Soil Test Interpretations
Guide A-122
Esteban Herrera, Extension HorticulturistCollege of Agriculture, Consumer and Environmental Sciences New Mexico State University
This publication is scheduled to be updated and reissued 5/05.A soil test can be an important management tool in developing an efficient soil fertility program, as well as monitoring a field for potential soil and water management problems. A soil test provides basic information on the nutrient supplying capacity of the soil. However, a test is not reliable if the soil sample is taken incorrectly or is improperly handled after collection. If you need help taking a soil sample properly, see your county Extension agent for publications on the proper soil sampling methods, and for a soil sampling kit.
Because analytical techniques vary among laboratories, the number values reported may vary from lab to lab. Numbers used by each have specific meanings for the laboratory. The interpretations discussed here are for the Soil, Plant and Water Testing Lab at New Mexico State University.
Fertilizer and soil management recommendations shown on the soil test report are based on the soil test and information provided on the information sheet which accompanies the soil sample to the lab. Items on the information sheet include cropping history, previous yields, fertilizer used, depth of soil and water table, water quality, and irrigation practices. Additional comments made on the information sheet can include general appearance of the crop, yield practices, or problems that may have a bearing on the crop. Fertilization requirements can vary with overall crop management program. Complete and accurate information is essential to get a fertilizer recommendation that will ensure the maximum yield for the least cost.
Individual Soil Tests
The following classifications are used for the standard soil test conducted by NMSU Soil, Plant and Water Testing Lab. Analyses for other factors are available upon request and require additional fees. Except for pH, the classifications are categorized as very low, low, moderate, high, and very high. For fertility factors (N, P, K, micronutrients) very low and low classifications indicate a high probability for obtaining a fertilizer response; moderate classifications indicate a fertilizer response may or may not occur; high and very high classifications indicate a fertilizer response is not likely to occur.
pH. Most crops will grow satisfactorily on soils with a pH ranging from 6.2 to 8.3. Crops susceptible to iron and zinc deficiencies may be affected at pH levels above 7.5.
Soils with a pH of 8.3 or higher usually have a high sodium content. Applications of sulfuric acid usually lower the pH for only a short period due to the high buffering capacity of the soils.
pH | Classification |
> 8.5 | strongly alkaline |
7.9-8.5 | moderately alkaline |
7.3-7.9 | slightly alkaline |
6.7-7.3 | neutral |
6.2-6.7 | slightly acid |
5.6-6.2 | moderately acid |
3.0-5.6 | strongly acid |
Salts, Electrical Conductivity (E.C. x 103). When the electrical conductivity is less than 2, few salinity problems are evident. Problems may become evident in highly sensitive crops when the E.C. x 103 is from 2 to 4, although problems are usually minor. When the E.C. x 103 is from 4 to 8, problems usually are evident. When the E.C. x 103 is greater than 8, crops with moderate salt tolerance will usually show signs of reduced growth, foliage burn or chlorosis. Leaching can decrease the salinity hazard if soil permeability is adequate. Tables 1 and 2 list the salt tolerances of some crops and ornamental plants.
E.C. x 103 | Classification |
< 2 | very low |
2-4 | low |
4-8 | moderate |
8-16 | high |
> 16 | very high |
Table 1. Relative salt tolerance of selected crops, in order of decreasing tolerance within each group. | ||||
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Good salt tolerance | Moderate salt tolerance | Poor salt tolerance | ||
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- - - - - - - - - - - - - - - - - - - - - - - Field Crops - - - - - - - - - - - - - - - - - - - - - - - - - | ||||
barley (grain)
sugar beet rape cotton |
rye (grain)
wheat (grain) oats (grain) alfalfa sorghum (grain) corn (grain) foxtail millet sunflower |
vetch
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- - - - - - - - - - - - - - - - - - - - - - - Forage Crops - - - - - - - - - - - - - - - - - - - - - - - - | ||||
alkali sacaton
saltgrass bermudagrass Canada wild rye western wheatgrass |
white sweetclover
yellow sweetclover perennial ryegrass mountain bromegrass barley (hay) birdsfoot trefoil strawberry clover dallisgrass sudangrass hubam clover alfalfa tall fescue rye (hay) wheat (hay) oats (hay) |
white Dutch clover
meadow foxtail alsike clover red clover ladino clover |
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- - - - - - - - - - - - - - - - - - - - - - - Truck Crops - - - - - - - - - - - - - - - - - - - - - - - - | ||||
garden beet
kale asparagus |
tomato
broccoli cabbage cauliflower lettuce potatoes (White Rose) sweetcorn carrot peas onion squash canteloupe cucumber |
radish
spinach celery green beans |
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- - - - - - - - - - - - - - - - - - - - - - -Fruit and Nut Crops - - - - - - - - - - - - - - - - - - - - | ||||
pistachio
palm |
grape
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pear
apple prune plum apricot peach strawberry pecan |
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Table 2. Tolerance of selected ornamental plants to soil salinity. | ||
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Tolerance and range at
which plants are affected |
Ornamental plant |
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Extremely sensitive
E.C. x 103 = < 2 |
Southern yew
Glossy abelia Photinia Rose Chinese holly Star jasmine Pyrenees cotoneaster |
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Sensitive
E.C. x 103 = 2-3 or 4 |
Laurustinus
Chinese hibiscus Heavenly bamboo Japanese pittosporoum Algerian ivy |
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Moderatley tolerant
E.C. x 103 = 4-5 or 6 |
Spreading juniper
Pyracantha Thorny elaeagnus Oriental arborvitae Indian hawthorn Japanese black pine Japanese boxwood Yellow sage Glossy privet |
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Tolerant
E.C. x 103 = 6-8 |
Aleppo pine
European fan palm Rosemary Spindle tree Blue dracaena Oleander |
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Most tolerant
E.C. x 103 = 8-10 |
Croceum iceplant
Purple iceplant Rosea iceplant White iceplant Ceniza Bougainvillea Natal plum |
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Exchangeable Sodium. Sodium problems arise when the exchangeable sodium is 20% or more. High sodium soils (sodic soils) can be reclaimed if the sodium can be replaced by another element, usually calcium. Applications of gypsum, elemental sulfur, or sulfuric acid have successfully reclaimed calcareous soils which are high in sodium, providing good permeability is present. Notations are made on the soil test report if either a sodium or salinity hazard exists. Table 3 lists the exchangeable sodium tolerances of some crops.
Sodium % | Classification |
< 10 | low |
10-20 | moderate |
20-30 | high |
> 30 | very high |
Table 3. Tolerance of various crops to exchangeable-sodium-percentage. | ||
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Tolerance to ESP1 and range at which crop is affected | Growth response under field conditions | |
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Extremely sensitive
(ESP = 2-10) Deciduous fruits Nuts Citrus Avocado |
Sodium toxicity symptoms
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Sensitive (ESP = 10-20)
Beans |
Stunted growth at low ESP values even though the physical condition of the soil may be good | |
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Moderately tolerant
(ESP = 20-40) Clover Oats Tall fescue Rice Dallisgrass |
Stunted growth due to both nutritional factors and adverse soil
conditions
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Tolerant (ESP = 40-60)
Wheat Cotton Alfalfa Barley Tomatoes Beets |
Stunted growth usually due to adverse physical condition of soil
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Most tolerant
(ESP = more than 60) Crested wheatgrass Fairway wheatgrass Tall wheatgrass Rhodesgrass |
Stunted growth usually due to adverse physical condition
of soil |
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1ESP = exchangeable-sodium-percentage. |
Organic Matter. Percentage of organic matter can be used toestimate nitrogen in the soil. This method alone is not always a dependable measure of available nitrogen, but is used with nitrate nitrogen to make nitrogen fertilizer recommendations on many crops.
Sand % | Clay % | Classification |
< .5 | < 1.0 | very low |
.5-1.0 | 1.0-2.0 | low |
1.0-1.5 | 2.0-3.0 | moderate |
> 1.5 | > 3.0 | high |
Texture. Coarse-textured soils lack both nutrient and water holding capacities. Fine-textured soils often have structural and infiltration problems.
Material | Texture |
Sand, loamy sand | coarse |
Sandy loam, fine sandy loam | moderately coarse |
Very fine sandy loam, loam, silt loam, silt | medium |
Sandy clay, silty clay, clay | fine |
Nitrate Nitrogen. Nitrate nitrogen is the measure of readily available nitrogen in the soil and is used with percentage of organic matter to make a nitrogen fertilizer recommendation. Because nitrate-N is highly soluble, it is subject to leaching in all soils, especially in coarse to medium textured soils. A fertilizer recommendation for nitrogen is more accurate if the subsoil is sampled 18 to 36 inches deep and tested for nitrate-N. Split applications of nitrogen fertilizer help reduce the potential for leaching. This practice is particularly important for sandy soils.
Parts per million | Classification |
< 10 | low |
10-30 | moderate |
> 30 | high |
Bicarbonate Phosphorus. Soils in New Mexico are usually low in available phosphorus because phosphorus is quickly tied up in calcareous soils. Bicarbonate phosphorus, also known as NaHCO3-P or Olsen-P, measures water soluble P, highly soluble calcium P, and organic P.
Parts per million | Classification |
< 7 | very low |
8-14 | low |
15-22 | moderate |
23-30 | high |
>31 | very high |
Soluble Potassium. Adequate potassium is usually available in the strongly weathered soils of New Mexico which have not been leached by high rainfall. Potassium does not readily tie up in calcareous soils and may be found at elevated levels in some saline soils. Potassium fertilizer responses may sometimes be observed on sandy soils with low cation-exchange capacities.
Parts per million | Classification |
< 30 | low |
300-60 | moderate |
< 60 | high |
DTPA Extractable Iron. Iron deficiency is often a problem with sensitive crops grown in soils with pH values over 7.5. Although the critical level of iron in soils is 4.5 ppm, iron-sensitive crops often can be grown satisfactorily down to levels of 2.5 ppm if rooting is not restricted by caliche or gypsum, and care is taken not to over-irrigate. Some crop varieties are more susceptible to iron deficiency than other varieties.
Parts per million | Classification |
< 2.5 | low |
2.5-4.5 | moderate |
> 4.5 | high |
DPTA Extractable Zinc. Zinc deficiency is an important problem in some crops, especially corn and grain sorghum. It is especially a problem in soils with pH values over 7.5 or soils that have a long history of heavy P fertilization. Some crop varieties may be more sensitive to zinc deficiency than other varieties.
Parts per million | Classification |
< 0.5 | low |
0.5-1.0 | moderate |
> 1.0 | high |
DPTA Extractable Copper. Copper deficiencies have not been verified in New Mexico. Factors contributing to copper deficiencies include high organic matter, sandy texture, and high pH.
Parts per million | Classification |
< 0.3 | low |
0.3-1.0 | moderate |
> 1.0 | high |
DTPA Extractable Manganese. Manganese deficiencies have not been verified in New Mexico. They usually occur under conditions similar to those in which iron and zinc deficiencies occur. Manganese levels in the soil can also vary with the soil moisture content.
Parts per million | Classification |
< 1.0 | low |
1.0-2.5 | moderate |
> 2.5 | high |
Conversion Factors
Soil test results can be converted from parts per million (ppm) to pounds per acre by multiplying ppm by a conversion factor based on the depth to which the soil was sampled. Because a slice of soil 1 acre in area and 3 inches deep weighs approximately 1 million pounds, the following conversion factors can be used:
Soil sample depth inches | Multiply ppm by |
3 | 1 |
6 | 2 |
7 | 2.33 |
8 | 2.66 |
9 | 3 |
10 | 3.33 |
12 | 4 |
Fertility Considerations
A good soil sample and an accurate soil test interpretation are not the only considerations for good yields and maximum profit in crop production. Although the appropriate amounts of fertilizer based on a soil test are recommended and applied, other factors override the effects of fertilizer by limiting the yield potential of a crop. These factors include 1) the soil type in the field, 2) proper insect and disease control, 3) irrigation water quality, and 4) irrigation water management. Of these factors, the soil type and irrigation water quality are difficult for the grower to control. However, insect and disease control and water management are under the direct control of the grower and his management skills. Favorable fertilizer response is usually related to how well a crop is managed.
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Reprinted May 2000Electronic Distribution June 2000