Soil Survey Manual - Chapter Two (Part 1 of 3)

Soil Systematics

Table of Contents

Page 1
Introduction
Pedon and Polypedon
Soil Series
    New Series, Variants, and Taxadjuncts
    Phases
    Miscellaneous Areas

Page 2
Map Units
    Designing Map Units
    Kinds of Map Units
    Inclusions within Map Units
    Naming Map Units

Page 3
Miscellaneous Areas
Records and Definitions of Soil Taxa
    Soil Series Definitions
    Other Taxa
Orders of Soil Surveys
    Two Orders of Soil Survey in the Same Project
Soil Maps Made by Other Methods
    Generalized Soil Maps
    Schematic Soil Maps

Introduction

The factors of soil formation are discussed in chapter 1. Climatic and biological factors generally produce broad geographic patterns. These have given rise to a zonal concept and definition of soil distribution. Parent materials contribute to soil variations within climatic and vegetative zones. Local topographic patterns add further complexity, affecting both the time of exposure to processes of soil formation and the kinds of processes. The complex interactions among these factors occur in repetitive patterns which lead to the formation of repetitive combinations. These are the basis for defining, identifying, and mapping soils.

The repetitive patterns imprinted in soils by the soil-forming factors can be observed at scales ranging from continental to microscopic. The patterns are the basis for soil identification and mapping at vastly different scales. A system of terminology, definitions, and operations can be ascribed to the various scales. Hierarchical systems of classes and subclasses can be set up to produce groupings at the different scales.

The National Cooperative Soil Survey of the United States has systems of descriptive terminology, class definitions, hierarchical soil groupings, and operations that are applicable to various scales and appropriate to a wide variety of uses. Development of such flexibility has, in turn, required a fairly complex system in which it is important to understand a number of philosophical, conceptual, and operational relationships. Foremost among these are the relationships between mapping units and taxonomic units, site data and mapping unit data, conceptual models and the real entities in the landscape.

Map units are designed to carry important information for the more common uses of soils in the survey area such as small grain, rural living, and small community development. The map units must also be easily recognizable and mappable at scales compatible with the available base maps, the time allocated to collecting the data, and the skills of the surveyors.

In the United States, soil surveys vary in scale and in intensity of observations. The components of map units are designated by taxa in Soil Taxonomy (Soil Survey Staff, 1975) and miscellaneous areas if they are present. Soil taxa are modified with phases, such as slope and stoniness to convey more specific information. The phase often is a portion of the range of properties exhibited by the taxon. For example, a certain soil series may have slopes from 3 to more than 60 percent, but map units are shown with narrower ranges—such as 3 to 8, 8 to 15, 15 to 25—in order to provide information that is useful in managing the soils in the area.

Kinds of soils are known best by the characteristics embodied in small samples. Field descriptions of soil include horizon designation; depth and thickness; color; moist and dry; features of ped surfaces and interiors; texture; structure; consistence at several water states; and special features such as roots, pores, nodules, salts, rock fragments, pH, and boundary conditions. Site characteristics of the soil and its immediate surrounding are usually described. Chemical and physical data are obtained from horizon samples analyzed in a laboratory. Special features of microbial entities and activities are not routinely determined but may be carried out for research or special purposes.

All samples and all models of soils have a purpose in the soil survey program. Small samples from peds and profiles help us describe the properties of points and how they are organized. This mainly gives us a perspective of the results of soil genesis. We only know what is present by the techniques of measurement that are used, although we may speculate about what has been removed, changed, added, or translocated. Profile features are combined into models of soil formation and the processes and events of geomorphology that have influenced and helped shape the hypothesized features. These mental processes of model building permit us to shift readily from considering an ion in solution to the arrangement of horizons in profiles and their stratigraphic relationships across landscapes.

Purposive sampling of soil map units depends on whether the answers or relationships we desire are related in a meaningful way with the features of the soil map units. The actual clues are not necessarily soil properties at all but are features of identification that we associate with the unseen soil models. Mapping in most surveys involves delineating segments of the landscape, cutting out geographic areas, and putting the boundaries on base maps. Tonal shades and patterns on aerial photographs are used to indicate possible changes of vegetation, drainage conditions, materials, and so forth. The patterns of the gray tones are used to delineate areas on maps. As we look at the existing vegetation, we see differences of tones and composition of the species makeup, and we verify or modify the boundary locations of the units accordingly. Configurations of the visible surface of the land, stones, and other features are used as evidence of changes important enough to be recognized as separate areas. Finally, the soils are examined at a few locations to verify the models being used in the mapping process.

Soil surveys are conducted so that all the clues, features, and pieces of evidence that support the delineations that are called soil map units are in fact surrogates for the models that have been established. The measure of models of landscape evolution and soil formation relative to observable landscape areas is provided by the constant testing that goes on in the soil survey. The outdoors is a laboratory in which variability is subject to some level of systematic portrayal. Thus the small items that are used to assist in locating, verifying, modifying, and developing soil models are similar to the criteria used to identify the basis of differentiation in the classification of soils.

Predictions of properties that exist in soil map units and the predictions made about the qualities and suitabilities and responses of areas of land are all based on the relationships that exist between the desired or expected result and the actuality that is represented by the models used in mapping.

Many schemes have been proposed and tested for determining the composition of map units. The same can be said for the distribution of properties that exist, or are thought to exist, in areas of the landscape that can be delineated consistently on base maps. It is fairly well accepted that certain features of soils and of landscapes are not in accord with existing models of distributions in systematic and predictable ways. The frequency of random events can readily be predicted and tested; however, the location of the occurrences associated with events is, and likely will remain, a probabilistic phenomenon. Such aberrant features are what gives rise to most of the inclusions in map units because their occurrence cannot be predicted and mapped with models even at larger scales. It is the nonsystematic features that make all models approximations of what actually takes place. The composition of map units can never be known. It can only be approximated from samples of them.

It is common to employ transects to estimate the composition of map units. The first aspect of composition is to identify the taxonomic components because they are the things that we have learned to identify and recognize. These can be translated or interpreted as responses or properties or whatever has an acceptable relationship. If results are not satisfactory or favorable, it is because the models are being used beyond their capability. As long as one is aware of the probability of accuracy or known variability, then soil survey information can have relevant use.

Transecting carries with it some important assumptions when it is used as a means of purposive sampling for map unit composition. A major assumption is that the points observed along a line will be representative of areas on the land that are shown as map units. This line varies in utility with different landscapes and models developed for soil surveys.

Transects crossing landscape units must cross all parts of the landscape, rather than line up with known patterns of variability as this would unduly bias the results. For example, transects that follow small steam valleys overweigh the alluvial-colluvial material relative to what occurs on the hillslopes and uplands. The differences between accuracy of line and point transects for grid points or sample plots is not the same for all landscapes and is better determined by some preliminary checking. Point transects have proven to be satisfactory for most surveys in the United States. Observations made at points along a transect are usually identified as belonging to a class... often a particular taxon, but they could be a combination of properties such as depth, a thickness, a color, or a structure. Because the assumption that point-to-area relationships are satisfactory, the number of observations in the various classes are handled as samples, and statistical inferences are made about the mean values and the ranges that are thought to exist.

Pedon and Polypedon

In soil surveys the individual parts that make up the soil continuum are classified. The classes are defined to include bodies of soil of significant kinds and sizes. The classes are concepts, not real soils, but they are related to their representatives in nature—the pedon and the polypedon.

Pedon.—The pedon is presented in Soil Taxonomy (Soil Survey Staff, 1975) as a unit of sampling within a soil. The limits on the area of a pedon establish rules for deciding whether to consider one or two or more kinds of soil within a small-scale pattern of local lateral variability. A pedon is regarded as the smallest body of one kind of soil large enough to represent the nature and arrangement of horizons and variability in the other properties that are preserved in samples.

A pedon extends down to the lower limit of a soil. It extends through all genetic horizons and, if the genetic horizons are thin, into the upper part of the underlying material. The pedon includes the rooting zone of most native perennial plants. For purposes of most soil surveys, a practical lower limit of the pedon is bedrock or a depth of about 2 m, whichever is shallower. A depth of 2 m provides a good sample of major soil horizons, even in thick soil. It includes much of the volume of soil penetrated by plant roots, and it permits reliable observations of soil properties.

The surface of a pedon is roughly polygonal and ranges from 1 m² to 10 m² in area, depending on the nature of the variability in the soil. Where the cycle of variations is less than 2 m and all horizons are continuous and nearly uniform in thickness, the pedon has an area of approximately 1 m². Where horizons or other properties are intermittent or cyclic over an interval of 2 to 7 m, the pedon includes one-half of the cycle (1 to 3 1/2 m). If horizons are cyclic over an interval greater than 7 m, each cycle is considered to contain more than one soil. The range in size, 1 to 10 m², permits consistent classification by different observers where important horizons are cyclic or repeatedly interrupted over short distances.

Polypedon.—The pedon is considered too small to exhibit more extensive features, such as slope and surface stoniness. The polypedon is presented in Soil Taxonomy as a unit of classification, a soil body, homogeneous at the series level, and big enough to exhibit all the soil characteristics considered in the description and classification of soils (fig. 2-1).

Figure 2-1 (Click here or on picture for a larger and more detailed 118 KB image)

A schematic diagram of a polypedon as a unit soil body on the Earth's surface, illustrating (a) its characteristic landscape and (b) its unique set of internal properties. Its margins represent the geographic limits of a set of soil properties defined for a soil series. (courtesy of Walter M. Simonson)

In practice, the concept of polypedon has been largely ignored and many soil scientists consider a pedon or some undefined body of more or less similar soil represented by a pedon large enough to classify. Polypedons seldom, if ever, serve as the real thing we want to classify because of the extreme difficulty of finding the boundary of a polypedon on the ground and because of the self-contradictory and circular nature of the concept. Soil scientists have classified pedons, regardless of their limited size, by deliberately or unconsciously transferring to the pedon any required extensive properties from the surrounding area of soil.

George Holmgren incorporated this pragmatic, flexible view of the pedon in his proposal of the point pedon which combines the fixed position of a pedon with consideration of whatever area is needed to identify and measure the properties under consideration (Holmgren, 1988). This concept, combined with criteria for the scale of lateral variability to be considered within one kind of soil, could establish the pedon as the basic unit of classification and eliminate the need for the polypedon; however, the term "polypedon" will be used in this manual.

Polypedons link the real bodies of soil in nature to the mental concepts of taxonomic classes.

Soil Series

The soil series category is the most homogeneous category in the taxonomy used in the United States. As a class, a series is a group of soils or polypedons that have horizons similar in arrangement and in differentiating characteristics. The soils of a series have a relatively narrow range in sets of properties.

Soil series are differentiated on all the differentia of the higher categories plus those additional and significant characteristics in the series control section. Some of the characteristics commonly used to differentiate series are the kind, thickness, and arrangement of horizons and their structure, color, texture, reaction, consistence, content of carbonates and other salts, content of humus, content of rock fragments, and mineralogical composition. A significant difference in any one of these can be the basis for recognizing a different series. Very rarely, however, do two soil series differ in just one of these characteristics. Most characteristics are related, and generally several change together.

New series, variants, and taxadjuncts

Some soils are outside the limits of any recognized soil series and have unique sets of properties. These are potential new series. When such a soil is first recognized, it is described and identified as a taxon of the lowest category in which it can be classified. A phase of that taxon can be used to identify a map unit. In some surveys, including virtually all detailed surveys, greater refinement of definition is needed. For these, the soil is proposed as a new series, but the new series remains tentative until its properties can be described in detail, its extent determined, and any conflicts with established series resolved. If the soil proves to be unique and significant in extent, it is established as a new series.

Before October 1988, a soil that had characteristics outside the limits of any defined series and was less than 800 hectares (2,000 acres) in extent was designated as a variant. Variants differed enough in one or more properties from the series for which they were named that major interpretations for comparable phases were different. They were named by adding the word "Variant" to the name of a closely related series, preferably one within the survey area. Variants were potential soil series, and the soil was established as a new series if a significant area of a variant was eventually recognized.

Taxadjuncts are polypedons that have properties outside the range of any recognized series and are outside higher category class limits by one or more differentiating characteristics of the series. The differences in properties are small so that major interpretations are not affected. A taxadjunct is given the name of an established series that is most similar in characteristics. It is an adjunct to, but not part of, the named series. It is treated as if it were a member of the named series, and its interpretations are similar to those for comparable phases of the series for which it is named. The difference from the established series is described. Example: A potential series is in a fine-silty family particle size class, marginal to fine-loamy; however, it differs from an established fine-loamy series in only particle size and no appropriate fine-silty series has been identified. The potential series is given the name of the established series, and a new series is not proposed.

Phases

If a property of a taxon has too wide a range for the interpretations needed or if some feature outside the soil itself is significant for use and management, these are bases for defining phases. Phases commonly include only part of the range of features exhibited by a taxon, but phases can be based on attributes such as frost hazard, character of the deeper substratum, or physiographic position that are not characteristics used to identify taxa but, nevertheless, affect use and management. If these vary from place to place within the survey area, phases can be defined to accommodate the differences.

A soil map unit that bears the name of a phase of a taxon consists dominantly of that phase of the taxon, but it also includes other soil components. The other components are included because of the limitations imposed by the scale of mapping and the number of points that can be examined. When the limits of soil taxa are superimposed on the pattern of soil in nature, areas of taxonomic classes rarely, if ever, coincide precisely with mappable areas. Some polypedons are too small to be drawn on the map and are included in delineations and named for another soil. The boundaries between polypedons are not always so obvious that they can be plotted precisely on a map, so part of one polypedon is commonly included in the delineation of an adjacent polypedon. Some polypedons are so intimately intermingled that mappable areas are necessarily identified in terms of two or more taxa. Other polypedons are not easily distinguished from similar adjacent ones and are inadvertently or deliberately included in delineations named for other soils because apparent differences in use and management are small.

Classes of soil properties are not necessarily used directly as phases. Defined class limits of properties are designed for a convenient description of soil, and they can also be used to define phases of soil where appropriate. But they are not useful for all soils. Distinctions significant for one kind of soil are not significant for every other kind. Any single property is significant only through its interactions with other properties. The usefulness of each phase must be repeatedly tested and verified during a survey. Separate phases of a taxon must differ significantly in behavior. If no useful purpose is served by separating them in mapping, similar phases of different taxa may be combined, and the combination described. The interpretations prepared during the course of a survey provide evidence of similarities and differences among map units.

The justification for most phases rests on the behavior of the soils under use. At least one statement about soil behavior must be unique to each phase of a taxon, and the differences of soil properties must exceed normal errors of observation.

Miscellaneous Areas

Some land areas have little or no soil and thus support little or no vegetation without major reclamation. Rock outcrop is an example. Such areas are called miscellaneous areas. The names of the different kinds of miscellaneous areas (discussed later) are used in the same manner as the names of soil taxa to identify map units.