).
       
   2. Entries.—Enter the appropriate fragment kind name for the record of fragments populated in the Component Horizon Fragments Table in the NASIS database. The class names appear in a choice list and can also be viewed in the NASIS data dictionary.
       
8. Fragment Roundness
    
   1. Definition.—“Fragment roundness” is an expression of the sharpness of edges and corners of fragments.
       
   2. Significance.—The roundness of fragments impacts water infiltration, root penetration, and macropore space.
       
   3. Classes.—The fragment roundness classes are:
       

      Roundness Class	Definition
      Very angular	Strongly developed faces with very sharp, broken edges.
      Angular	Strongly developed faces with sharp edges (Soil Survey Manual (SSM)).
      Subangular	Detectable flat faces with slightly rounded corners.
      Subrounded	Detectable flat faces with well-rounded corners (SSM).
      Rounded	Flat faces absent or nearly absent with all corners rounded (SSM).
      Well-rounded	Flat faces absent with all corners rounded.

   
   
   4. Entries.—Enter the appropriate fragment roundness class name for the record of fragments populated in the Component Horizon Fragments Table in the NASIS database.
       
9. Fragment Hardness
    
   1. Definition.—“Fragment hardness” is equivalent to the “rupture resistance cemented” of a fragment of specified size that has been air dried and then submerged in water.
       
   2. Measurements.—Measurements are made using the procedures and classes of cementation that are listed with the rupture resistance classes in the Soil Survey Manual. Classes are described for block-like specimens about 25-30 mm on edge, which are air dried and then submerged in water for at least 1 hour. The specimen is compressed between an extended thumb and forefinger, between both hands, or between a foot and a nonresilient flat surface. If the specimen resists compression, a weight is dropped onto it from progressively greater heights until it ruptures. Failure is considered at the initial detection of deformation or rupture. Stress applied in the hand should be over a 1-second period. The tactile sense of the class limits may be learned by applying force to top-loading scales and sensing the pressure through the tips of the fingers or through the ball of the foot. Postal scales may be used for the resistance range that is testable with the fingers. A bathroom scale may be used for the higher rupture resistance range.
       
   3. Significance.—The hardness of a fragment is significant where the rupture resistance class is strongly cemented or higher. These classes can impede or restrict the movement of soil water vertically through the soil profile and have a direct impact on the quality and quantity of ground water and surface water.
       
   4. Classes.—The fragment hardness (rupture resistance) classes are—
       
      1. Extremely weakly cemented
      2. Very weakly cemented
      3. Weakly cemented
      4. Strongly cemented
      5. Very strongly cemented
      6. Indurated
          
   5. Entries.—Enter the appropriate class namefor each record of fragments populated in the Component Horizon Fragments Table in the NASIS database. Choose the term without the word “cemented” (i.e., choose “moderately” to represent the moderately cemented class).
       
10. Fragment Shape
     
    1. Definition.—“Fragment shape” is a description of the overall shape of the fragment.
        
    2. Significance.—Fragment shape is important for fragments that are too large to be called channers or flagstones.
        
    3. Classes.—The fragment shape classes are flat and nonflat.
        
    4. Entries.—Enter the appropriate fragment shape class name for each record of fragments populated in the Component Horizon Fragments Table in the NASIS database.
        
11. Fragment Size
     
    1. Definition.—“Fragment size” is based on the multiaxial dimensions of the fragment.
        
    2. Significance.—The size of fragments is significant to the use and management of the soil. Fragment size is used as a criterion in naming map units. It affects equipment use, excavation, construction, and recreational uses.
        
    3. Classes.—Classes of fragment size are subdivided as flat or nonflat based on the shape of the fragments (described above).
        
       1. Flat fragment classes are:
           

          Flat Fragment Class	Length of Fragment (mm)
          Channers	2-150
          Flagstones	150-380
          Stones	380-600
          Boulders	≥600

          
          
       2. Nonflat fragment classes are:
           

          NonFlat Fragment Class	Diameter (mm)
          Gravel	2-75
          fine gravel	2-5
          medium gravel	5-20
          coarse gravel	20-75
          Cobbles	75-250
          Stones	250-600
          Boulders	≥600

          
          
       3. Gravel is a collection of fragments having a diameter ranging from 2 to 75 mm. Individual fragments in this size range are properly referred to as pebbles, not “gravels.” For fragments that are less than strongly cemented, “para” is added as a prefix to the above terms (e.g., paracobbles or fine paragravel).
           
    4. Entries.—Enter the high, low, and representative values of each size class populated in the Component Horizon Fragments Table in the NASIS database. The range of valid entries is from 2 to 3,000 millimeters, and only whole numbers (integers) are allowed.
        
12. Fragment Volume
     
    1. Definition.—“Fragment volume” is the volume percentage of the horizon occupied by the 2 mm or larger fraction (20 mm or larger for wood fragments) on a whole soil base.
        
    2. Significance.—The volume occupied by the 2 mm or larger fraction is important in selecting texture modifiers (i.e., gravelly, very gravelly, extremely paragravelly).
        
    3. Entries.—Enter the high, low, and representative values for the percent volume present of each size class and kind of fragment populated in the Component Horizon Fragments Table in the NASIS database. The range of valid entries is from 0 to 100 percent, and only whole numbers (integers) are allowed.
        

618.28  Free Iron Oxides

1. Definition.—“Free iron oxides” are secondary iron oxides, such as goethite, hematite, ferrihydrite, lepidocrocite, and maghemite. These forms of iron may occur as discrete particles, as coatings on other soil particles, or as cementing agents between soil mineral grains. They consist of iron extracted by dithionite-citrate from the fine-earth fraction.
    
2. Significance.—The amount of iron that is extractable by dithionite-citrate is used in the ferritic, ferruginous, parasesquic, and sesquic mineralogy classes defined in Soil Taxonomy. The ratio of dithionite-citrate (free) iron to total iron in a soil is a measure of the degree of soil weathering. Free iron oxides are important in the soil processes of podzolization and laterization and play a significant role in the phosphorous fixation ability of soils.
    
3. Measurement.— Free iron oxides are measured as the amount extracted by a solution of sodium dithionite and sodium-citrate using a method outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
    
4. Entries.—Enter high, low, and representative values as percentages for each horizon for which data is available. The range of valid entries is from 0 to 100 percent, and hundredths (two decimal places) are allowed.
    

618.29  Frost Action, Potential

1. Definition.—“Potential frost action” is a rating of the susceptibility of the soil to upward or lateral movement by the formation of segregated ice lenses. It rates the potential for frost heave and the subsequent loss of soil strength when the ground thaws.
    
2. Classes.—Classes are used in regions where frost action is a potential problem. Refer to Part 618, Subpart B, Exhibits, Section 618.84 for more information. The classes are low, moderate, and high and are defined as follows:
    

   Potential Frost Action Class	Definition
   Low	Soils are rarely susceptible to the formation of ice lenses.
   Moderate	Soils are susceptible to the formation of ice lenses, which results in frost heave and subsequent loss of soil strength.
   High	Soils are highly susceptible to the formation of ice lenses, which results in frost heave and subsequent loss of soil strength.



3. Significance.—Damage from frost action results from the formation of segregated ice crystals and ice lenses in the soil and the subsequent loss of soil strength when the ground thaws. Frost heave damages highway and airfield pavements. It is less of a problem for dwellings and buildings that have footings which extend below the depth of frost penetration. In cold climates, unheated structures that have concrete or asphalt floors can be damaged by frost heave. Driveways, patios, and sidewalks can heave and crack. The thawing of the ice causes a collapse of surface elevation and produces free-water perches on the still-frozen soil below. Soil strength is reduced. Backslopes and side slopes of cuts and fills can slough during thawing. Seedlings and young plants of clover, alfalfa, wheat, and oats can be raised out of the soil or have their root systems damaged by frost heave.
    
4. Estimates
    
   1. Freezing temperatures, soil moisture, and susceptible soils are needed for the formation of segregated ice lenses. Ice crystals begin to form in the large pores first. Water in small pores or water that was adsorbed on soil particles freezes at lower temperatures. This supercooled water is strongly attracted to the ice crystals, moves toward them, and freezes on contact with them. The resulting ice lens continues to grow in width and thickness until all available water that can be transported by capillary has been added to the ice lens and a further supply cannot be made available because of the energy requirements.
       
   2. Soil temperatures must drop below 0° C for frost action to occur. Generally, the more slowly and deeply the frost penetrates, the thicker the ice lenses are and the greater the resulting frost heave is. Part 618, Subpart B, Exhibits, Section 618.85 is a map that shows the design freezing index values in the continental United States. The values are the number of degree days below 0° C for the coldest year in a period of 10 years. The values indicate duration and intensity of freezing temperatures. The 250 isoline is the approximate boundary below which frost action ceases to be a problem. Except on the West Coast, the frost action boundary corresponds closely to the functional boundary between the mesic and thermic soil temperature regimes as defined in Soil Taxonomy. More information is provided in the U.S. Army Engineer School, Student Reference, 1967, Soil Engineering, Section I, Volume II, Chapters VI-IX, Fort Belvoir, Virginia.
       
   3. Water necessary for the formation of ice lenses may come from a high water table or from infiltration at the surface. Capillary water in voids and adsorbed water on particles also contribute to ice lens formation but, unless this water is connected to a source of free water, the amount generally is insufficient to produce significant ice segregation and frost heave.
       
   4. The potential intensity of ice segregation is dependent to a large degree on the effective soil pore size and soil saturated hydraulic conductivity, which are related to soil texture. Ice lenses form in soils in which the pores are fine enough to hold quantities of water under tension but coarse enough to transmit water to the freezing front. Soils that have a high content of silt and very fine sand have this capacity to the greatest degree and hence have the highest potential for ice segregation. Clayey soils hold large quantities of water but have such slow saturated hydraulic conductivity that segregated ice lenses are not formed unless the freezing front is slow moving. However, sandy soils have large pores and hold less water under lower tension. As a result, freezing is more rapid and the large pores permit ice masses to grow from pore to pore, entombing the soil particles. Thus, in coarse grained soils, segregated ice lenses are not formed and less displacement can be expected.
       
   5. Estimates of potential frost action generally are made for soils in mesic or colder temperature regimes. Exceptions are on the West Coast, where the mesic-thermic temperature line crosses below the 250 isoline, as displayed in Part 618, Subpart B, Exhibits, Section 618.85, and along the East Coast, where the soil climate is moderated by the ocean. Mesic soils that have a design freezing index of less than 250 degree days should not be rated because frost action is not likely to occur. The estimates are based on bare soil that is not covered by insulating vegetation or snow. They are also based on the moisture regime of the natural soil. The ratings can be related to manmade modifications of drainage or to irrigation systems on an onsite basis. Frost action estimates are made for the whole soil to the depth of frost penetration, to bedrock, or to a depth of 2 meters (6.6 feet), whichever is shallowest. Part 618, Subpart B, Exhibits, Section 618.84 is a guide for making potential frost action estimates. It uses the soil moisture regimes and taxonomic family particle-size classes as defined in Soil Taxonomy.
       
5. Entries.—Enter one of the following classes for the whole soil: low, moderate, or high. If frost action is not a problem, enter “none.”
    

618.30  Gypsum

1. Definition.—“Gypsum” is the percent, by weight, of hydrated calcium sulfates in the <20 mm fraction of soil.
    
2. Significance.—Gypsum is partially soluble in water and can be dissolved and removed by water. Soils with more than 10 percent gypsum may collapse if the gypsum is removed by percolating water. Gypsum is corrosive to concrete. Corrosion of concrete is most likely to occur in soils that are more than about 1 percent gypsum when wetting and drying occurs.
    
3. Measurement.— Gypsum percentage is measured by a method that uses precipitation in acetone as outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
    
4. Entries.—Enter the high, low, and representative values to represent the range in gypsum content as a weight percent of the soil fraction less than 20 mm in size. Round values to the nearest 5 percent for layers that are more than 5 percent gypsum and to the nearest 1 percent for layers that are less than 5 percent gypsum (for example, 0-1, 1-5, 5-10). If the horizon does not have gypsum, enter “0.” The range of valid entries is from 0 to 120 percent, and only whole numbers (integers) are allowed.
    

618.31  Horizon Depth to Bottom

1. Definition.—“Horizon depth to bottom” is the distance from the top of the soil to the base of the soil horizon.
    
2. Measurement.— Distance is measured from the top of the soil, which is defined as the top of the mineral soil, or, for soil with “O” horizons, the top of any “O” layer that is at least slightly decomposed. For soils that are covered by 80 percent or more rock or pararock fragments, the top of the soil is the surface of the fragments. See pages 63-64 in the Soil Survey Manual for a complete discussion. Measurement should be estimated to a depth of 200 cm for most soils and to a depth at least 25 cm below a lithic contact if the contact is above 175 cm. For soils, including those that have a root-restricting contact such as a paralithic contact, the lowest horizon bottom should extend to a depth of at least 25 cm below the contact or to a depth of 200 cm, whichever is shallower.
    
3. Entries.—Horizon depth to bottom values used to populate component data in NASIS are not specific to any one point but rather are a reflection of commonly observed values based on field observations and are intended to model the component as it occurs throughout the map unit. Enter the high, low, and representative values in whole centimeters. The high value represents either the greatest depth to which the base of the horizon extends or, for horizons extending beyond the limit of field observation, is the depth to which observation was made (usually no more than 200 cm but at least 150 cm).
    

618.32  Horizon Depth to Top

1. Definition.—“Horizon depth to top” is the distance from the top of the soil to the upper boundary of the soil horizon.
    
2. Measurement.— Distance is measured from the top of the soil, which is defined as the top of the mineral soil, or, for soils with “O” horizons, the top of any “O” layer that is at least slightly decomposed. For soils that are covered by 80 percent or more rock or pararock fragments, the top of the soil is the surface of the fragments. See pages 63-64 in the Soil Survey Manual for a complete discussion.
    
3. Entries.—Horizon depth to top values used to populate component data in NASIS are not specific to any one point but rather are a reflection of commonly observed values based on field observations and are intended to model the component as it occurs throughout the map unit. Enter the high, low, and representative values in whole centimeters. See Section 618.33, “Horizon Designation,” for a discussion as to how to list E/B and E and Bt type horizons.
    

618.33  Horizon Designation

1. Definition.—“Horizon designation” is a concatenation of three kinds of symbols used in various combinations to identify layers of soil that reflect the investigator’s interpretations of genetic relationships among layers within a soil.
    
2. Significance.—Soils vary widely in the degree to which horizons are expressed. The range is from little or no expression to strong expression. Layers of different kinds are identified by symbols. Designations are provided for layers that have been changed by soil formation and for those that have not. Designations are assigned after comparison of the observed properties of the layer with properties inferred for the material before it was affected by soil formation. Designations of genetic horizons express a qualitative judgment about the kind of changes that are believed to have taken place. A more detailed discussion can be reviewed in the latest edition of the Keys to Soil Taxonomy and in the Soil Survey Manual, Chapter 3. Horizon designations shown in field pedon descriptions (point data) represent a specific location on the landscape. Horizon designations used to populate component data in NASIS are not specific to any one point but rather are a reflection of commonly observed horizon sequences based on field observations and are intended to model the component as it occurs throughout the map unit so that accurate interpretations can be derived.
    
3. Entries.—Enter combinations of symbols to reflect master horizons and their vertical subdivisions. Commonly occurring master horizon sequences, identified in field pedon descriptions (e.g., A-Bt1-Bt2-Btk-2Bk-2C), are used for soil components in NASIS. Generalized horizon layer designations (e.g., H1-H2-H3 etc.) may be used instead of genetic horizon designations. Users of generalized horizon layer designations must be cognizant of the fact that certain master horizon designations such as O, Cr, and R, are still required entries in NASIS for proper soil interpretations.
    
   1. It is not possible to include all master horizon sequences observed in individual pedon descriptions when populating the interpretive horizons of soil components. For example, if an Ap horizon is present and the former E horizon is incorporated into the Ap horizon, a sequence of Ap-E should not be used because the horizons would not normally occur together. Judgment is required when determining how much detail is required to represent the component adequately for interpretations. Care must be taken to maintain important differences between generic horizons whose range in properties are specified separately on the official series descriptions. Some general guidance follows. Master horizons (i.e., O, A, E, B) represent unique sets of pedogenic processes and therefore should not be combined when aggregating data, even if their basic properties are similar.
       
   2. Transitional horizons, such as EB and BC, should be recorded if they are commonly more than about 10 cm thick. After applying careful judgment, some horizons may be combined in order to avoid overly complex horizon sequences representing map unit components. Keep in mind, however, that once combined, any useful information gained by their separation is lost. Transitional horizons thinner than about 10 cm may be combined with an adjacent master horizon if they are not considered important to the interpretation of the component. Master horizon subdivisions showing genetic variations not deemed significant to interpretations may be combined. For example, a horizon sequence of Bt1-Bt2-Bt3, based solely on color variation and having no other significant differences, may be combined and shown simply as Bt. Master horizon subdivisions showing genetic variations that are deemed significant to interpretations should not be combined. For example, a horizon sequence of Bt-Btx-BC should not be combined. Do not combine horizons that straddle the criteria break to a diagnostic horizon. For example, a Bk1 with insufficient calcium carbonate content to qualify as a calcic horizon should not be combined with a Bk2 that is a calcic horizon. Enter only what the documentation can support.
       
   3. Combination horizons (E and Bt, B/E, E/B, etc.) should be entered as two separate horizon records, such as one for the E part of the horizon and the second for the Bt part of the horizon. Both records would have the same horizon depths and horizon designations assigned. Soil property data elements would be populated for each part to describe the characteristics of that part of the horizon.
       
   4. Allowable codes are listed in the NASIS data dictionary (see Part 618, Subpart B, Exhibits, Section 618.82 for the Web address of the current version). The rules for use of horizon designations are in the latest edition of the Keys to Soil Taxonomy and in the Soil Survey Manual, Chapter 3.
       

618.34  Horizon Thickness

1. Definition.—“Horizon thickness” is a measurement from the top to bottom of a soil horizon throughout its areal extent.
    
2. Measurement.— Soil horizon thickness varies on a cyclical basis. Measurements should be made to record the range in thickness as it normally occurs in the soil.
    
3. Entries.—Horizon Depth to Bottom values used to populate component data in NASIS are not specific to any one point but rather are a reflection of commonly observed values based on field observations and are intended to model the component as it occurs throughout the map unit. Enter the high, low, and representative values in whole centimeters. The minimum allowable entry is 1 cm. For horizons extending beyond the limit of field observation, thickness is only populated to the depth at which an observation was made.
    

618.35  Hydrologic Group

1. Definition
    
   1. The complete definition and official criteria for hydrologic soil groups are available online at (Title 210, National Engineering Handbook, Part 630, Chapter 7, “Hydrologic Soil Groups”).
       
   2. “Hydrologic group” is a group of soils having similar runoff potential under similar storm and cover conditions. Soil properties that influence runoff potential are those that influence the minimum rate of infiltration for a bare soil after prolonged wetting and when not frozen. These properties are depth to a seasonal high water table, saturated hydraulic conductivity after prolonged wetting, and depth to a layer with a very slow water transmission rate. Changes in soil properties caused by land management or climate changes also cause the hydrologic soil group to change. The influence of ground cover is treated independently.
       
2. Classes.—The soils in the United States are placed into four groups, A, B, C, and D, and three dual classes, A/D, B/D, and C/D.
    
3. Significance.—Hydrologic groups are used in equations that estimate runoff from rainfall. These estimates are needed for solving hydrologic problems that arise in planning watershed-protection and flood-prevention projects and for planning or designing structures for the use, control, and disposal of water.
    
4. Measurements.—The original classifications assigned to soils were based on the use of rainfall-runoff data from small watersheds and infiltrometer plots. From these data, relationships between soil properties and hydrologic groups were established.
    
5. Estimates.— Assignment of soils to hydrologic groups is based on the relationship between soil properties and hydrologic groups. Wetness characteristics, water transmission after prolonged wetting, and depth to very slowly permeable layers are properties used in estimating hydrologic groups.
    
6. Entries.—Enter the soil hydrologic group, such as A, B, C, D, A/D, B/D, or C/D.
    

618.36  Landform

1. Definition.—“Landform” is any physical, recognizable form or feature of the earth’s surface having a characteristic shape and produced by natural causes.
    
2. Significance.—Geographic order suggests natural relationships. Running water, with weathering and gravitation, commonly sculptures landforms within a landscape. Over the ages, earthy material is removed from some landforms and deposited on others. Landforms are interrelated. An entire area has unity through the interrelationships of its landform.
    
   1. Each landform may have one kind of soil present or several. Climate, vegetation, and time of exposure to weathering of the parent materials are commonly about the same throughout the extent of the landform, depending on the relief of the area. Position on the landform may have influenced the soil-water relationships, microclimate, and vegetation.
       
   2. The proper identification of the landform is an important part of understanding the formative history of the soil and the materials from which the soil formed. This aids in the development of the soil mapping model and in the transfer of information between areas.
       
   3. Landform terms are also used as phase criteria for separating mapping components or phases of a soil taxon.
       
3. Classes.—The allowable list of landform terms is included in the NASIS data dictionary. Definitions of the terms are included in Part 629 of this handbook.
    
4. Entries.—Enter the appropriate class name for the landforms on which each map unit component occurs. A representative value (term) may be indicated. It is possible to indicate the presence of one landform occurring on another landform (e.g., a dune on a flood plain).
    

618.37  Linear Extensibility Percent

1. Definition.—“Linear extensibility percent” is the linear expression of the volume difference of natural soil fabric at 1/3-bar or 1/10-bar water content and oven dryness. The volume change is reported as percent change for the whole soil.
    
2. Classes.—Shrink-swell classes are based on the change in length of an unconfined clod as moisture content is decreased from a moist to a dry state. If this change is expressed as a percent, the value used is LEP, linear extensibility percent. If it is expressed as a fraction, the value used is COLE, coefficient of linear extensibility. The shrink-swell classes are defined as follows:
    

   Shrink-Swell Class	LEP	COLE
   Low	<3	<0.03
   Moderate	3-6	0.03-0.06
   High	6-9	0.06-0.09
   Very High	≥9	≥0.09



3. Significance.—If the shrink-swell potential is rated moderate to very high, shrinking and swelling can damage buildings, roads, and other structures. The high degree of shrinkage associated with high and very high shrink-swell potentials can damage plant roots.
    
4. Measurement.— Coefficient of linear extensibility is measured directly as the change in clod dimension from moist to dry conditions and is expressed as a percentage of the volume change to the dry length:

   COLE = ^{(moist length - dry length)}/_{dry length}

   When expressed as LEP (linear extensibility percent):

   LEP = COLE x 100

   Linear extensibility may be determined by any of the following methods:

   1. For the core method of measurement, select a sample core from a wet or moist soil. Carefully measure the wet length of the cores and set the core upright in a dry place. If the core shrinks in a symmetrical shape without excessive cracking or crumbling, its length can be measured and linear extensibility percent calculated. If the core crumbles or cracks, measurements cannot be accurately determined by this method.
       
   2. In the coated clod method of measurement, shrink-swell potential can be estimated from the bulk density of soil measured when moist and when dry. The coated clod method is widely used and is the most versatile procedure for determining the bulk density of coherent soils. Procedures and calculations are given in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
       
   3. Linear extensibility percent can be calculated from bulk density moist (Dbm) and bulk density dry (Dbd) using the following formula:

      LEP = 100 [(Dbd/Dbm)1/3 -1] [1-(Volume % > 2 mm/100)]

      This equation is used to simplify the determination of shrink-swell potential classes. The classes are as follows:

      Dbd/Dbm	Shrink-Swell Potential
      < 1.10	Low
      1.10 - 1.20	Moderate
      1.20 - 1.30	High
      ≥ 1.30	Very High



5. Estimates.— Field estimates of shrink-swell potential can be made by observing desiccation cracks, slickensides, gilgai, soil creep, and leaning utility poles. Shrink-swell potential correlates closely with the kind and amount of clay. The greatest shrink-swell potential occurs in soils that have high amounts of 2:1 lattice clays, such as clay minerals in the smectite group. Illitic clays are intermediate, and kaolinitic clays are least affected by volume change as the content in moisture changes.
    
6. Entries.—Enter the low, high, and representative linear extensibility percent values. The high and low values correspond to the high and low limits of the appropriate class. The range of valid entries is from 0 to 30 percent, and tenths (one decimal place) are allowed.
    

618.38  Liquid Limit

1. Definition.—“Liquid limit” is the water content of the soil material, which passes a no. 40 sieve, at the change between the liquid and plastic states.
    
2. Significance.—The plasticity chart, given in ASTM Designation D 2487, is a plot of liquid limit (LL) versus plasticity index (PI) and is used in classifying soil in the Unified soil classification system. The liquid limit is also a criterion for classifying soil in the AASHTO classification system, as shown in Part 618, Subpart B, Exhibits, Section 618.83. Generally, the amount of clay- and silt-size particles, the organic matter content, and the type of minerals determine the liquid limit. Soils that have a high liquid limit have the capacity to hold a lot of water while maintaining a plastic or semisolid state.
    
3. Measurement.— Tests are made on thoroughly puddled soil material that has passed a no. 40 (425 mm) sieve and is expressed on a dry weight basis, according to ASTM Designation D 4318. This procedure requires the use of a liquid limit device, a special tool designed to standardize the arbitrary boundary between a liquid and plastic state of a soil. Estimates of liquid limit are made on soils during soil survey investigations and mapping activities. The liquid limit is usually inferred from clay mineralogy and clay content. If soils are tested later in the laboratory, adjustments are made to the field estimates as needed. Generally, experienced personnel can estimate these values with a reasonable degree of accuracy.
    
4. Estimates.—The formula in Part 618, Subpart B, Exhibits, Section 618.86 is used in the NASIS database to provide default calculated values if no measurements are available.
    
5. Entries.—Enter the high, low, and representative values as a range for each horizon. The range of valid entries is from 0 to 400 percent, and tenths (one decimal place) are allowed. However, entries should be rounded to the nearest 10 percent unless they represent measured values. Enter “0” for nonplastic soils. The liquid limit for organic soil material is not defined and is assigned “null.” A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.102.
    

618.39  Organic Matter

1. Definition.—“Organic matter percent” is the weight of decomposed plant and animal residue and expressed as a weight percentage of the soil material less than 2 mm in diameter.
    
2. Significance
    
   1. Organic matter influences the physical and chemical properties of soils far more than the proportion to the small quantities present would suggest. The organic fraction influences plant growth through its influence on soil properties. It encourages granulation and good tilth, increases porosity and lowers bulk density, promotes water infiltration, reduces plasticity and cohesion, and increases the available water capacity. It has a high capacity to adsorb and exchange cations and is important to pesticide binding. It furnishes energy to micro-organisms in the soil. As it decomposes, it releases nitrogen, phosphorous, and sulfur. The distribution of organic carbon according to depth indicates different episodes of soil deposition or soil formation.
       
   2. Soils that are very high in organic matter have poor engineering properties and subside upon drying.
       
3. Measurement.— Laboratory measurements are made using a dry combustion method to determine percent total carbon. For an estimate of organic carbon in calcareous soils, the carbon present in carbonate compounds, such as CaCO₃, must be calculated and then subtracted from the total carbon. This is done using the equation: percent organic carbon = percent total carbon - [% <2 mm CaCO₃ x 0.12]. The results are given as the percent of organic carbon in dry soil. To convert the figures for organic carbon to those for organic matter, multiply the organic carbon percentage by 1.724. The detailed procedures are outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
    
4. Estimates.— Color and “feel” are the major properties used to estimate the amount of organic matter. Color comparisons in areas of similar materials can be made against laboratory data so that a soil scientist can make estimates. In general, black or dark colors indicate high amounts of organic matter. The contrast of color between the A horizon and subsurface horizons is also a good indicator.
    
5. Entries.—Enter the high, low, and representative values for the range in organic matter in each horizon. The range of valid entries is from 0 to 100 percent and hundredths (two decimal places) are allowed.
    

618.40  Parent Material, Kind, Modifier, and Origin

Parent material is the unconsolidated material, mineral or organic, from which the soil develops. The soil surveyor considers parent material in developing a model to be used for soil mapping. Soil scientists and specialists in other disciplines use parent material data to help interpret soil boundaries and project performance of the material below the soil. Many soil properties relate to parent material. Among these properties are proportions of sand, silt, and clay; chemical content; bulk density; structure; and the kinds and amounts of fragments. These properties affect interpretations and may be criteria used to separate soil series. Soil properties and landscape information infer parent material. Three data elements – parent material kind, parent material modifier, and parent material origin – describe parent material.

1. Parent Material Kind
    
   1. Definition.—“Parent material kind” is a term describing the general physical, chemical, and mineralogical composition of the material, mineral or organic, from which the soil develops. Mode of deposition, weathering, or both may be implied or implicit.
       
   2. Classes.—The list of allowable entries is included in the NASIS data dictionary. Definitions of many of these terms are included in part 629 of this handbook.
       
   3. Entries.—Enter the applicable class names for each map unit component. Multiple entries are permissible. Multiple rows of parent materials may also be indicated for a single component, such as loess over till over residuum.
       
2. Parent Material Modifier
    
   1. Definition.—“Parent material modifier” is the general description of the texture of the parent material. Class limits correspond to those of the general texture defined in the Soil Survey Manual and the family category particle-size classes defined in Soil Taxonomy.
       
   2. Classes.—The classes of parent material modifiers are as follows:
       
      • Clayey
      • Coarse-loamy
      • Coarse-silty
      • Fine-loamy
      • Fine-silty
      • Gravelly
      • Loamy
      • Sandy
      • Sandy and gravelly
      • Sandy and silty
      • Silty
      • Silty and clayey
         
   3. Entries.—Enter the appropriate class name to modify the corresponding row of parent material kind.
       
3. Parent Material Origin
    
   1. Definition.—“Parent material origin” is the type of bedrock from which the parent material is derived.
       
   2. Classes.—The allowable class names are included in the NASIS data dictionary and are the same as for the “bedrock kind” data element.
       
   3. Entries.—Enter the appropriate parent material origin class names that correspond to each parent material kind. Although this data element is intended to be used when “residuum” is the chosen parent material kind, it may also be used with other kinds of parent material.
       

618.41  Particle Density

1. Definition.—“Particle density” is the mass per unit of volume of the solid soil particle, either mineral or organic. Also known as specific gravity.
    
2. Significance.—Particle density is used in the calculation of weight and volume for soil (porosity). The relationship between bulk density, percent pore space, and the rate of sedimentation of solid particles in a liquid depends on particle density. The term “particle density” indicates wet particle density or specific gravity.
    
3. Measurement.—The standard methods of measurement for particle density are: the ASTM standard test method for specific gravity of soils, ASTM Designation D 854-92, which uses soil materials passing a no. 4 sieve; the method described by Blake and Hartge in Methods of Soil Analysis, Part 1, Agronomy 9; or the method for volcanic soils described by Bielders and others in Soil Science Society of America Journal 54, pages 822-826.
    
4. Estimates
    
   1. Particle density is often assumed to be 2.65 g cm^-3; however, many minerals and material of various origins exhibit particle densities less than or greater than this standard. Particle density (Dp) may be calculated using the extractable iron and the organic carbon percentages in the following formula:

      
       

   2. OC is the organic carbon percentage and Fe is the percent extractable iron determined by dithionite-citrate extraction, or by an equivalent method. The particle density of the organic matter (Dp1) is assumed to be 1.4 g cm^-3, that of the minerals from which the extractable iron originates (Dp2) is assumed to be 4.2 g cm^-3, and that of the material exclusive of the organic matter and the minerals contributing to the extractable Fe (Dp3) is assumed to be 2.65 g cm^-3.
       
5. Entries.—Enter the representative value for the horizon. The range of valid entries is from 0.01 to 5 g cm^-3, and hundredths (two decimal places) are allowed.
    

618.42  Particle Size

1. Definition.—“Particle size” is the effective diameter of a particle as measured by sedimentation, sieving, or micrometric methods. Particle sizes are expressed as classes with specific effective diameter class limits. The broad classes are clay, silt, and sand, ranging from the smaller to the larger of the less than 2 mm mineral soil fraction. It includes fragments of weathered or poorly consolidated fragments that disperse to particles less than 2 mm.
    
2. Significance.—The physical behavior of a soil is influenced by the size and percentage composition of the size classes. Particle size is important for most soil interpretations, for determination of soil hydrologic qualities, and for soil classification.
    
3. Measurement.— Particle size is measured by sieving and sedimentation. The method used is method 3A1, which is outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.103.
    
4. Classes.—The USDA uses the following size separates for the <2 mm mineral material:
    

   USDA Particle Size Separates	Size (mm)
   Clay, total	<0.002
   Silt, total	0.002-0.05
   Silt, fine	0.002-0.02
   Silt, coarse	0.02-0.05
   Sand, total	0.05-2.00
   Very fine sand	0.05-0.10
   Fine sand	0.10-0.25
   Medium sand	0.25-0.50
   Coarse sand	0.50-1.00
   Very coarse sand	1.00-2.00

   Part 618, Subpart B, Exhibits, Section 618.87 compares the USDA system with the AASHTO and Unified soil classification systems and shows the U.S. standard sieve sizes.
    

5. Clay Percentage
    
   1. Definition.—“Total clay percentage” is the weight percentage of the mineral particles less than 0.002 mm in equivalent diameter in the less-than-2-mm soil fraction. Most of the material is in one of three groups of clay minerals or in a mixture of these clay minerals. The groups are kaolinite, smectite, and hydrous mica, the best known member of which is illite.
       
   2. Significance.—Physical and chemical activities of a soil are related to the kind and amount of clay minerals. Clay particles may have thousands of times more surface area per gram than silt particles and nearly a million times more surface area than very coarse sand particles. Thus, clay particles are the most chemically and physically active part of mineral soil.
       
      1. Clay mineralogy and clay percentage have a strong influence on engineering properties and the behavior of soil material when it is used as construction or foundation material. They influence linear extensibility, compressibility, bearing strength, and saturated hydraulic conductivity.
          
      2. The kind and amount of clay influence plant growth indirectly by affecting available water capacity, water intake rate, aeration, cation-exchange capacity, saturated hydraulic conductivity, erodibility, and workability. Up to a certain point, an increase in the amount of clay in the subsoil is desirable. Clay can increase the amount of water and nutrients stored in that zone. By slightly slowing the rate of water movement, it can reduce the rate of nutrient loss through leaching. If the amount of clay is great, it can impede water and air movement, restrict root penetration, increase runoff and, on sloping land, result in increased erosion.
          
      3. Clay particles are removed by percolating water from surface and subsurface horizons and deposited in the subsoil horizons. The amount of clay accumulation and its location in the profile provide clues for the soil scientist about soil genesis. Irregular clay distribution as related to depth may indicate lithologic discontinuities, especially if accompanied by irregular sand distribution.
          
   3. Measurement.— Clay content is measured in the laboratory by the pipette or hydrometer methods after the air-dry soil is pretreated to remove organic matter and soluble salts. Field estimates of clay content are made by manual methods. The way a wet soil ribbons (develops a long continuous ribbon) when pressed between the thumb and fingers gives a good idea of the amount of clay present. Excessive amounts of sodium can toughen the soil, making the soil feel more clayey. Care should be taken not to overestimate the amount of clay in sodic soils. Accuracy depends largely on frequent and attentive observation. Texture reference samples determined in the laboratory are used by soil scientists to calibrate the feel of soils with various percentages of clay.
       
   4. Entries.—Enter the high, low, and representative values of the clay total separate as a percent of the material less than 2 mm in size for each horizon. Enter a “0” if the amount is not significant, as in organic layers or in some andic soil materials. The range of valid entries is from 0 to 100 percent and tenths (one decimal place) are allowed. The representative value chosen should equate to a valid clay total content for the representative texture class posted for each horizon.
       
6. Sand Percentage
    
   1. Definition.—“Sand percentage” is the weight percentage of the mineral particles less than 2 mm and greater than or equal to 0.05 mm in equivalent diameter in the less than 2 mm soil fraction. The sand separates recognized are very coarse, coarse, medium, fine, very fine, and total. Respective size limits are shown in Section 618.42(D) above. Much of the sand fraction is composed of fragments of rocks and primary minerals, especially quartz. Therefore, the sand fraction is quite chemically inactive.
       
   2. Significance.—Physical properties of the soil are influenced by the amounts of total sand and of the various sand fractions present in the soil. Sand particles, because of their size, have a direct impact on the porosity of the soil. This impact influences other properties, such as saturated hydraulic conductivity, available water capacity, water intake rates, aeration, and compressibility related to plant growth and engineering uses.
       
   3. Measurement.— Sand content is measured in the laboratory by the wet sieving method and then fractionated by dry sieving. Field estimates are made by manual methods. The degree of grittiness in a wet soil sample, when worked between the thumb and forefinger, gives an estimate of the sand content. The size of sand grains may be observed with the naked eye or with the aid of a hand lens.
       
   4. Entries.—Enter the high, low, and representative value of the sand total separate and each sand size separate (sand very coarse separate, sand coarse separate, sand medium separate, sand fine separate, and sand very fine separate) as a percent of the material less than 2 mm in size for each horizon. The sum of the representative values for the five sand size fractions must equal the representative value for the sand total separate. The range of valid entries is from 0 to 100 percent and tenths (one decimal place) are allowed. Enter a “0” if the amount is not significant, as in organic layers or in some andic soil materials. The representative values chosen should equate to a valid sand total content and sand size fraction content for the representative texture class posted for each horizon.
       
7. Silt Percentage
    
   1. Definition.—“Silt percentage” is the weight percentage of the mineral particles greater than or equal to 0.002 mm but less than 0.05 mm in the less than 2 mm soil fraction. The silt separates recognized are fine, coarse, and total. The respective size limits are listed in paragraph 618.42(D) above. The silt separate is dominated by primary minerals, especially quartz, and therefore has a low chemical activity.
       
   2. Significance.—The silt separate possesses some plasticity, cohesiveness, and absorption, but to a much lesser degree than the clay separate. Silt particles act to slow water and air movement through the soil by filling voids between sand grains. A very high content of silt in a soil may be physically undesirable for some uses unless supplemented by adequate amounts of sand, clay, and organic matter.
       
   3. Measurement
       
      1. The silt content is measured in the laboratory in two phases. The fine silt is measured using the pipette method on the suspension remaining from the wet sieving process. Aliquots of the diluted suspension are removed at predetermined intervals based on Stokes Law. The aliquots are then dried and weighed. The coarse silt fraction is the difference between 100 percent and the sum of the sand, clay, and fine silt percentages.
          
      2. The silt content may be estimated in the field using the ribbon test as described for clay. The content of silt is usually estimated by first estimating the clay and sand portions and then subtracting that number from 100 percent. Silt tends to give the soil a smooth feel.
          
   4. Entries.—Enter the high, low, and representative value of the silt total separate and each silt size separate (silt coarse separate and silt fine separate) as a percent of the material less than 2 mm in size for each horizon. The sum of the representative values for the two silt size fractions must equal the representative value for the silt total separate. The range of valid entries is from 0 to 100 percent, and tenths (one decimal place) are allowed. Enter a “0” if the amount is not significant, as in organic layers or in some andic soil materials. The representative value chosen should equate to a valid silt total content for the representative texture class posted for each horizon.
       

618.43  Percent Passing Sieves

1. Definition.—The percent passing sieve numbers 4, 10, 40, and 200 is the weight of material that passes through these sieves, based on the material less than 3 inches (75 mm) in size and expressed as a percentage.
    
2. Significance.—Data for the percent passing sieves are used to classify the soil in the engineering classifications and to make judgments on soil properties and performance. Many soil characteristics are influenced by the depth distribution of grain sizes for the soil as well as the soil’s mode of deposition, stress history, density, and other features.
    
3. Measurement.— Measurements involve sieve analysis for the determination of grain size distribution of that portion of the soil having particle diameters between 3 inches and 0.074 mm (no. 200 sieve). ASTM Designations D 422, C 136, and C 117 are applicable procedures. Measurements are made on most benchmark soils and other representative soils in survey areas.
    
4. Estimates
    
   1. Estimates of the content of sand, silt, clay, and rock fragments that are made for soils during soil survey investigations and mapping activities are used to estimate percent passing sieves. If samples are tested later in a laboratory, adjustments are made to the field estimates as needed. Generally, experienced personnel can estimate these values with a high degree of accuracy. Estimates for percent passing sieves can be made from soil texture using the following general guidance:
       
      1. Percent passing #200 = clay + silt + 1/2 very fine sand.
      2. Percent passing #40 = 1/2 very fine sand + fine sand + 1/2 medium sand + percent passing #200.
          
   2. The percent passing #10 equals the less-than-2-mm fraction, and soil texture is based on the less than 2 mm fraction. Since sieves represent the less-than-3-inch fraction, the #40 and #200 sieve estimates must be adjusted when the percent passing #10 is less than 100 percent. The percent passing #40 and #200 that is determined by texture must be adjusted by multiplying the percent passing #40 and percent passing #200 by the percent passing #10. Pararock fragments are not cemented strongly enough to be retained on sieves. They are crushed and estimated into percent passing sieves. ASTM procedures use a roller crusher as a pretreatment of the soil material prior to sieving. Field estimates should try to replicate this procedure.
       
5. Entries.—Enter the high, low, and representative values to represent the range of percent passing each sieve size for each horizon. The range includes the estimating accuracy as well as the range of values for a soil. The range of valid entries is from 0 to 100 percent, and tenths (one decimal place) are allowed. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.104.
    

618.44  Plasticity Index

1. Definition.—The plasticity index is the numerical difference between the liquid limit and the plastic limit. It is the range of water content in which a soil exhibits the characteristics of a plastic solid. The plastic limit is the water content that corresponds to an arbitrary limit between the plastic and semisolid states of a soil.
    
2. Significance.—The plasticity index, when used in connection with the liquid limit, serves as a measure of the plasticity characteristics of a soil. The plasticity chart, given in ASTM Designation D 2487, is a plot of the liquid limit (LL) versus the plasticity index (PI) and is used in classifying soil in the Unified soil classification system. The plasticity index is also a criterion for classifying soil in the AASHTO classification system, as shown in Part 618, Subpart B, Exhibits, Section 618.83. Soils that have a high plasticity index have a wide range of moisture content in which the soil performs as a plastic material. Highly and moderately plastic clays have large PI values.
    
3. Measurements.—Tests are made on that portion of the soil having particles passing the no. 40, (425 micrometer) sieve, according to ASTM Designation D 423. Measurements are made on most benchmark soils and on other representative soils in survey areas. Estimates of plasticity index are made on all soils during soil survey investigations and mapping activities. The plasticity index is usually not estimated directly: a position on the plasticity chart in ASTM Designation D 2487 is estimated and the plasticity index is determined from the chart. If soils are later tested in the laboratory, adjustments are made to the field procedures as needed. Generally, experienced personnel can estimate these values with a reasonable degree of accuracy. Estimates are expressed in ranges that include the estimating accuracy as well as the range of values from one location to another within the map unit.
    
4. Estimates.—The formula in Part 618, Subpart B, Exhibits, Section 618.86 is used in the NASIS database to provide default calculated values if no measurements are available.
    
5. Entries.—Enter the high, low, and representative values to represent the range of plasticity index for each horizon. The range of valid entries is from 0 to 130 percent, and tenths (one decimal place) are allowed. However, entries should be rounded to the nearest 5 percent unless they represent measured values. Enter “0” for nonplastic soils. The plasticity index for organic soil material is not defined and is assigned “null.” A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.102.
    

618.45  Ponding Depth, Duration Class, Frequency Class, and Month

1. Ponding is standing water in a closed depression. The water is removed only by deep percolation, transpiration, evaporation, or by a combination of these processes. Ponding of soils is classified according to depth, frequency, duration, and the beginning and ending months in which standing water is observed.
    
2. Ponding Depth
    
   1. Definition.—“Ponding depth” is the depth of the surface water that is ponding on the soil.
       
   2. Entries.—Enter the high, low, and representative values for the ponding depth, in centimeters, for the map unit component. The range of valid entries is from 0 to 185 cm, and only whole numbers (integers) are allowed.
       
3. Ponding Duration Class
    
   1. Definition.—“Ponding duration class” is the average duration, or length of time, of the ponding occurrence.
       
   2. Classes.—The ponding duration classes are:
       

      Ponding Duration Class	Duration of the Ponding Occurrence
      Very brief	4 to 48 hours
      Brief	2 to 7 days
      Long	7 to 30 days
      Very long	> 30 days

   
   
   3. Entries.—Enter very brief, brief, long, or very long for the map unit component. Only use entries if ponding frequency (defined below) occurs more often than rare.
       
4. Ponding Frequency Class
    
   1. Definition.—“Ponding frequency class” is the number of times ponding occurs over a period of time.
       
   2. Classes.—The ponding frequency classes are:
       

      Ponding Frequency Class	Definition
      None	No reasonable possibility of ponding; near 0 percent chance of ponding in any year.
      Rare	Ponding is unlikely but possible under unusual weather conditions; nearly 0 to 5 percent chance of ponding in any year or nearly 0 to 5 times in 100 years.
      Occasional	Ponding is expected infrequently under usual weather conditions; 5 to 50 percent chance of ponding in any year or nearly 5 to 50 times in 100 years.
      Frequent	Ponding is likely to occur under usual weather conditions; more than a 50 percent chance in any year (i.e., 50 times in 100 years).

   
   
   3. Entries.—Enter none, rare, occasional, or frequent as appropriate for the map unit component.
       
5. Ponding Month
    
   1. Definition.—“Ponding month” is the calendar months in which ponding is expected.
       
   2. Classes.—The time of year when ponding is likely to occur is expressed in months for the expected beginning to expected end of the ponding period. The time period expressed includes two-thirds to three-fourths of the occurrences.
       
   3. Entries.—Enter the name of each month of the year in which ponding is expected.
       
6. Significance.—The susceptibility of soils to ponding is important for homes, building sites, and sanitary facilities. Time and duration of the ponding are critical factors in determining plant species. Ponding during the dormant season has few if any harmful effects on plant growth or mortality and may even improve growth.
    
7. Estimates.— Generally, estimates of ponding frequency and duration can be made for each soil. Where the natural infiltration, saturated hydraulic conductivity, and surface and subsurface drainage of soils is altered, ponding studies are needed to reflect present ponding characteristics.
    
   1. Evidence of ponding events should be gathered during soil survey fieldwork. High water lines and other signs of maximum water height are recorded. Other records may also exist.
       
   2. Certain landform features are subject to ponding. These features are characteristics of closed drainage systems and include potholes, playas, sloughs, and backswamps. Most of these features are recognizable when correlating features on aerial photographs with ground observations. Different kinds of vegetation and soils are normally associated with these geomorphic features.
       
   3. The vegetation that grows in ponded areas may furnish clues to past ponding and indicate the potential for ponding in the future. Generally, native vegetation in ponded areas consists of obligate and facultative wet hydrophytes. Some plant species are intolerant of ponding and do not grow in areas that are ponded.
       
   4. The soil provides clues to past ponding, but characteristics vary according to climate and soil conditions. Some of the clues (alone or in any combination) are—
       
      1. A dark surface horizon or layer overlying a gleyed subsoil.
      2. Many prominent redoximorphic features that have low value and chroma.
      3. Capillary transport and concentrations of carbonates or sulfates, or both, in the upper soil horizons.
      4. Dark colors and high levels of organic matter throughout the profile.
          

618.46  Pores

1. “Pore space” is a general term for voids in the soil material. The term includes matrix, nonmatrix, and interstructural pore space. For water movement at low suction and conditions of satiation, the nonmatrix and interstructural porosity have particular importance.
    
   1. Matrix Pores.—Matrix pores are formed by the agents that control the packing of the primary soil particles (i.e., primary packing voids). These pores are usually smaller than nonmatrix pores. Additionally, their aggregate volume and size can change markedly according to water state for soil horizons or layers with high extensibility.
       
   2. Nonmatrix Pores.—Nonmatrix pores are relatively large voids that are expected to be present both when the soil is moderately moist or wetter as well as in drier states. The voids are not bounded by the planes that delimit structural units. Nonmatrix pores may be formed by roots, animals, the action of compressed air, and other agents. The size of the distribution of nonmatrix pores usually bears no relationship to the particle-size distribution and the related matrix pore-size distribution.
       
   3. Interstructural Pores.—Interstructural pores are delimited by structural units. Inferences as to the interstructural porosity may be obtained from the structure description. Commonly, interstructural pores are at least crudely planar.
       
2. Nonmatrix pores are described by quantity, size, shape, and vertical continuity (generally in that order).
    
   1. Pore Quantity
       
      1. Definition.—“Pore quantity” is defined by classes that pertain to the number of a selected size of pores per unit area of undisturbed soils. The unit area that is evaluated varies according to the size class of the pores: 1 cm² for very fine and fine pores, 1 dm² for medium and coarse pores, and 1 m² for very coarse pores.
          
      2. Classes.—The pore quantity classes are:
          

         Pore Quantity Class	Number of Pores per Unit Area
         Few	< 1
         Common	≥ 1-5
         Many	≥ 5

         
         
      3. Entries.—Enter pore quantity as pores/area. Enter the high, low, and representative values as whole numbers between 0 and 99 for the horizon.
          
   2. Pore Size
       
      1. Definition.—“Pore size” is the average diameter of the pore.
          
      2. Classes.—The pore size classes are:
          

         Pore Size Class	Pore Size (mm)
         Very fine	< 1
         Fine	1 - <2
         Medium	2 - <5
         Coarse	5 - <10
         Very coarse	≥ 10

         
         
      Entries.—Enter a single class or classes for the horizon.
       
   3. Pore Shape
       
      1. Definition.—“Pore shape” is a description of the multiarial shape of the pore. The shapes of nonmatrix pores are dendritic tubular (approximately cylindrical, elongated, and branching), irregular (nonconnected cavities or chambers), tubular (approximately cylindrical and elongated), or vesicular (approximately spherical or elliptical). The primary packing voids between soil particles or rock fragments are referred to as interstitial pores.
          
      2. Classes.—The pore shape classes are:
          
         • Dendritic tubular
         • Interstitial
         • Irregular
         • Tubular
         • Vesicular
            
      3. Entries.—Enter one of the classes from the pore shape list for the horizon.
          
   4. Vertical Continuity
       
      1. Definition.—“Vertical continuity” is the average vertical distance through which the minimum pore diameter exceeds 0.5 mm when the soil layer is moist or wetter.
          
      2. Classes.—The vertical continuity classes are:
          

         Vertical Continuity Class	Vertical Distance (cm)
         Low	< 1
         Moderate	1 - <10
         High	≥ 10

         
         
      3. Entries.—Enter one of the vertical continuity classes.
          

618.47  Reaction, Soil (pH)

1. Definition.—“Soil reaction” is a numerical expression of the relative acidity or alkalinity of a soil.
    
2. Classes.—The descriptive terms for reaction and their respective ranges in pH are:
    

   Reaction Class	Range in pH
   Ultra acid	1.8-3.4
   Extremely acid	3.5-4.4
   Very strongly acid	4.5-5.0
   Strongly acid	5.1-5.5
   Moderately acid	5.6-6.0
   Slightly acid	6.1-6.5
   Neutral	6.6-7.3
   Slightly alkaline	7.4-7.8
   Moderately alkaline	7.9-8.4
   Strongly alkaline	8.5-9.0
   Very strongly alkaline	9.1-11.0



3. Significance
    
   1. A principal value of soil pH is the information it provides about associated soil characteristics. Two examples are phosphorus availability and base saturation. Soils that have a pH of approximately 6 or 7 generally have the most ready availability of plant nutrients. Strongly acid or more acid soils have low extractable calcium and magnesium; a high solubility of aluminum, iron, and boron, and a low solubility of molybdenum. In addition, these soils may possibly have organic toxins and generally have a low availability of nitrogen and phosphorus. At the other extreme are alkaline soils. Calcium, magnesium, and molybdenum are abundant where there is little or no toxic aluminum and nitrogen is readily available. If pH is above 7.9, the soils may have an inadequate availability of iron, manganese, copper, zinc, and especially phosphorus and boron.
       
   2. Soil reaction is one of several properties used as a general indicator of soil corrosivity or the soil’s susceptibility to dispersion. In general, soils that are either highly alkaline or highly acid are likely to be corrosive to steel. Soils that have pH <5.5 are likely to be corrosive to concrete. Soils that have pH >8.5 are likely to be highly dispersible and may have a piping problem.
       
4. Measurement.—The most common soil laboratory measurement of pH is the 1:1 water method. In this method, a crushed and sieved soil sample is mixed with an equal amount of water and a measurement is made of the suspension using a pH meter. Another method, used for mineral and organic soils, is the 0.01M calcium chloride method. In NASIS, these two methods are shown as separate data elements.
    
   1. The pH values derived from water suspension are affected by field applications of fertilizer or other salts in the soil, the content of carbon dioxide in the soil, and the moisture content at the time of sampling. The 0.01M calcium chloride method reduces these influences.
       
   2. The laboratory methods are described in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
       
5. Estimates.— A variety of field test kits are available for determination of pH in the field. The methods include a water-soluble dye, which is mixed with soil and thus produces a color that is compared with a chart; a dye-impregnated paper, which changes color according to differences in pH; and portable glass electrodes. Each State office can recommend a suitable pH method for the soils in the State. If requested, the Soil Survey Laboratory makes suggestions for suitable methods for field measurements and furnishes NRCS soil scientists with the proper chemicals.
    
6. Entries.—Soil reaction (pH) is time and moisture dependent, and water pH can vary up to a whole unit during the growing season. The range of pH should reflect the variations. The 1:1 water method generally is used with mineral soils. Organic soils are measured in a 1:2 0.01 M calcium chloride solution. Separate entries are made for pH 1:1 water and pH 1:2 0.01 M calcium chloride, depending on whether the horizon is mineral or organic. Enter the high, low, and representative values of the appropriate estimated pH range for each horizon. The high and low values must correspond with the class limits, such as1.8-3.4, 3.5-4.4, 4.5-5.0, 5.1-5.5, 5.6-6.0, 6.1-6.5, 6.6-7.3, 7.4-7.8, 7.9-8.4, 8.5-9.0, and 9.1-11.0, or a combination of classes, such as 4.5-5.5, must be entered.
    

618.48  Restriction Kind, Depth, Thickness, and Hardness

1. Restriction Kind
    
   1. Definition.—“Restriction kind” is the type of nearly continuous layer that has one or more physical, chemical, or thermal properties that significantly reduce the movement of water and air through the soil or that otherwise provide an unfavorable root environment. Bedrock (e.g., limestone), cemented horizons (e.g., duripan), densic material (e.g., dense till), frozen horizons or layers (e.g., permanent ground ice), and horizontally oriented, human-manufactured materials (e.g., concrete) are examples of layers that are kinds of restrictions.
       
   2. Significance.—Restrictive layers limit plant growth by restricting the limits of the rooting zone. They also impede or restrict the movement of soil water vertically through the soil profile and have a direct impact on the quality and quantity of ground water and surface water. Restrictions are important for both soil interpretations and soil classification.
       
   3. Measurement.—Identify and describe restrictive soil layers in the field. Observe, measure, and record the restriction kind along with their depth, thickness, and hardness (defined below). When describing pedons, identify types or kinds of restrictions by suffix symbols, such as "d," "f," "m," "r," "v," or "x," or by the master layers “M” or "R." Use measurements or observations made throughout the extent of occurrence of a soil as a basis for estimates of restriction kind.
       
   4. Entries.—Enter the appropriate choice for the kind of restrictive horizon or layer from the following list—
       
      1. Abrupt textural change
      2. Bedrock, densic
      3. Bedrock, lithic
      4. Bedrock, paralithic
      5. Cemented horizon
      6. Densic material
      7. Duripan
      8. Fragipan
      9. Human-manufactured materials
      10. Natric
      11. Ortstein
      12. Permafrost
      13. Petrocalcic
      14. Petroferric
      15. Petrogypsic
      16. Placic
      17. Plinthite
      18. Salic
      19. Strongly contrasting textural stratification
      20. Sulfuric
2. Restriction Depth
    
   1. Definition.—“Restriction depth” is the vertical distance from the soil surface to the upper and lower boundaries of the restriction.
       
   2. Measurement.—Use measurements or observations made throughout the extent of occurrence of a soil as a basis for estimates of restriction depth.
       
   3. Entries.—Restriction depth values used to populate component data in NASIS are not specific to any one point. They are a reflection of commonly observed values based on field observations and are intended to model the component as it occurs throughout the map unit. Enter the high, low, and representative values for the top and bottom restriction depths in centimeters using whole numbers (integers).
       
3. Restriction Thickness
    
   1. Definition.—“Restriction thickness” is the distance from the top to the bottom of a restrictive layer.
       
   2. Significance.—Restriction thickness has a significant impact on the ease of mechanical excavation.
       
   3. Measurement.—Use observations made throughout the extent of occurrence of a soil as a basis for estimates of restriction thickness.
       
   4. Entries.—Restriction thickness values used to populate component data in NASIS are not specific to any one point. They are a reflection of commonly observed values based on field observations and are intended to model the component as it occurs throughout the map unit. Enter the high, low, and representative values for the thickness in centimeters. The range of valid entries is from 1 to 999, and only whole numbers (integers) are allowed.
       
4. Restriction Hardness
    
   1. Definition.—“Restriction hardness” is the rupture resistance cemented of an air-dried, then submerged block-like specimen of mineral material. Ice is not applicable.
       
   2. Significance.—Restriction hardness has a significant impact on the ease of mechanical excavation. Use excavation difficulty classes (defined above) to evaluate the relationships of restriction layers to excavations.
       
   3. Measurement.—Use observations made throughout the extent of occurrence of a soil as a basis for estimates of restriction hardness. For measurements of the restriction hardness, use the procedures and classes of cementation listed with the rupture resistance classes. Classes are described for like specimens about 25-30 mm on edge that are air-dried and then submerged in water for at least 1 hour. Compress the specimen between extended thumb and forefinger, between both hands, or between the foot and a nonresilient flat surface. If the specimen resists compression, drop a weight onto it from progressively greater heights until it ruptures. Failure is the point of the initial detection of deformation or rupture. Stress applied in the hand should be over a 1-second period. Learn the tactile sense of the class limits by applying force to top-loading scales and sensing the pressure through the tips of the fingers or through the ball of the foot. Use postal scales for the resistance range that is testable with the fingers. Use a bathroom scale for the higher rupture resistance range.
       
   4. Classes.—Restriction hardness is rated using the following classes and operation descriptions:
       

      Restriction Hardness (Rupture Resistance) Class	Operation Description
      Noncemented	Fails under very slight force applied slowly between thumb and forefinger (<8N).
      Extremely weakly cemented	Fails under slight force applied slowly between thumb and forefinger (8 to 20N).
      Very weakly cemented	Fails under moderate force applied slowly between thumb and forefinger (20 to 40N).
      Weakly cemented	Fails under strong force applied slowly between thumb and forefinger (about 80N maximum force can be applied) (40 to 80N).
      Moderately cemented	Cannot be failed between thumb and forefinger but can be failed between both hands or by placing specimen on a nonresilient surface and applying gentle force underfoot (80 to 160N).
      Strongly cemented	Cannot be failed in hands but can be failed underfoot by full body weight (about 800N) applied slowly (160 to 800N).
      Very strongly cemented	Cannot be failed underfoot by full body weight but can be failed by <3J blow (800N to 3J).
      Indurated	Cannot be failed by blow of 3J (≥ 3J).

      Both force (Newtons, N) and energy (joules, J) are employed. The number of Newtons is 10 times the kilograms of force. One joule is the energy delivered by dropping a 1 kg weight a distance of 10 cm.
       

618.49  Saturated Hydraulic Conductivity

1. Definition.—“Saturated hydraulic conductivity” is the ease with which pores of a saturated soil transmit water. Formally, it is the proportionality coefficient that expresses the relationship of the rate of water movement to hydraulic gradient in Darcy's Law (a law that describes the rate of water movement through porous media). It is expressed in micrometers per second. To convert micrometers per second to inches per hour, multiply micrometers per second by 0.1417. The historical definition of “saturated hydraulic conductivity” is the amount of water that would move vertically through a unit area of saturated soil in unit time under unit hydraulic gradient.
    
2. Significance.—Saturated hydraulic conductivity is used in soil interpretations. It is also known as Ksat.
    
3. Measurement.—Means of measurement, such as the Amoozemeter and double ring infiltrometers, provide some basis for estimation of saturated hydraulic conductivity. No method has been accepted as a standard. Since measurements are difficult to make and are only available for relatively few soils, estimates of saturated hydraulic conductivity are based on soil properties.
    
4. Estimates.— The soil properties that affect saturated hydraulic conductivity are distribution, continuity, size, and shape of pores. Since the pore geometry of a soil is not readily observable or measurable, observable properties related to pore geometry are used to make estimates of saturated hydraulic conductivity. These properties are texture, structure, pore size, density, organic matter content, and mineralogy. Part 618, Subpart B, Exhibits, Section 618.88 provides a guide for estimating saturated hydraulic conductivity according to soil texture and bulk density or according to specified overriding conditions.
    
   1. In making estimates, the soil characteristic that exerts the greatest control for many soils is texture.
       
   2. The general relationships shown in Part 618, Subpart B, Exhibits, Section 618.88 are adjusted up or down depending on bulk density. Structure, pore size, organic matter content, clay mineralogy, and other features observed within the soil profile, such as consistency, dry layers in wet seasons, root mats or absence of roots, and evidence of perched water levels or standing water, are good field indicators for adjusting estimates.
       
   3. (3) Water movement through bedrock for layers designated as R and Cr can be estimated from the guide in Part 618, Subpart B, Exhibits, Section 618.89 of this handbook.
       
5. Entries.—Enter the high, low, and representative values of saturated hydraulic conductivity for each horizon. The range of valid entries is 0 to 705 µm s^-1, and four decimal places are allowed.
    

618.50  Slope Aspect

1. Definition.—“Slope aspect” is the direction toward which the surface of the soil faces.
    
2. Significance.—Slope aspect may affect soil temperature, evapotranspiration, winds received, and snow accumulation.
    
3. Measurement.— Slope aspect is measured clockwise from true north as an angle between 0 and 360 degrees.
    
4. Entries.—Enter the slope aspect counterclockwise, slope aspect clockwise, and slope aspect representative for the map unit component. The range of valid entries is from a minimum of 0 degrees to a maximum of 360 degrees. Record values to the nearest whole number (integer).
    
   1. Slope aspect counterclockwise is one end of the range in characteristics for the slope aspect of a component. This end of the range is expressed in degrees measured clockwise from true north, but in the direction counterclockwise from the representative slope aspect.
       
   2. Slope aspect clockwise is one end of the range in characteristics for the slope aspect of a component. This end of the range is expressed in degrees measure clockwise from true north, and in the direction clockwise from the representative slope aspect.
       
   3. Slope aspect representative is the common, typical, or expected direction toward which the surface of the soil faces, measured in degrees clockwise from true north.
       

618.51  Slope Gradient

1. Definition.—“Slope gradient” is the difference in elevation between two points and is expressed as a percentage of the distance between those points. For example, a difference in elevation of 1 meter over a horizontal distance of 100 meters is a slope of 1 percent.
    
2. Significance.—Slope gradient influences the retention and movement of water, the potential for soil slippage and accelerated erosion, the ease with which machinery can be used, soil-water states, and the engineering uses of the soil.
    
3. Measurement.— Slope gradient is usually measured with a hand level or clinometer. The range is determined by summarizing data from several sightings.
    
4. Entries.—Enter the high, low, and representative values to represent the range of slope gradient as a percentage for the map unit component. The range of valid entries is from 0 to 999 percent, and tenths (one decimal place) are allowed but should only be used for values less than 1 percent.
    

618.52  Slope Length, USLE

1. Definition.—“Slope length” is the horizontal distance from the origin of overland flow to the point where either the slope gradient decreases enough that deposition begins or runoff becomes concentrated in a defined channel. Refer to Agriculture Handbook 703.
    
2. Significance.—Slope length has considerable control over runoff and potential accelerated water erosion. Slope length is combined with slope gradient in erosion prediction equations to account for the effect of topography on erosion.
    
3. Measurement
    
   1. Slope length is measured from the point of origin of overland flow to the point where the slope gradient decreases enough that deposition begins or runoff becomes concentrated in a defined channel. In cropland, defined channels are usually ephemeral gullies or, in rare instances where they are near a field edge, are a classic gully or stream. Surface runoff will usually concentrate in less than 400 feet (120 meters), although longer slope lengths of up to 1000 feet are occasionally found. The maximum distance allowed in erosion equations is 1000 feet (305 meters). Conversion to the horizontal distance is made in the conversion process within the equation model.
       
   2. Assume no support practices. Ignore practices such as terraces or diversions. Slope length is best determined by pacing or measuring in the field. Do not use contour maps to estimate slope lengths unless contour intervals are 1 foot or less. Slope lengths estimated from contour maps are usually too long because most maps do not have the detail needed to indicate all ephemeral gullies and concentrated flow areas that end the slope lengths. Refer to figures 4-1 through 4-10 within Agriculture Handbook 703 for more landscape guidance.
       
4. Entries.—Enter the high, low, and representative values for the range for each map unit component. Enter a whole number that represents the slope length in meters, from the point of origin of overland flow to the point of deposition or concentrated flow, of the slope on which the component lies. The slope length may be fully encompassed within one map unit or may cross several map units. The minimum value is 0, and the maximum value used in erosion equations is 305 meters. The NASIS database allows valid entries from 0 to 4000 meters.
    

618.53  Sodium Adsorption Ratio

1. Definition.—“Sodium adsorption ratio” (SAR) is a measure of the amount of sodium (Na⁺) relative to calcium (Ca²⁺) and magnesium (Mg²⁺) in the water extracted from a saturated soil paste. It is the ratio of the Na concentration divided by the square root of one-half of the Ca + Mg concentration. SAR is calculated from the equation:

   SAR = Na⁺ / [(Ca²⁺ + Mg²⁺)/2]^0.5
    

2. Significance.—Soils that have values for sodium adsorption ratio of 13 or more may have an increased dispersion of organic matter and clay particles, reduced saturated hydraulic conductivity and aeration, and a general degradation of soil structure.
    
3. Measurement.— The concentration of Na, Ca, and Mg ions is measured in a water extract from a saturated soil paste. The method is described in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS. The sodium adsorption ratio is then calculated from the molar concentrations of the three cations using the equation shown above in Section 618.53 (A).
    
4. Entries.—Enter the high, low, and representative values for the range of sodium adsorption ratio for each horizon. Enter “0” where SAR is negligible. The range of valid entries is from 0 to 9999, and tenths (one decimal place) are allowed.
    

618.54  Soil Erodibility Factors, USLE, RUSLE2

1. Definition.—Soil erodibility factors (Kw) and (Kf) quantify soil detachment by runoff and raindrop impact. These erodibility factors are indexes used to predict the long-term average soil loss from sheet and rill erosion under crop systems and conservation techniques. Factor Kw applies to the whole soil and factor Kf applies only to the fine-earth (less than 2.0 mm) fraction. The procedure for determining the Kf factor is outlined in Agriculture Handbook 703, Predicting Soil Erosion by Water: A Guide to Conservation Planning With the Revised Universal Soil Loss Equation (RUSLE), USDA, Agricultural Research Service, 1997. The K factors for soils in Hawaii and the Pacific Basin were extrapolated from local research. The nomograph, shown in Part 618, Subpart B, Exhibits, Section 618.91, was not used to determine K factors for soils in Hawaii.
    
2. Classes.—Experimentally measured Kw factors vary from 0.02 to 0.69. For soil interpretations, the factors are grouped into 14 classes. The classes are identified by a representative class value as follows: 0.02, 0.05, 0.10, 0.15, 0.17, 0.20, 0.24, 0.28, 0.32, 0.37, 0.43, 0.49, 0.55, and 0.64.
    
3. Significance.—Soil erodibility factors Kw or Kf are used in the erosion prediction equations USLE and RUSLE. Soil properties that influence rainfall erosion are those that affect—
    
   1. Infiltration rate, movement of water through the soil, and water storage capacity.
       
   2. Dispersion, detachability, abrasion, and mobility by rainfall and runoff. Some of the most important properties are texture, organic matter content, structure size class, and the saturated hydraulic conductivity of the subsoil.
       
4. Estimates
    
   1. The Kw factor is measured by applying a series of simulated rainstorms on freshly tilled plots. Direct measurement of K factors is both costly and time consuming and is conducted only for a few selected soils.
       
   2. Reliable estimates of Kf factor are obtained from the soil erodibility nomograph, which is presented on page 11 of Agriculture Handbook 537 and reproduced in Part 618, Subpart B, Exhibits, Section 618.91, or by using the soil erodibility equation. The nomograph integrates the relationship between the Kf factor and the following five soil properties:
       
      1. Percent silt plus very fine sand
      2. Percent sand greater than 0.10 mm
      3. Organic matter content
      4. Soil structure
      5. Saturated hydraulic conductivity
          
   3. The soil erodibility equation which follows also provides an estimate of Kf. K factor = {2.1 x M^1.14 X 10^-4 x (12-a)+3.25 x (b-2)+2.5 x (c-3)}/100 where: M = (percent si + percent vfs) X (100 - percent clay). Example: For a soil with 29.0% silt, 12.3% very fine sand, and 36% clay M = (29.0+12.3) x (100-36) = 2,643.20. a = percent organic matter (0, 1, 2, 3, or 4). Use worse case organic matter content assuming long-term cultivation. b = soil structure code (1, = very fine granular, 2, = fine granular, 3, = med or coarse granular, or 4 = blocky, platy, or massive) c = profile saturated hydraulic conductivity code (1, 2, 3, 4, 5, or 6). Use the layer with the lowest K_sat rv (representative value) in the permeability control section. The permeability control section is the zone from the top of the mineral soil layer being evaluated to a depth of 50 cm below the top of that soil layer but should not exceed a profile depth of 200 cm. The permeability control section guarantees that a specific zone is only considered relative to the mineral soil layer being evaluated. Include the permeability of any bedrock or other non-soil layers in the permeability control section. Note that the codes were initially established using the 1951 Soil Survey Manual. The codes correspond to the following saturated hydraulic conductivity ranges:
       

      Profile permeability class code	Permeability class of 1951	Saturated hydraulic conductivity range μm/sec	Saturated hydraulic conductivity classes 1993
      6	Very slow	<0.30	very low or mod. low
      5	Slow	0.30 to <1.20	mod. low
      4	Slow or mod.	1.20 to <4.80	mod. high
      3	Moderate	4.80 to <15.00	mod. high or high
      2	Mod. or rapid	15.00 to <30.00	high
      1	Rapid	≥30.00	high or very high

   
   
   4. The accuracy of the nomograph and equation has been demonstrated for a large number of soils in the United States. However, the nomograph and the equation may not be applicable to some soils having properties that are uniquely different from those used in developing the nomograph. For example, the nomograph does not accurately predict Kf factors for certain Oxisols in Puerto Rico or the Hawaiian Islands; some soils with andic properties, organic soil materials, or low activity clays; and some calcareous or micaceous soils. In these cases, Kf factors are estimated using the best information at hand and knowledge of the potential for rainfall erosion. See Agriculture Handbook 703 for more information.
       
   5. When using the nomograph and the equation, care should be taken to select an organic matter percentage that is most representative of the horizon being considered, assuming long-term cultivation. It is acceptable to use linear interpolations between plotted lines on the nomograph and values in hundredths (two decimal places) for organic matter content in the equation. For horizons that have organic matter content greater than 4 percent, use the 4 percent curve in the nomograph and exactly 4 percent in the equation.
       
   6. Rock or pararock fragments are not taken into account in the nomograph or the soil erodibility equation. If fragments are substantial, they have an armoring effect. Pararock fragments are assumed to break down with cultivation or other manipulation and so are not used in determining Kw factors. If a soil has mixtures of rock and pararock fragments, the Kw factor should reflect the degree of protection afforded only by the rock fragments. Guidelines for determining Kw factors are as follows:
       
      1. First use the soil erodibility nomograph shown in Part 618, Subpart B, Exhibits, Section 618.91 or the soil erodibility equation shown above to determine the Kf factor for the soil material less than 2 mm in diameter.
          
      2. Then use the table in Part 618, Subpart B, Exhibits, Section 618.92 to convert the Kf value of the soil fraction less than 2 mm in diameter, which is derived from either the nomograph in Part 618, Subpart B, Exhibits, Section 618.91 or from the soil erodibility equation, to a Kw factor adjusted for the total volume of rock fragments. The Kw factor is adjusted only when the total content of rock fragment values in the layer, by volume, is equal to or greater than 15 percent. If total rock fragment content, by volume, is less than 15 percent, the Kw factor equals the Kf factor. In practice, the representative values (rv’s) for rock fragment volume, as populated in the NASIS Horizon Fragments Table, are summed for each size fraction to compute the total rock fragment content for the layer.
          
      3. If the soil on site contains more or less rock fragments than the mean of the range reported, adjustments can be made in Kf by using Part 618, Subpart B, Exhibits, Section 618.92. Select the estimates of total rock fragment volume percentages, and then use Part 618, Subpart B, Exhibits, Section 618.92. Enter Part 618, Subpart B, Exhibits, Section 618.92 in line with the rock fragment volume percentage and find, in the appropriate line, the nearest value to the Kf factor. Within that column, read the Kw factor on the line with the percentage of rock fragments of the soil for which you are making the estimate. Round the K factor displayed in the table to the closest acceptable K factor class entry, as shown below. This is the new Kw factor adjusted for rock fragments on site.
          
5. Entries.—Enter the coordinated values for Kw and Kf factor classes for each horizon posted, except organic horizons.
    
   1. The acceptable entries for Kw and Kf classes are 0.02, 0.05, 0.10, 0.15, 0.17, 0.20, 0.24, 0.28, 0.32, 0.37, 0.43, 0.49, 0.55, and 0.64. Use the comparison reports and calculation script in NASIS for help in populating Kf and Kw factors. Use the reports to print or export the currently stored values for Kf and Kw factor classes for each component in a selected set for comparison with the computed Kf and Kw factor classes using the calculation script formulas. The comparison reports give a preview of the results of the K factor calculations and should be used before the decision is made to run the calculation and save the new data.
       
   2. Soil horizons that do not have rock fragments are assigned equal Kw and Kf factors. In horizons where total rock fragments are 15 percent or more, by volume, the Kw factor is always less than the Kf factor. For example:
       

      Depth (in)	USDA Texture	Kw	Kf
      0-5	GR-L	0.20	0.32
      0-5	L	0.32	0.32
      0-5	GRV-L	0.10	0.32
      0-46	CL	0.28	0.28
      46-60	SL	0.20	0.20

   
   
   3. Soils that have similar properties and erosivity should be grouped in similar K factor classes.
       

618.55  Soil Erodibility Factors for WEPP

1. Soil erodibility factors for WEPP include interrill erodibility (K_i), rill erodibility (K_r), and critical hydraulic shear stress (T_c). These erodibility factors for the WEPP erosion model quantify the susceptibility of soil detachment by water. These erodibility factors predict the long-term average soil loss which results from sheet and rill erosion under various alternative combinations of crop systems and conservation techniques.
    
2. The K_i, K_r, and T_c factors are used in a continuous simulation computer model which predicts soil loss and deposition on a hillslope. Reference NSERL Report No. 9, USDA, Agricultural Research Service, National Erosion Research Laboratory, August 1994, documentation version 94.7. This procedure does not include data for soils with highly weathered material (e.g., oxic horizons) and those with andic soil properties. These factors are quantitative and calculated using experimental equations. They are different than the soil erodibility factors used in USLE and RUSLE.
    
   1. Interrill erodibility (K_i)
       
      1. Definition.—“Interrill erodibility (K_i)” is the susceptibility of detachment and transport of soil particles by water. It is the susceptibility of the soil to movement to a rill carrying runoff.
          
      2. Significance.—Interrill erodibility (K_i) is a measure of sediment delivery rate to rills as a function of rainfall intensity. The K_i values for soil need to be adjusted if factors that influence the resistance of soil to detachment occur. These factors include live and dead root biomass, soil freezing and thawing, and mechanical and livestock compaction.
          
      3. Measurement.— Interrill erodibility (K_i) measurements are determined from rainfall simulation experiments. These experiments require the use of specialized equipment and specialized measurement techniques in a research setting.
          
      4. Calculations.—Use the following equations:
          
         • For cropland soils with 30 percent or more sand:
           K_i = 2,728,000 + 192,100 x (% very fine sand)
           Where:
           Very fine sand must be less than or equal to 40 percent; if very fine sand is greater, use 40 percent.
            
         • For cropland soils with less than 30 percent sand:
           K_i = 6,054,000 - 55,130 x (% clay)
           Where:
           Clay must not exceed 50 percent; if clay is greater, use 50 percent.
            
      5. Entries.—The computer generates entry values using the above formulas. Allowable K_i values range from 2,000,000 to 11,000,000.
          
   2. Rill erodibility (K_r)
       
      1. Definition.—“Rill erodibility (K_r)” is a measure of the susceptibility of a soil to detachment by flowing water. As rill erodibility (K_r) increases, rill erosion rates increase.
          
      2. Significance.—Rill erodibility (K_r) is often defined as the soil detachment per unit increase in shear stress of clear water flow. The rate of soil detachment in rills varies because of a number of factors, including soil disturbance by tillage, living root biomass, incorporated residue, fragments, soil consolidation, freezing and thawing, and wheel and livestock compaction.
          
      3. Measurement.— Rill erodibility (K_r) measurements are determined by rainfall simulation and flow simulation experiments. These experiments require the use of specialized equipment and specialized measurement techniques in a research setting.
          
      4. Calculations.—Use the following equations:
          
         • For cropland soils with 30 percent or more sand:
           K_r = 0.00197 + 0.00030 x (% very fine sand) + 0.03863 x EXP(-1.84 x ORGMAT)
           Where:
           Organic matter (ORGMAT) is the organic matter in the surface soil (assuming that organic matter equals 1.724 times organic carbon content). Organic matter must exceed 0.35 percent; if less, use 0.35 percent. Very fine sand must be less than or equal to 40 percent; if greater, use 40 percent.
            
         • For cropland soils with less than 30 percent sand:
           K_r = 0.0069 + 0.134 x EXP(-0.20 x % Clay)
           Where:
           Clay must be 10 percent or greater; if less, use 10 percent.
            
      5. Entries.—The computer generates the value by using the above formulas. Allowable K_r values range from 0.002 to 0.045 s/m.
          
   3. Critical shear stress (T_c)
       
      1. Definition.—“Critical shear stress (T_c)” is the hydraulic shear that must be exceeded before rill erosion can occur.
          
      2. Significance. Critical shear stress (T_c) is important in the rill detachment equation. It is the shear stress below which no soil detachment occurs. Critical shear stress (T_c) is the shear intercept on a plot of detachment by clear water versus shear stress in rills.
          
      3. Measurements.—Critical shear stress (T_c) is derived from a specialized research project.
          
      4. Calculations.—Use the following equations:
          
         • For cropland soils with 30 percent or more sand:
           T_c = 2.67 + 0.065 x (% clay) - 0.058 x (% very fine sand)
           Where:
           Very fine sand must be less than or equal to 40 percent; if greater, use 40 percent.
            
         • For cropland soils with less than 30 percent sand:
           T_c = 3.5
            
      5. Entries.—No manual entry is needed. The value is computer generated using the above formulas. Allowable T_c values range from 1 to 6 N/m².
          

618.56  Soil Moisture Status

1. Definition.—“Soil moisture status” is the mean monthly soil water state at a specified depth.
    
2. Classes.—The water state classes used in soil moisture status are dry, moist, and wet. These classes are defined as follows:
    

   Water State Class	Definition
   Dry	≥15 bar suction
   Moist	<15 bar to ≥0.0 bar (moist plus nonsatiated wet)
   Wet	<0.0 bar; free water present (satiated wet)



3. Significance.—Soil moisture status is a recording of the generalized water states for a soil component. Soil moisture greatly influences vegetation response, root growth, excavation difficulty, albedo, trafficability, construction, conductivity, soil chemical interactions, workability, chemical transport, strength, shrinking and swelling, frost action, seed germination, and many other properties, qualities, and interpretations. Soil moisture states are significant to soil taxonomic classification, wetland classification, and other classification systems. The recording of soil moisture states helps to document the soil classification as well as convey information useful for crop and land management models.
    
4. Measurement
    
   1. Soil water status can be measured using tensiometers or moisture tension plates. Soil water status also can be field estimated. Chapter 3 of the Soil Survey Manual provides more information. It is important to note that the three water state classes and eight subclasses described in the Soil Survey Manual are used to describe the moisture state at a point in time for individual pedons (spatial and temporal point data), while the water state classes discussed here are used to estimate the mean monthly aggregated moisture conditions for a map unit component. As a consequence, only three classes are used and the definitions for the moist and wet classes are modified from the definitions in the Soil Survey Manual. The wet class used here includes only the satiated wet class and corresponds to a free water table. The moist class is expanded to include the nonsatiated wet class given in the Soil Survey Manual.
       
   2. Dry is separated from moist at 15 bar suction. Wet satiated has a tension of 0.0 bar or less (zero or positive pore pressure).
       
   3. Changes in natural patterns of water movement from dams and levees are considered in evaluating and entering soil moisture status. Infiltration, saturated hydraulic conductivity, and organic matter, which affect soil moisture movement, are strongly impacted by land cover and land use. Land use and land cover should be considered as a mapping tool for separating map units or map unit components. The difference in soil moisture status resulting from differences in land use and land cover constitute a difference in soil properties. However, conservation practices, such as irrigating and fallowing the land, alter the soil moisture status but are not considered in the map unit component data. Use-dependent databases may allow entries for these altered states in the future. Permanent installations, such as drainage ditches and tile, affect soil moisture status, and the drained condition should be reflected in the soil moisture status entries for map unit components that are mapped as “drained.” Undrained areas are mapped as “undrained” components, and the entries for soil moisture status reflect the undrained condition.
       
   4. Irrigation and drainage canals are shown on soil maps; their effects on the soil should be shown in the properties of the soils in mapping and in the property records. Soils that are now wet because of excessive irrigation and leaking canals should be mapped, and their properties should reflect the current soil moisture status.
       
5. Guiding Concepts
    
   1. The intent is to describe a mean moisture condition, by month, for a soil component. Layer depths may or may not be the same as horizon depths in the Component Horizon Table. Layers define the zone having a specific soil moisture state. If the soil is wet throughout 0 to 200 cm, then one entry (“wet”) is made for 0 to 200 cm for that month.
       
   2. For frozen soils, enter the appropriate soil moisture state that the soil would have if thawed. For example, if the soil is frozen and then determined to be wet when thawed, enter “wet.”
       
   3. The horizons can be subdivided or combined, as appropriate, into layers for the various soil moisture states as needed. Remember that these are monthly averages for the extent of the component across the landscape.
       
   4. The entries are expected to come from the best estimates that local knowledge can provide. If local knowledge is supported by data, so much the better. The information as aggregated data is not expected to be exact but should be generalized and reflect an average condition.
       
   5. Entries for the representative values (rv) on distance to the upper and lower boundary of the moisture layer should reflect the soil moisture conditions expected in a normal year, as defined in the latest edition of the Keys to Soil Taxonomy.
       
   6. Make entries for each month by layer. Enter the dominant condition for the month. This is the condition that exists for more than 15 days on the long-term average. The low and high values represent the depth range within the component for the normal year; they should not represent the extremes, such as years of drought.
       
   7. If the depth to free water fluctuates during the month, use the depth for the average between the high and low levels.
       
6. Entries.—Enter the soil moisture status as dry, moist, or wet for each soil layer for each month. Enter only one soil moisture state for a given layer. The number of layers depends upon the number of changes of soil moisture status in the profile. Enter the values for component soil moisture depth to top and depth to bottom that represents the distance, in whole centimeters, from the soil surface to the top and bottom respectively, of each soil layer for each month. Part 618, Subpart B, Exhibits, Section 618.97 contains examples of entries in a worksheet format that graphs soil moisture status by month and depth.
    

618.57  Soil Slippage Potential

1. Definition.—“Soil slippage potential” is the hazard that a mass of soil will slip when vegetation is removed, soil water is at or near saturation, and other normal practices are applied. Conditions that increase the hazard of slippage but are not considered in this rating are undercutting lower portions or loading the upper parts of a slope or altering the drainage or offsite water contribution to the site, such as through irrigation. The publication Landslides Investigation and Mitigation Special Report 247 (Transportation Research Board, National Research Council, 1996) provides additional information on landscape slippage.
    
2. Significance.—Slippage is an important consideration for engineering practices, such as constructing roads and buildings, and for forestry practices.
    
3. Estimates.— Soil slippage potential classes are estimated by observing slope; lithology, including contrasting lithologies; strike and dip; surface drainage patterns; and occurrences of such features as slip scars and slumps.
    
4. Guides.—Use Part 618, Subpart B, Exhibits, Section 618.96, "Key Landforms and Their Susceptibility to Slippage," as a guide for rating the hazard of slippage.
    
5. Entries.—Enter one of the following soil slippage potential classes for the component:
    
   1. High (unstable)
   2. Medium (moderately unstable)
   3. Low (slightly unstable to stable)
       

618.58  Soil Temperature

1. Definition.—“Soil temperature” is the mean monthly soil temperature at the specified depth (the average of the daily high and daily low temperature for the month).
    
2. Significance.—Soil temperature is important to many biological and physical processes that occur in the soil. Plant germination and growth are closely related to soil temperature. Cold soil temperatures effectively create a thermal pan in the soil. Roots cannot uptake moisture or nutrients below the threshold temperatures specific to plant species. Chemical reactions are temperature sensitive. Pesticide breakdown, residue breakdown, microbiological activity in the soil, and nutrient conversions relate to soil temperature. Soil temperature gradients affect soil moisture and salt movement. Soil temperatures below freezing especially affect soil saturated hydraulic conductivity, excavation difficulty, and construction techniques. Soil temperature is used in soil classification and hydric soil determinations. Additional information is provided in chapter 3 of the Soil Survey Manual.
    
3. Estimates.— Soil temperature according to depth can be estimated from measured soil temperatures of the vicinity. Air temperature fluctuations, soil moisture, aspect, slope, color, snow cover, plant cover, and residue cover affect soil temperature. Estimates of soil temperature should take these factors into account when soil temperatures are extrapolated from one soil map unit component to another.
    
4. Measurement.— Soil temperature can be measured by many types of thermometers, including mercury, bimetallic, thermisters, and thermocouples. Many types of thermometers can be configured for remote, unattended operation.
    
5. Entries
    
   1. Enter soil temperature for the component as the long-term monthly average of the mean daily soil temperature at a specified depth for the month in question. The long-term average is generally considered to be a 30-year average. The range of valid entries is from -25 to 50 degrees Celsius, and only whole numbers (integers) are allowed. The number of layers populated depends upon the number of changes of soil temperature status in the profile.
       
   2. Enter the value for soil temperature depth to top that represents the distance, in centimeters, from the soil surface to the top of each soil temperature layer for each month. Enter the value for soil temperature depth to bottom that represents the distance, in centimeters, from the soil surface to the bottom of each soil temperature layer for each month.
       

618.59  Subsidence, Initial and Total

1. Definition.—“Subsidence” is the decrease in surface elevation as a result of the drainage of wet soils that have organic layers or semifluid, mineral layers. Initial subsidence is the decrease of surface elevation that occurs within the first 3 years of the drainage of these wet soils. Total subsidence is the potential decrease of surface elevation as a result of the drainage of these wet soils.
    
2. Significance
    
   1. The susceptibility of soils to subsidence is an important consideration for organic soils that are drained. If these soils are drained for community development, special foundations are needed for buildings. Utility lines, sidewalks, and roads that lack special foundations may settle at different rates, thus causing breakage, high maintenance costs, and inconvenience. If the soils are drained for farming, the long-term effects of subsidence, the possible destruction of land if it subsides below the water table, and possible legal implications where the soils are in wetlands must be considered.
       
   2. Subsidence as a result of drainage is attributed to the following factors. The first three factors are responsible for the initial subsidence that occurs rapidly, specifically within about 3 years after the water table is lowered.
       
      1. Shrinkage from drying
      2. Consolidation because of the loss of ground-water buoyancy.
      3. Compaction from tillage or manipulation
      4. Wind erosion
      5. Burning
      6. Biochemical oxidation
          
   3. After the initial subsidence, a degree of stability is reached and the loss of elevation declines to a steady rate, primarily because of oxidation. The oxidation and subsidence continue at this slower rate until stopped by the water table or underlying mineral material. The rate of subsidence depends on—
       
      1. Ground-water depth.
      2. Amount of organic matter.
      3. Kind of organic matter.
      4. Soil temperature.
      5. pH.
      6. Biochemical activity.
          
3. Estimates
    
   1. A number of studies have been made to measure actual subsidence. Other useful studies have measured the bulk density of organic soils after drainage. Based on these studies, some general guidelines can be given for initial and total subsidence.
       
   2. Initial subsidence generally is about half of the depth to the lowered water table or to mineral soil, whichever is shallower. It occurs within about 3 years after drainage. Total subsidence is the total depth to the water table or the thickness of the organic layer, whichever is shallower. It is rarely reached, except where organic layers are thin or where drainage systems have been installed for a long time.
       
4. Measurement.— After organic soils have been drained and cultivated for a number of years, they reach a nearly steady rate of subsidence that is reflected by the rather stable bulk density. Unpublished studies by the Soil Survey Laboratory have shown that the bulk density of the organic component, such as that with the percent mineral calculated out, stabilizes at around 0.27 g/cc for surface layers and 0.18 g/cc for subsurface layers. These values can be used to calculate the amount of subsidence at some time in the future as compared to the thickness of soil at the time of observation or measurement. The procedure is as follows:
    
   1. Sample the surface and subsurface layers for field state bulk density. Methods are described in the Handbook of Soil Survey Investigations Field Procedures, I 4-2, 1971, USDA, Soil Conservation Service, and in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
       
   2. Calculate out the weight contribution of the mineral component to obtain the bulk density of the organic component (DbOM). This manipulation allows bulk densities to be on a common base so that various layers can be compared. The formula for the computation is as follows:

      DbOM = Db (1 - percent mineral/100), where Db is the field state bulk density.
       

   3. Calculate the subsidence percent (SP) for surface and subsoil horizons as follows:

      For surface horizons:
      SP = 100 - [(DbOM/0.27) x 100]

      For subsurface horizons:
      SP = 100 - [(DbOM/0.18) x 100]

      Where DbOM is obtained from step (2).
       

   4. Convert initial subsidence percent to depth of subsidence in inches as follows:

      S = SPsur x Tsur + SPsub x Tsub
      Where:
      S = depth of subsidence in inches
      SPsur = subsidence percent of the surface horizon
      Tsur = thickness of the surface horizon
      SPsub = subsidence percent of the subsurface horizon
      Tsub = thickness of the subsurface horizon above the water table or the mineral soil, whichever is shallower
       

5. Entries.—Enter the high, low, and representative values that represent the range for initial and total subsidence, in centimeters, for the map unit component. The range of valid entries is from 0 to 999, and only whole numbers (integers) are allowed. If subsidence is not a concern, enter “0.”

618.60  Sum of Bases

1. Definition.—“Sum of bases” is the sum of the basic cations calcium, magnesium, potassium, and sodium that are extractable from the < 2 mm soil fraction using a solution of ammonium acetate (NH₄OAc, pH 7).
    
2. Significance.—Sum of bases is important for soil classification and for certain evaluations of soil nutrient availability or of the effect of waste additions to the soil.
    
3. Measurement.—Sum of bases is calculated from the results of methods outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
    
4. Entries.—Enter the range of sum of bases as milliequivalents per 100 grams (meq 100 g^-1) of soil for the horizon. The range of valid entries is from 0 to 30.0 and tenths (one decimal place) are allowed.
    

618.61  Surface Fragments

1. Definition.—“Surface fragments” are unattached, cemented pieces of bedrock, bedrocklike material, durinodes, concretions, nodules, or pedogenic horizons (e.g., petrocalcic fragments) 2 mm or larger in diameter and woody material 20 mm or larger in diameter that are exposed at the surface of the soil. Surface fragments can be rock fragments, pararock fragments, or wood fragments, as defined in Section 618.27. Vegetal material other than wood fragments, whether live or dead, is not included.
    
2. Surface Fragment Cover Percent
    
   1. Definition.—“Surface fragment cover percent” is the percent of ground covered by fragments 2 mm or larger in diameter (20 mm or larger in diameter for wood fragments).
       
   2. Significance.—Fragments on the soil surface are used as map unit phase criteria and greatly affect the use and management of the soil. They affect equipment use, erosion, excavation, and construction. They act as a mulch, slowing evaporation and armoring the soil against rainfall impact. They also affect the heating and cooling of soils.
       
   3. Estimates.— An estimation of cover by surface fragments can be made visually without quantitative measurement, by transect techniques, or by some combination of visual and quantitative measures. Chapter 3 of the Soil Survey Manual provides more information.
       
   4. Entries.—Enter the high, low, and representative values for the percent of the surface covered by each size class and kind of fragment populated in the Component Surface Fragments Table of the NASIS database. The range of valid entries is from 0 to 100 percent, and hundredths (two decimal places) are allowed.
       
3. Surface Fragment Kind
    
   1. Definition.—“Surface fragment kind” is the lithology/composition of the surface fragments 2mm or larger in diameter (20 mm or larger in diameter for wood fragments).
       
   2. Significance.—Fragments vary according to their resistance to weathering. Consequently, fragments of some lithologies are more suited than others for use as building stone, road building material, or riprap to face dams and stream channels.
       
   3. Entries.—Enter the appropriate fragment kind name for the record of fragments populated in the Component Surface Fragments Table in the NASIS database. The class names are present in a choice list and can also be viewed in the NASIS data dictionary.
       
4. Surface Fragment Size
    
   1. Definition.—“Surface fragment size” is the size based on the multiaxial dimensions of the surface fragments.
       
   2. Significance.—The size of surface fragments is significant to the use and management of the soil. The adjective form of fragment size is used as phase criteria for naming map units. The size affects equipment use, excavation, construction, and recreational uses.
       
   3. Classes
       
      1. Classes of surface fragment size are subdivided based on the shape of the fragments (described below).

         Flat fragment classes are:

         Flat fragment class	Length of fragment (mm)
         Channers	2-150
         Flagstones	150-380
         Stones	380-600
         Boulders	≥600

         Nonflat fragment classes are:

         Nonflat fragment class	Diameter (mm)
         Gravel	2-75
         Fine gravel	2-5
         Medium gravel	5-20
         Coarse gravel	20-75
         Cobbles	75-250
         Stones	250-600
         Boulders	≥600

      
      
      2. Gravel is a collection of fragments having a diameter ranging from 2 to 75 mm. Individual fragments in this size range are properly referred to as pebbles, not “gravels.” For fragments that are less than strongly cemented, “para” is added as a prefix to the above terms, i.e., paracobbles or fine paragravel.
          
   4. Entries.—Enter the high, low, and representative values for each size class populated in the Component Surface Fragments Table in the NASIS database. Valid entries are 2 millimeters (mm) or larger, and only whole numbers (integers) are allowed.
       
5. Mean Distance Between Rocks
    
   1. Definition.—“Mean distance between rocks” is the average distance between surface stones, boulders, or both, measured between edges.
       
   2. Significance.—The mean distance between rocks is a field clue for naming stony or bouldery map units. The closer the distance, the more equipment limitations there are for harvesting forestland or soil cultivation.
       
   3. Estimates.— Table 3-12 of the Soil Survey Manual shows the distance between stones and boulders if the diameter is 0.25 m, 0.6 m, or 1.2 m. This table should be used with caution because stones and boulders are rarely equally spaced or have the same diameter.
       
   4. Entries.—Enter the high, low, and representative values for the mean distance between rocks. The range of valid entries is from 0 to 50 meters, and hundredths (two decimal places) are allowed.
       
6. Surface Fragment Roundness
    
   1. Definition.—“Surface fragment roundness” is an expression of the sharpness of edges and corners of surface fragments.
       
   2. Classes.—The surface fragment roundness classes are:

Roundness class	Definition
Very angular	Strongly developed faces with very sharp, broken edges
Angular	Strongly developed faces with sharp edges (SSM)
Subangular	Detectable flat faces with slightly rounded corners
Subrounded	Detectable flat faces with well-rounded corners (SSM)
Rounded	Flat faces absent or nearly absent with all corners rounded (SSM)
Well rounded	Flat faces absent with all corners rounded

3. Entries.—Enter the appropriate surface fragment roundness class name for the record of surface fragments populated in the Component Surface Fragments Table in the NASIS database.

7. Surface Fragment Hardness
    
   1. Definition.—“Surface fragment hardness” is equivalent to the rupture resistance cemented of a surface fragment of specified size that has been air-dried and then submerged in water.
       
   2. Measurements.—Procedures and classes of cementation are listed with the rupture resistance classes in the Soil Survey Manual. Classes are described for similar specimens about 25-30 mm on edge which are air-dried and then submerged in water for at least 1 hour. The specimen is compressed between extended thumb and forefinger, between both hands, or between the foot and a hard flat surface. If the specimen resists compression, a weight is dropped onto it from progressively greater heights until it ruptures. Failure is considered at the initial detection of deformation or rupture. Stress applied in the hand should be over a 1-second period. The tactile sense of the class limits may be learned by applying force to top-loading scales and sensing the pressure through the tips of the fingers or through the ball of the foot. Postal scales may be used for the resistance range that is testable with the fingers. A bathroom scale may be used for the higher rupture resistance range.
       
   3. Significance.—The hardness of a surface fragment is significant where the rupture resistance class is strongly cemented or higher. These classes can impede or restrict the movement of soil water vertically through the soil profile and have a direct impact on the quality and quantity of ground water and surface water.
       
   4. Classes.—The surface fragment hardness (rupture resistance) classes are the following:
       
      1. Extremely weakly cemented
      2. Very weakly cemented
      3. Weakly cemented
      4. Moderately cemented
      5. Strongly cemented
      6. Very strongly cemented
      7. Indurated
   5. Entries.—Enter the appropriate class name for each record of surface fragments populated in the Component Surface Fragments Table in the NASIS database. Choose the term without the word “cemented” (e.g., choose the “moderately” class to represent the moderately cemented class).
       
8. Surface Fragment Shape
    
   1. Definition.—“Surface fragment shape” is a description of the overall shape of the surface fragment.
       
   2. Classes.—The surface fragment shape classes are “flat” and “nonflat.”
       
   3. Entries.—Enter the appropriate surface fragment shape class name for each record of surface fragments populated in the Component Surface Fragments Table in the NASIS database.
       

618.62  T Factor

1. Definition.—The “T factor” is the soil loss tolerance (in tons per acre). It is defined as the maximum amount of erosion at which the quality of a soil as a medium for plant growth can be maintained. This quality of the soil to be maintained is threefold in focus. It includes maintaining the surface soil as a seedbed for plants, the atmosphere-soil interface to allow the entry of air and water into the soil and still protect the underlying soil from wind and water erosion, and the total soil volume as a reservoir for water and plant nutrients, which is preserved by minimizing soil loss. Erosion losses are estimated by USLE and RUSLE2.
    
2. Classes.—The classes of T factors are 1, 2, 3, 4, and 5.
    
3. Significance.—Soil loss tolerances commonly serve as objectives for conservation planning on farms. These objectives assist in the identification of cropping sequences and management systems that can maximize production and also sustain long-term productivity. T factors represent the goal for maximum annual soil loss.
    
4. Guidelines.—Conservation objectives for soil loss tolerance include maintaining a suitable seedbed and nutrient supply in the surface soil, maintaining an adequate depth and quality of the rooting zone, and minimizing unfavorable changes in water status throughout the soil. A single T factor is assigned to each map unit component.
    
5. Estimates.— The T factor is assigned to soils without respect to land use or cover. T factors are assigned to compare soils and do not imply differences to vegetation response directly. Many of the factors used to assign a T factor are also important to vegetation response, but the T factor is not assigned to imply vegetation sensitivity to all vegetation. The general guideline given in Part 618, Subpart B, Exhibits, Section 618.93 is used to assign T factors but more specific criteria are used to select limiting soil properties.
    
6. Entries.—The estimated soil loss tolerance should be calculated from the soil properties and qualities posted in the database for each map unit component based generally on the guideline given in Part 618, Subpart B, Exhibits, Section 618.93. Acceptable values are 1, 2, 3, 4, and 5.
    

618.63  Taxonomic Family Temperature Class

1. Definition
    
   1. The soil temperature classes are part of the family categorical level as defined in Soil Taxonomy. They differ from “soil temperature regimes,” (Data Element: taxonomic temp regime), in that the cryic temperature regime is divided between the frigid and isofrigid classes based on differences in mean winter and mean summer soil temperatures. Soil temperature classes are based on mean annual and mean seasonal soil temperatures using the Celsius (centigrade) scale and taken either at a depth of 50 cm from the soil surface or at a lithic or paralithic contact, whichever is shallower.
       
   2. For soil families in Gelisols, Gelic suborders, and Gelic great groups the soil temperature classes, defined in terms of the mean annual soil temperature, are as follows:
       
      1. Hypergelic: -10°C or lower
      2. Pergelic: -4°C to -10°C
      3. Subgelic: +1°C to -4°C
          
   3. For soil families that have a difference of 6°C or more between mean summer (June, July, and August in the northern hemisphere) temperature and mean winter (December, January, and February in the northern hemisphere) temperature, the soil temperature classes, defined in terms of the mean annual soil temperature, are as follows:
       
      1. Frigid: Lower than 8°C
      2. Mesic: 8°C to 15°C
      3. Thermic: 15°C to 22°C
      4. Hyperthermic: 22°C or higher
          
   4. For soil families that have a difference of less than 6°C between the mean summer and mean winter soil temperatures, the soil temperature classes, defined in terms of the mean annual soil temperature, are as follows:
       
      1. Isofrigid: Lower than 8°C
      2. Isomesic: 8°C to 15°C
      3. Isothermic: 15°C to 22°C
      4. Isohyperthermic: 22°C or higher


2. Significance.—All soils have a taxonomic soil temperature class. Soil temperature classes are used as family differentiae in all the orders defined in Soil Taxonomy. The names are used as part of the family name unless the criteria for a higher taxon carry the same limitation. The frigid or isofrigid class is implied in all cryic suborders and great groups, but the class is not used as part of the family name because it would be redundant.
    
3. Estimates.— Estimates of soil temperature classes are made with models that use climatic data including mean annual and mean seasonal air temperatures, precipitation, and evapotranspiration. Some models include snow cover, topographic, and vegetative inputs.
    
4. Measurement.— The Celsius (centigrade) scale is the standard. It is assumed that the temperature is that of a nonirrigated soil. The soil temperature classes are based on long-term averages of mean annual and mean seasonal soil temperatures taken either at a depth of 50 cm from the soil surface or at a lithic or paralithic contact, whichever is shallower.
    
5. Entries.—Enter the appropriate soil temperature class from the following list:
    
   1. Frigid
   2. Hypergelic
   3. Hyperthermic
   4. Isofrigid
   5. Isohyperthermic
   6. Isomesic
   7. Isothermic
   8. Mesic
   9. Pergelic
   10. Subgelic
   11. Thermic
   12. Not used
        

618.64  Taxonomic Moisture Class

1. Definition.—Soil moisture classes refer to the soil moisture regimes defined in Soil Taxonomy. Soil moisture regimes are defined by the presence or absence either of ground water or of water held at a tension of less than 1500 kPa, in the soil or in specific horizons, by periods of the year.
    
2. Significance.—All soils have a soil moisture regime. Soil moisture regimes are used as differentiae in all the orders defined in Soil Taxonomy. Data on the moisture regime are used for making interpretations for cropland agriculture, correlating soils to ecological sites, and determining suitability for wildlife habitat. The moisture regime of some soils is not apparent in the classification given in Soil Taxonomy. Ustolls and Xerolls, for example, can have an aridic moisture regime. Some soils have more than one moisture regime. An example is a soil that meets the requirements of the aquic moisture regime in the wet season and also meets the requirements of the ustic regime.
    
3. Estimates.— Estimates of soil moisture regimes are made with models that use climatic data, including mean annual and mean seasonal air temperatures, precipitation, and evapotranspiration. Some models include topographic and vegetative inputs. The soil moisture control section, also defined in Soil Taxonomy, is used to facilitate the estimation of soil moisture regimes. See Soil Survey Technical Note 9 for more guidance: http://soils.usda.gov/technical/technotes/.
    
4. Measurement.— The soil moisture regimes are based on annual and seasonal soil moisture measurements taken in the soil moisture control section. The soil should not be irrigated, fallowed, or influenced by other moisture-altering practices.
    
5. Entries.—Enter the appropriate soil moisture regimes from the following list:
    
   1. Aquic
   2. Aridic (torric)
   3. Peraquic
   4. Perudic
   5. Udic
   6. Ustic
   7. Xeric

618.65  Taxonomic Moisture Subclass (Subclasses of Soil Moisture Regimes)

1. Definition.—Subclasses of soil moisture regimes are defined at the subgroup categorical level in Soil Taxonomy. The criteria differ among the great groups. For example aquic, aridic, and udic are subclasses of the soil moisture regime in Haplustalfs. A subclass is entered for all soils in a great group that meet the subclass criteria, even if the subclass is not part of the taxonomic classification. For example, aquic, aridic, udic, or typic should be used as a subclass of the soil moisture regime in Lithic Haplustalfs if the criteria are met.
    
2. Significance.—Subclasses of soil moisture regimes are used at the subgroup categorical level in all orders in Soil Taxonomy except Histosols. They typically indicate an intergrade between two moisture regimes that affect the use and management of the soil. The subclasses of soil moisture regimes are used for making interpretations for cropland agriculture, correlating soils to ecological sites, and determining suitability for wildlife habitat.
    
3. Estimates.— Estimates of subclasses of soil moisture regimes are made with models that use climatic data, including mean annual and mean seasonal air temperatures, precipitation, and evapotranspiration. Some models include topographic and vegetative inputs. The soil moisture control section, also defined in Soil Taxonomy, is used to facilitate estimation of some subclasses of soil moisture regimes. For more guidance, see Soil Survey Technical Note 9 (available online at http://soils.usda.gov/technical/technotes/).
    
4. Measurement.— The subclasses of soil moisture regimes are based on annual and seasonal soil moisture measurements taken in the soil moisture control section. The soil should not be irrigated, fallowed, or influenced by other moisture-altering practices.
    
5. Entries.—Enter the appropriate subclass of soil moisture regimes from the following list:
    
   1. Aeric
   2. Anthraquic
   3. Aquic
   4. Aridic (torric)
   5. Oxyaquic
   6. Typic
   7. Udic
   8. Ustic
   9. Xeric

618.66  Taxonomic Temperature Regime (Soil Temperature Regimes)

1. Definition.—“Soil temperature regimes” refer to the temperature regimes as defined in Soil Taxonomy.
    
2. Significance.—Soil temperature regimes are used as differentiae above the family categorical level in all orders in Soil Taxonomy. (Soil temperature classes, defined above, are used as family differentiae.) Soil temperature regimes greatly affect the use and management of soils, particularly the selection of adapted plants. Temperature regimes are used for making interpretations for cropland agriculture, correlating soils to ecological sites, and determining suitability for wildlife habitat.
    
3. Estimates.— Estimates of soil temperature regimes are made with models that use climatic data including mean annual and mean seasonal air temperatures, precipitation, and evapotranspiration. Some models include topographic and vegetative inputs.
    
4. Measurement.— The soil temperature regime is based on mean annual and seasonal soil temperatures using the Celsius (centigrade) scale and taken either at a depth of 50 cm from the soil surface or at a lithic or paralithic contact, whichever is shallower.
    
5. Entries.—Enter the appropriate soil temperature regimes from the following list:
    
   1. Gelic
   2. Cryic
   3. Frigid
   4. Mesic
   5. Thermic
   6. Hyperthermic
   7. Isofrigid
   8. Isomesic
   9. Isothermic
   10. Isohyperthermic

618.67  Texture Class, Texture Modifier, and Terms Used in Lieu of Texture

1. Definition.—“Texture class” refers to the soil texture classification used by the U.S. Department of Agriculture as defined in the Soil Survey Manual. Soil texture is the relative proportion, by weight, of the particle separate classes finer than 2 mm in equivalent diameter. The material finer than 2 mm is the fine-earth fraction. Material 2 mm or larger is rock or pararock fragments.

   Click Interactive Online Soil Texture Calculator to enter the percent sand and clay, and let the calculator do the rest.
    

2. Significance.—Soil texture influences engineering works and plant growth and indicates how soils formed. Soil texture has a strong influence on soil mechanics and the behavior of soil when it is used as construction or foundation material. It influences such engineering properties as bearing strength, compressibility, saturated hydraulic conductivity, shrink-swell potential, and compaction. Engineers are also particularly interested in rock and pararock fragments. Soil texture influences plant growth by its affect on aeration, the water intake rate, the available water capacity, the cation-exchange capacity, saturated hydraulic conductivity, erodibility, and workability. Changes in texture as related to depth are indicators of how soils formed. When texture is plotted with depth, smooth curves indicate translocation and accumulation. Irregular changes in particle-size distribution, especially in the sand fraction, may indicate lithologic discontinuities, specifically differences in parent material.
    
3. Measurement.— USDA texture can be measured in the laboratory by determining the proportion of the various size particles in a soil sample. The analytical procedure is called particle-size analysis or mechanical analysis. Stone, gravel, and other material 2 mm or larger are sieved out of the sample and thus are not considered in the analysis of the sample. Their amounts are measured separately. Of the remaining material smaller than 2 mm, the amount of the various sizes of sand is determined by sieving. The amount of silt and clay is determined by a differential rate of settling in water. Either the pipette or hydrometer method is used for the silt and clay analysis. Organic matter and dissolved mineral matter are removed in the pipette procedure but not in the hydrometer procedure. The two procedures are generally very similar, but a few samples, especially those with high organic matter or high soluble salts, exhibit wide discrepancies. The detailed procedures are outlined in Soil Survey Investigations Report No. 42, Soil Survey Laboratory Methods Manual, Version 4.0, November 2004, USDA, NRCS.
    
4. Estimates
    
   1. The determination of soil texture for the less than 2 mm material is made in the field mainly by feeling the soil with the fingers. The soil must be well moistened and rubbed vigorously between the fingers for a proper determination of texture class by feel. This method requires skill and experience but good accuracy can be obtained if the field soil scientist frequently checks his or her estimates against laboratory results. Many NRCS offices collect reference samples for this purpose. The content of particles larger than 2 mm cannot be evaluated by feel. The content of the fragments is determined by estimating the proportion of the soil volume that they occupy. Fragments in the soil are discussed in Section 618.27.
       
   2. Each soil scientist must develop the ability to determine soil texture by feel for each genetic soil group according to the standards established by particle-size analysis. Soil scientists must remember that soil horizons that are in the same texture class but are in different subgroups or families may have a different feel. For example, natric horizons generally feel higher in clay than “non-natric” horizons. Laboratory analysis generally shows that the clay in natric horizons is less than the amount estimated from the field method. The scientist needs to adjust judgment and not the size distribution standards.
       
5. Entries.—Texture is displayed by the use of five data elements in NASIS: texture class, texture modifier, texture modifier and class, stratified texture flag, and terms used in lieu of texture. As many as four entries can be made for each horizon for each of these data elements. However, only one texture for a surface horizon should be entered for each component. Only use multiple textures if they interpret the same for the horizon. Only textures that represent complete horizons should be entered. A representative value is also identified for each horizon. This choice should match the representative values of the various soil particle-size separates posted elsewhere in the database.
    
6. Texture Class
    
   1. Definition
       
      1. “Texture class” is an expression, based on the USDA system of particle sizes, for the relative portions of the various size groups of individual mineral soil grains less than 2 mm equivalent diameter in a mass of soil.
          
      2. Each texture class has defined limits for each particle separate class of mineral particles less than 2 mm in effective diameter. The basic texture classes, in the approximate order of increasing proportions of fine particles, are sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay. The sand, loamy sand, and sandy loam classes may be further subdivided into coarse, fine, or very fine. The basic USDA texture classes are given graphically in Part 618, Subpart B, Exhibits, Section 618.87 as a percentage of sand, silt, and clay. The chart at the bottom of the figure shows the relationship between the particle size and texture classes among the AASHTO, USDA, and Unified soil classification systems.
          
   2. Entries.—Enter the texture class for each horizon using the list in Part 618, Subpart B, Exhibits, Section 618.94.
       
7. Terms Used in Lieu of Texture
    
   1. Definition.—“Terms used in lieu of texture” are substitute terms applied to materials that do not fit into a texture class because of high organic matter content, high fragment content, high gypsum content, cementation, or another reason. Examples include artifacts, bedrock, gravel, and muck. Part 618, Subpart B, Exhibits, Section 618.94 provides a list of these terms and their codes. Some of these terms may be modified with terms from the list of texture modifiers, such as cemented, code CEM. For example, cemented diagnostic horizons such as duripans, petrocalcic horizons, and petrogypsic horizons are coded CEM-MAT, using the texture modifier “cemented” with the term in lieu of texture “material.” The concatenated texture term for such horizons in pedon descriptions is “cemented material.”
       
   2. Application
       
      1. The terms used in lieu of texture “highly decomposed plant material,” “moderately decomposed plant material,” and “slightly decomposed plant material” (codes HPM, MPM, and SPM), should only be used to describe near surface horizons composed of plant material in various stages of decomposition that are saturated with water for less than 30 cumulative days in normal years and are not artificially drained. The terms “muck,” “mucky peat,” and “peat” (codes MUCK, MPT, and PEAT) should be used to describe histic epipedons and organic horizons of any thickness that are saturated with water for 30 or more cumulative days in normal years or are artificially drained, including those in Histels and Histosols, except for Folists.
          
      2. For soil materials with 40 percent or more, by weight, gypsum in the fine-earth fraction, gypsum dominates the physical and chemical properties of the soil and particle-size classes are not meaningful. Two terms in lieu of texture are used. “Coarse gypsum material” (code CGM) is used for these materials where 50 percent or more of the fine-earth fraction is comprised of particles ranging from 0.1 to 2.0 mm in diameter. “Fine gypsum material” (code FGM) is applied to materials where less than 50 percent of the fine-earth fraction is comprised of particles ranging from 0.1 to 2.0 mm in diameter.
          
      3. “Material,” which uses the code MAT, is generic and requires the use of a texture modifier. Texture modifier terms, such as “coprogenous,” “gypsiferous,” and “marly,” are used to describe material.
          
   3. Entries.—Enter the term used in lieu of texture for each horizon, if applicable, from the list in Part 618, Subpart B, Exhibits, Section 618.94.
       
8. Texture Modifier
    
   1. Definition.—“Texture modifier” is a term used to denote the presence of a condition or component other than sand, silt, or clay.
       
   2. Application.—Texture modifier terms may apply to both texture and terms used in lieu of texture. Some may apply to both, others only apply to one or the other. Combinations of some texture modifiers are allowed. A list of allowable texture modifier terms and their codes is given in Part 618, Subpart B, Exhibits, Section 618.94. Some rules of application are given below.
       
      1. If the content of fragments equals 15 percent or more, by volume, texture modifiers are used. An example is gravelly loam or parachannery loam. The adjectives “very” and “extremely” are used when the content of fragments equals 35 to less than 60 percent and 60 to less than 90 percent, by volume, respectively.
          
      2. “Mucky” and “peaty” are used to modify near surface horizons of mineral soils that are saturated with water for 30 or more cumulative days in normal years or are artificially drained. An example is mucky loam. Excluding live roots, the horizon has an organic carbon content (by weight) of one of the following:
          
         • 5 to < 12 percent if the mineral fraction contains no clay
         • 12 to < 18 percent if the mineral fraction contains 60 percent or more clay
         • [5 + (clay percentage multiplied by 0.12)) to < (12 + (clay percentage multiplied by 0.10)] if the mineral fraction contains less than 60 percent clay
            
      3. “Highly organic” is used to modify near surface horizons of mineral soils that are saturated with water for less than 30 cumulative days in normal years and are not artificially drained. Excluding live roots, the horizon has an organic carbon content (by weight) of one of the following:
          
         • 5 to < 20 percent if the mineral fraction contains no clay
         • 12 to < 20 percent if the mineral fraction contains 60 percent or more clay
         • [(5 + (clay percentage multiplied by 0.12)] to < 20 percent if the mineral fraction contains less than 60 percent clay
            
      4. When modifying the texture of soils with greater than 15 percent, by volume, artifacts, the following classes are used:
          

         Less than 15 percent:	No texture modifier terms are used.
         15 to < 35 percent:	The adjectival term “artifactual” is used as a modifier of the texture class, such as “artifactual loam.”
         35 to < 60 percent:	The adjectival term “very artifactual” is used as a modifier of the texture class, such as “very artifactual loam.”
         60 percent or more:	If enough fine earth is present to determine the texture class (approximately 10 percent or more, by volume), the adjectival term “extremely artifactual” is used as a modifier of the texture class, such as “extremely artifactual loam.”

         If there is too little fine earth to determine the texture class (less than about 10 percent, by volume) the term in lieu of texture “artifacts” is used.

         In some instances, the soil may contain a combination of both artifacts and rock fragments. In these cases, the rock fragments and artifacts are described separately. Where appropriate, compound texture modifiers can be used; the modifier for artifacts comes before the modifier for rock fragments. Examples are artifactual very gravelly sandy loam and very artifactual channery mucky clay.
          

      5. Compound texture modifiers may be used. For example, a term may be used to indicate the presence of fragments and another used to indicate some nonfragment condition. The term used to indicate fragments should be listed first. Examples are very gravelly mucky silt loam and paragravelly ashy loam.
          
      6. Texture modifiers, such as paragravelly and paracobbly, are used to identify the presence of pararock fragments. The size, shape, and amounts of pararock fragments required for these terms are the same as for rock fragments.
          
      7. When a horizon includes both rock and pararock fragments, use the following rules for selecting texture modifiers:
          
         • Describe the individual kinds and amounts of rock and pararock fragments.
            
         • Do not use a fragment texture modifier when the combined volume of rock and pararock fragments is less than 15 percent.
            
         • When the combined volume of rock and pararock fragments is more than 15 percent and the volume of rock fragments is less than 15 percent, assign pararock fragment modifiers based on the combined volume of fragments. For example, use paragravelly as a texture modifier for soils with 10 percent rock and 10 percent pararock gravel-sized fragments.
            
         • When the volume of rock fragments is 15 percent or greater, use the appropriate texture modifier for rock fragments (see Part 618, Subpart B, Exhibits, Section 618.90), regardless of the volume of pararock fragments. (Do not add the volume of rock and pararock fragments to determine the texture modifier.)
            
      8. The definitions of the following four compositional texture modifiers guide their usage:
          
         • Hydrous.—Material that has andic soil properties and an undried 15 bar (1500 kPa)water content of 100 percent or more of the dry weight.
            
         • Medial.—Material that has andic soil properties and has a 15 bar (1500 kPa) water content of less than 100 percent on undried samples and of 12 percent or more on air-dried samples.
            
         • Ashy.—Material that has andic soil properties and is neither hydrous nor medial or material that does not have andic soil properties and the fine-earth fraction contains 30 percent or more particles 0.02 to 2.0 mm in diameter, of which 5 percent or more is composed of volcanic glass and the [(aluminum plus 1/2 iron percent by ammonium oxalate) times 60] plus the volcanic glass percent is equal to or more than 30.
            
         • Gypsiferous.—Material that contains 15 to < 40 percent, by weight, gypsum.
            
      9. Woody, grassy, mossy, and herbaceous texture modifiers are only used to modify muck, peat, or mucky peat terms (used for histic epipedons and organic horizons of any thickness that are saturated with water for 30 or more cumulative days in normal years, or are artificially drained, including those in Histels and Histosols, except for Folists). The definitions of the following four compositional texture modifiers guide their usage:
          
         • Woody.—Any material that contains 15 percent or more wood fragments larger than 2 cm in size or organic soil materials other than SPM, MPM, or HPM, that contain 15 percent or more fibers that can be identified as wood origin and contain more wood fibers than any other kind of fiber.
            
         • Grassy.—Organic soil material that contains more than 15 percent fibers that can be identified as grass, sedges, cattails, and other grasslike plants and contains more grassy fibers than any other kind of fiber.
            
         • Mossy.—Organic soil material that contains more than 15 percent fibers that can be identified as moss and contains more moss fibers than any other kind of fiber.
            
         • Herbaceous.—Organic soil material that contains more than 15 percent fibers that can be identified as herbaceous plants other than moss and grass or grasslike plants and more of these fibers than any other kind of fiber.
            
      10. Some materials can be described by utilizing an apparent texture, even though they do not fit the requirements of texture. These materials use a texture modifier. An example is marly silt loam.
           
      11. Limnic materials have modifiers to texture to connote the origin of the material. The three kinds of limnic materials are coprogenous earth, diatomaceous earth, and marl. These materials were deposited in water by precipitation or through the action of aquatic organisms or derived from plants and organisms. Refer to the Keys to Soil Taxonomy for the complete definitions and taxonomic criteria of limnic materials. The following three compositional texture modifiers are used with limnic materials to indicate presence and origin without respect to any set quantity of pellets, grains, or particles:
           
          • Coprogenous.—Soil material that is a limnic layer containing many very small (0.1 to 0.001 mm) fecal pellets.
          • Diatomaceous.—Soil material that is a limnic layer composed of diatoms.
          • Marly.—Soil material that is a limnic layer that is light colored and reacts with HCl to evolve CO₂.
             
      12. “Permanently frozen” is a term applied to a soil layer in which the temperature is perennially at or below 0 degrees C, whether its consistence is very hard or loose.
           
   3. Entries.—Enter the applicable texture modifiers from the list in Part 618, Subpart B, Exhibits, Section 618.94. Multiple texture modifiers are used in some horizons based on the application rules for texture modifier presented above. They must be assigned sequence numbers in the Horizon Texture Modifier Table in the NASIS database for the proper calculated result.
       
9. Texture Modifier and Class
    
   1. Definition.—“Texture modifier and class” is a concatenation of texture modifier and texture class or texture modifier and a term used in lieu of texture. This data element indicates the full texture term of the horizon. If texture modifiers are used, they are attached to the texture class by a hyphen, for example, GR-SL. If a layer is stratified, enter SR as a texture modifier and the end members of the textural range and connect them by hyphens, for example, SR-C-L and SR-GR-S-GR-C.
       
   2. Entries.—Enter the appropriate texture modifier and class for each horizon. These entries are calculated in the Horizon Texture Group Table in the NASIS database.
       
10. Stratified Texture Flag
     
    1. Application.—A stratified texture flag is used to identify stratified textures in the Horizon Texture Group Table in the NASIS database.
        
    2. Entries.—A Boolean flag is set to “yes” by checking the box for the stratified texture flag. This indicates that the textures that comprise a particular record are stratified. The default entry is “no” and is displayed by keeping the box for stratified texture flag unchecked.
        
11. Representative Indicator Flag
     
    1. Application.—A representative indicator flag is used to identify one representative texture (comprised of texture modifier and class) in the Horizon Texture Group Table in the NASIS database.
        
    2. Entries.—A Boolean flag is set to “yes” by checking the box for the representative indicator flag. This indicates that the texture that comprises a record in the particular horizon texture group is representative. It also indicates that the selected texture validates the soil properties populated for the layer. The selected texture record must be in agreement with the representative values for important soil properties such as clay content, sand content, rock fragment content, and organic matter content. The flag must be set even when only one texture record is populated for a particular horizon (such as in surface layers or bedrock layers). The default entry is “no” and is displayed by keeping the box for representative indicator flag unchecked. Only one texture record may be selected as representative for a given horizon or layer.
        

618.68  Water, One-Tenth Bar

1. Definition.—“Water, one-tenth bar” is the amount of soil water retained at a tension of 1/10 bar (10 kPa), expressed as a percentage of the whole soil on a volumetric basis.
    
2. Significance.—Water retained at one-tenth bar is significant in the determination of soil water-retention difference, which is used as the initial estimation of available water capacity for some soils.
    
3. Measurement.—Measurement in the laboratory is done on natural clods using a pressure desorption method. Measurement for nonswelling soils, loamy sand or coarser soils, and some sandy loams is also done using a a pressure desorption method but sieved (< 2 mm) air-dry samples are used. Gravimetric water contents are reported in laboratory measurements as a percentage of the fine-earth (<2 mm) fraction. Conversion to a volumetric basis is made using bulk density and rock fragment content.
    
4. Entries.—Enter the low, high, and representative values for the horizon. The range of valid entries is from 0 to 100 percent, and tenths (one decimal place) are allowed. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.105.
    

618.69  Water, One-Third Bar

1. Definition.—Water, one-third bar” is the amount of soil water retained at a tension of 1/3 bar (33 kPa), expressed as a percentage of the whole soil on a volumetric basis.
    
2. Significance.—Water retained at one-third bar is significant in the determination of soil water-retention difference, which is used as the initial estimation of available water capacity for some soils.
    
3. Measurement.—Measurement in the laboratory is done on natural clods using a pressure desorption method. Measurement for nonswelling soils, loamy sand or coarser soils, and some sandy loams is also done using a pressure desorption method but sieved (< 2 mm) air-dry samples are used. Gravimetric water contents are reported in laboratory measurements as a percentage of the fine-earth (< 2 mm) fraction. Conversion to a volumetric basis is made using bulk density and rock fragment content.
    
4. Entries.—Enter the low, high, and representative values for the horizon. The range of valid entries is from 0 to 100 percent, and tenths (one decimal place) are allowed. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.105.
    

618.70  Water, 15 Bar

1. Definition.—“Water, 15 bar” is the amount of soil water retained at a tension of 15 bars (1500 kPa), expressed as a percentage of the whole soil on a volumetric basis.
    
2. Significance.—Water retained at 15 bar is significant in the determination of soil water-retention difference, which is used as the initial estimation of available water capacity for some soils. Water retained at 15 bar is an estimation of the wilting point.
    
3. Measurement.—Measurement in the laboratory is done on sieved (< 2 mm) air-dry samples using a pressure desorption method. Gravimetric water contents are reported in laboratory measurements as a percentage of the fine-earth (< 2 mm) fraction. Conversion to a volumetric basis is made using bulk density and rock fragment content.
    
4. Entries.—Enter the low, high, and representative values for the horizon. The range of valid entries is from 0 to 100 percent, and tenths (one decimal place) are allowed. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.105.
    

618.71  Water, Satiated

1. Definition.—“Water, satiated” is the estimated volumetric soil water content at or near zero bar tension, expressed as a percentage of the whole soil.
    
2. Significance.—Water, satiated, represents the total possible water content of the soil, including the amount in excess of field capacity, and is used to estimate the amount of water available for leaching and translocation. Satiated water content approximates the water content at saturated conditions. It is used in such resource assessment tools as Soil Hydrology, Water Budgets, Leaching, and Nutrient/Pesticide Loading models.
    
3. Estimation.—The values are derived by the following formula:
   
   Satiated water % = total porosity % - entrapped air %
   Where:
   Total porosity % = 100(1-bulk density moist/particle density).
   Assume approximately 3% entrapped air.
    
4. Entries.—Enter the high, low, and representative values for the horizon. The range of valid entries is from 0 to 100 percent, and only whole numbers (integers) are allowed. A NASIS calculation is available and can be viewed in Part 618, Subpart B, Exhibits, Section 618.105.
    

618.72  Wind Erodibility Group and Index

1. Definition.—A wind erodibility group (WEG) is a grouping of soils that have similar properties affecting their resistance to soil blowing in cultivated areas. The groups indicate the susceptibility to blowing. The wind erodibility index (I), used in the wind erosion equation, is assigned using the wind erodibility groups.
    
2. Significance.—There is a close correlation between soil blowing and the size and durability of surface clodiness, fragments, organic matter, and the calcareous reaction. The soil properties that are most important with respect to soil blowing are listed below. Soil moisture and the presence of frozen soil also influence soil blowing.
    
   1. Soil texture class
   2. Organic matter content
   3. Carbonates in the fine-earth fraction as determined by effervescence class
   4. Rock and pararock fragment content
   5. Mineralogy
       
3. Estimates.— Soils are placed into wind erodibility groups on the basis of the properties of the soil surface layer. Subpart B, Exhibits, Section 618.95 lists the wind erodibility index assigned to the wind erodibility groups. The wind erodibility index values are assigned because the dry soil aggregates are very use-dependent on crop management factors.
    
4. Entries.—Enter the wind erodibility group and wind erodibility index values for surface layers only. The range of valid entries for wind erodibility group data is 1, 2, 3, 4, 4L, 5, 6, 7, and 8. The lowest valid entry for wind erodibility index data is 0, and the highest is 310. The index values should correspond exactly to their wind erodibility group.

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