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Soil Survey Manual - Chapter Two (Part 2 of 3)Soil SystematicsTable of Contents
Map UnitsA map unit is a collection of areas defined and named the same in terms of their soil components or miscellaneous areas or both. Each map unit differs in some respect from all others in a survey area and is uniquely identified on a soil map. Each individual area on the map is a delineation. Map units consist of one or more components. An individual component of a map unit represents the collection of polypedons or parts of polypedons that are members of the taxon or a kind of miscellaneous area. Parts of polypedons are common when phases are used to divide a taxon. Classes of miscellaneous areas are treated the same as soil taxa in soil surveys. A taxonomic unit description describes the ranges in soil properties exhibited in the polypedon for the maps in a survey area that are referenced by that taxonomic unit. The limits of these ranges are set for the taxonomic class of which a taxonomic unit is a member, but generally the full range allowed by the taxonomic class is not exhibited in a small survey area (<200,000 ha). A delineation of a map unit generally contains the dominant components in the map unit name, but it may not always contain a representative of each kind of inclusion. A dominant component is represented in a delineation by a part of a polypedon, a complete polypedon, or several polypedons. A part of a polypedon is represented when the phase criteria, such as a slope, requires that a polypedon be divided. A complete polypedon is present when there are no phase criteria that require the subdivision of the polypedon or the features exhibited by the individual polypedon do not cross the limits of the phase. Several polypedons of a component may be represented if the map unit consists of two or more dominant components and the pattern is such that at least one component is not continuous but occurs as an isolated body or polypedon. Similarly, each inclusion in a delineation is represented by a part of a polypedon, a complete polypedon, or several polypedons. Their extent, however, is small relative to the extent of the dominant component(s). Soil boundaries can seldom be shown with complete accuracy on soil maps, hence parts and pieces of adjacent polypedons are inadvertently included or excluded from delineations. A few delineations of some map units may not contain any of the dominant components named in the map unit description, but contain very similar soils. In most survey areas there are a few soils that occur as mappable bodies, but they have very limited total extent. They are normally included with other map units, if, for all practical purposes, interpretations are the same. The kinds of map units used in a survey depend primarily on the purposes of the survey and the pattern of the soils and miscellaneous areas in the landscape. The pattern in nature is fixed and it is not exactly the same in each delineation of a given map unit. In soil surveys these patterns must be recognized and map units designed to meet the major objectives of the survey. It must be remembered that soil interpretations are made for areas of land and the most useful map units are those that group similarities. Designing Map UnitsWhile studying the soil patterns in different landscapes, the soil scientist must keep in mind how best to relate the patterns observed to appropriate map units. The kinds of map units, the level of soil taxa, and the phases needed to satisfy the survey objectives must be determined. This requires many judgments. Every map unit that is tentatively identified is evaluated by two tests: (1) Can it be mapped consistently? (2) Is it needed to meet the objectives of the survey? Designing map units to indicate significant differences in behavior among soils is particularly important to meet the current objectives of a survey. Reflecting differences in genesis and morphology is also important, even if no immediate differences in interpretations are known. Differences in soil properties that do not affect current interpretations may be important in the future; however, having too many delineations seriously reduces the immediate usefulness of a soil map. A potential benefit must be weighed carefully against the costs incurred in making additional separations. One objective of every soil survey is to record knowledge about soils, but this does not mean that the soil map must show the location of every kind of soil in a survey area or that the publication must record all that has been learned about the soils. Taxonomic classes provide the basic sets of soil properties with which soil map units are defined. They summarize an immense amount of research and experience related to the significance of soil properties and combinations of properties. They provide predefined sets of soil properties that have been tested for genetic relationships and for interpretative value. Taxa provide a firm base for recognizing the components of potential map units in an unfamiliar area. Using established taxa is much easier than independently sorting out sets of properties and determining significant class limits. The objectives of a survey determine the kind of map units and the taxonomic level used to identify components of map units. For the more detailed surveys, decisions must be made about what criteria to use to recognize phases of soil series, how broadly or narrowly to define the phases, and whether similar phases of different series have such similar interpretations that they can be combined. For the less detailed surveys, decisions must be made about how the complexities of soil in large areas can be best identified for purposes of the survey, what combinations of soils characterize useful and mappable units, what taxonomic level should be used in naming map units, and which phases contribute to the usefulness of the map units. The names of soil taxa, along with one or more modifying terms are used to identify the soils in map units. For example, the name "Tama silt loam, 2 to 5 percent slopes," indicates that soils of the Tama series (a Udoll) are dominant in that map unit. The names of taxa of higher categories are also used in map unit names, especially on small scale maps. "Udolls, rolling," for example, identifies a map unit consisting dominantly of soils of the Udoll suborder, which includes Tama and other series. The name of a taxon of the lowest category that accurately identifies the dominant soil is commonly used. Within each survey, soil maps can be designed with component taxa of low or high categories that reflect narrowly or broadly defined ranges of soil properties. In addition, soil map units can be designed with different compositions of soil taxonomic units and mapping inclusions. This flexibility permits the design of map units that will be most useful for the purposes of a specific survey as well as for the attainment of as much uniformity in mapping as possible. As methods of measuring soil properties are refined, as experience in the field increases, and as use and management requirements are intensified, progressively narrower ranges in soil properties can be recognized or established. Narrow ranges of properties are not established just because methods permit it. Unnecessary separations are time consuming to delineate consistently, and they make the survey difficult to use. Not separating two significantly different, mappable units, however, makes a survey less useful. The significance of each map unit in meeting the objectives of the survey must be constantly evaluated during the mapping process. Improper use of phases to designate map units and misinterpretations of soil survey procedures can result in unreasonably detailed soil maps delineating unnecessary map units or ones in less detail than is needed to accomplish the objectives of the survey. Using pre-established classes of selected soil properties—surface texture, depth, slope, accelerated erosion, and stoniness—as phase criteria and then using all combinations of these in defining phases, creates problems. Meaningless map units cause an unnecessary expense. Phase distinctions must be compatible with natural variability. To illustrate, a series may range in depth to bedrock from 1.5 m to more than 2 m. For some uses, a separation at 1.5 m would be significant. If within a survey area the soils of a series range in depth to bedrock from slightly less than 1.75 m to slightly more, designating two depth phases cannot be justified. The mapping is likely to be inconsistent and the difference of a few centimeters in depth is likely to be of minor significance. In this case it is far better to designate only one phase on the basis of depth to bedrock. The description of the map unit should, of course, give the depth range. Another example: In some areas a slope of 8 percent is about the upper limit for cropping many soils without special practices for erosion control; yet, in some series a large part of the soil has slopes of less than 3 percent, most of the rest has slopes of 6 to 10 percent, and a small acreage has slopes of 3 to 6 percent. Dividing phases of such soils at 8 percent slope would produce a large number of delineations having a gradient a little below 8 percent and a large number having a gradient a little above 8 percent. The differences in interpretations of the phases thus defined would probably not be consistently significant. For these soils, slope phases could be set at 0 to 3 percent, 3 to 6 percent, and 6 to 10 percent; or, if there is little or no significance of the break at 3 percent, they could be set at 0 to 6 percent and 6 to 10 percent. Phases must also be compatible with practical needs. In a hypothetical survey area that is relevant to farming, the polypedons of Alpha soils are similar in all properties except stoniness and slope. The areas range from nearly stone-free to very stony and from undulating to steep. The most important single distinction for farming is the distinction between areas that can be cultivated feasibly and areas that cannot. As many as three mappable classes of stoniness could be combined with four mappable classes of slope—a total of 12 potential phases. Four of these twelve phases might be used to distinguish combinations of degrees of slope and degrees of stoniness within the limits that permit cultivation. Using the remaining eight to subdivide the non-arable areas would confuse the user with unnecessary detail more than it would help. Perhaps two, and probably no more than three, phases are adequate for all significant distinctions among non-arable areas if the survey area is to be used primarily for farming. A list of potentially useful phases is as follows:
If the soil map is to be used for planning operations related to forestry or other non-farming activities, other distinctions may be needed. The object of surveying soils is to gather relevant facts, record the facts on maps, and then interpret the facts. Field records and observations together with other relevant information must be coordinated in defining phases and map units that meet the objectives of a soil survey. Kinds of Map UnitsSoils differ in size and shape of their areas, in degree of contrast with adjacent soils, and in geographic relationships. Four kinds of map units are used in soil surveys to show the relationships: consociations, complexes, associations, and undifferentiated groups. Consociations—In a consociation, delineated areas are dominated by a single soil taxon (or miscellaneous area) and similar soils. As a rule, at least one-half of the pedons in each delineation of a soil consociation are of the same soil components that provide the name for the map unit1. Most of the remainder of the delineation consists of soil components so similar to the named soil that major interpretations are not affected significantly. The total amount of dissimilar inclusions of other components in a map unit generally does not exceed about 15 percent if limiting and 25 percent if non-limiting. A single component of dissimilar limiting inclusion generally does not exceed 10 percent if very contrasting. The amount of dissimilar inclusions in an individual delineation of a map unit can be greater than this if no useful purpose would be served by defining a new map unit. The soil in a consociation may be identified at any taxonomic level. A consociation named for a kind of miscellaneous area is dominated by the kind of area for which it is named to the extent that inclusions do not significantly affect the use of the map unit. Generally, this means that less than about 15 percent of any delineation is soil or less than about 25 percent is other kinds of miscellaneous areas. Percentages may vary, depending on the kind of miscellaneous area and the kind, size, and pattern of the inclusions. Complexes and associations—Complexes and associations consist of two or more dissimilar components occurring in a regularly repeating pattern. Only the following arbitrary rule related to mapping scale determines whether the name complex or association should be used. The major components of a complex cannot be mapped separately at a scale of about 1:24,000 (fig. 2-2). The major components of an association can be separated at a scale of about 1:24,000 (fig. 2-3). In either case, the major components are sufficiently different in morphology or behavior that the map unit cannot be called a consociation. In each delineation of either a complex or an association, each major component is normally present, though their proportions may vary appreciably from one delineation to another. The total amount of inclusions in a map unit that are dissimilar to any of the major components does not exceed about 15 percent if limiting and 25 percent if nonlimiting, and a single kind of dissimilar limiting inclusion generally does not exceed 10 percent if very contrasting.
Undifferentiated groups—Undifferentiated groups consist of two or more taxa components that are not consistently associated geographically and, therefore, do not always occur together in the same map delineation. These taxa are included as the same named map unit because use and management are the same or very similar for common uses. Generally, they are included together because some common feature such as steepness, stoniness, or flooding determines use and management. If two or more very steep soils geographically separated are so similar in their potentials for use and management that defining two or more additional map units would serve no useful purpose, they may be placed in the same unit. Every delineation has at least one of the major components and some may have all of them. The same principles regarding proportion of inclusions apply to undifferentiated groups as to consociations. Inclusions Within Map UnitsIn all soil surveys, virtually every delineation of a map unit includes areas of soil components or miscellaneous areas that are not identified in the name of the map unit. Many areas of these components are too small to be delineated separately. The location of some components cannot be identified by practical field methods. Some mapping inclusions are deliberately placed in delineations identified as another map unit to avoid excessive detail of the map or the legend. Inclusions reduce the homogeneity of map units and may affect interpretations. The objective is to define map units that will contain as few inclusions as practical of components that behave differently from the naming components. Also, map units must be so defined that they can be recognized and delineated consistently in the field. The number of inclusions reflects the taxonomic purity of map units. The number and degree of contrast of inclusions with the reference taxa can be used to estimate the interpretative purity of map units. The actual amount of inclusions is estimated from observations made during the survey. Adjustments in mapping are made if appropriate. In the definition of map units, judgment must be exercised about the effects of inclusions on management and about how much effort is justified to keep the amount of inclusions small. In exercising these judgments, visualizing two kinds of differences between components is useful. If differences are small, the components are compared as similar. If differences are large, the components are contrasted as dissimilar. Similar components are alike or much alike in most properties and share limits of those diagnostic properties in which they differ. Differences are beyond the limits of the reference taxon or phase class, but they generally are within or slightly beyond normal errors of observation. Because only a few limits are shared or the range is small, interpretations for most common uses are alike or reasonably similar and the interpretative value of a map unit is not affected. Dissimilar components on the other hand, differ appreciably in one or more properties, and the differences generally are great enough to affect major interpretations. Some dissimilar components are limiting, and others are nonlimiting relative to the interpretations being considered. If an inclusion does not restrict the use of entire areas or impose limitations on the feasibility of management practices, its impact on predictions for the map unit is small. Inclusions of soil components that have less severe restrictions on use than the dominant soil of a map unit do not adversely affect predictions about the unit as a whole. They may even be beneficial. Such inclusions are nonlimiting and the interpretative purity of a map unit for most interpretations is not altered. For example, the inclusion of many small areas having slopes of 4 to 8 percent in an area having slopes mainly of 15 to 25 percent has no adverse effect on use of the area for most purposes; however, if an inclusion has significantly lower potential for use than the dominant component in the map unit or affects the feasibility of meeting management needs, a small amount in a map can affect predictions greatly. These are the most critical inclusions because they decrease the interpretative purity of map units. Even a small area having slopes of 15 to 25 percent in a map unit dominated by slopes of 4 to 8 percent can seriously affect the use of the area for many purposes. Even small inclusions of Typic Epiaqualfs in areas of Aquic Hapludalfs, for an example, may control and limit the uses of the dominant soil. Soils that cannot be used feasibly for the same purposes as the surrounding soils are especially critical. They are separately delineated if the map scale permits it and if showing them will improve the usefulness of the map for the major anticipated uses. Areas too small to delineate may be identified and located on the map by special symbols. Naming Map UnitsAll map units in a soil survey are named. Different conventions are used for each of the four kinds of map units so that the kind of unit can be determined at a glance. In general, names are as short as is practical; the name of a map unit should be only as long as is necessary to distinguish it from all others in the survey. At times an extra term, not needed to distinguish a phase from all others in the survey, is used so that comparable phases in other areas have the same name. Although some of the conventions for naming map units are discussed in this chapter, a more complete discussion is provided in the National Soils Handbook (Soil Conservation Service). Soil names indicate the categorical level used for identification, but soil taxonomic names are never used alone. Phases are groupings created to serve specific purposes in individual soil surveys. Phases can be defined for any class or classes of any category. The classes are helpful in describing the soil phases that are important for the survey. Differences in soil or environmental features that are significant to use, management, or behavior are the bases for designating soil phases. Any property or combination of properties that does not duplicate class limits for a taxon can be used to differentiate phases, and any value of a property can be set to divide phases. The choice of properties and limits are determined by the purpose of the survey and by how consistently the phase criteria can be applied. Because objectives differ from one soil survey to another, limits and ranges of a property or attribute may also differ from one survey to another. In general, phase criteria are given a smaller range where soil use is intensive (as for irrigated farming or urban development) and a larger range where use is extensive (as for forestry or grazing). The attributes most commonly used in defining phases in soil surveys are: Texture of the surface layer.—As the surface layer has special significance for use of the soil, texture of the surface layer is commonly indicated in the names of consociations bearing the names of soil series. These phases generally identify the dominant texture of a mineral layer about equal in thickness to that commonly mixed in tillage, which is generally 12 to 25 cm. If the layer has not been mixed, the texture that would be produced by mixing is estimated. If, after mixing, the layer is organic, terms for organic material are used to name the phase. In some areas such as deserts, where the surface layer is normally thin and cultivation is unlikely, the texture of the A horizon can be used in naming phases even if it is less than 12 cm thick. Some mineral soils have a thin layer at the surface that contrasts sharply with the next layer. Such soils may be designated as separate phases if they are unlikely to be tilled and if clearly significant to use or management. On rangeland, for example, the texture of the uppermost few centimeters of soil is important. A thin layer of sandy material at the surface may mean the difference between success and failure of seeding on some soils. For some surveys, such layers need to be recognized and their areas delineated. Moreover, a thin cover of silt or clay is an important distinction. Conceivably, other kinds of phases based on texture could be useful in mapping, but they should be used only if the indicated property has a major and lasting effect on interpretations. Deposits on the surface.—Some soils have received deposits of material thick enough to influence interpretations of the soil but not thick enough to change the classification. Depositional phases of the buried soils may be recognized:
Texture terms in the names of map units describe the material currently at the surface. Phases may be recognized for soils covered by a thin layer of volcanic ash. Such phases are generally used only if needed to distinguish the phase from another phase that lacks the ash cover. Rock fragments.—Rock fragments at the surface and in the surface layer are commonly used as phase distinctions. Kinds of rock fragments are defined by shape and size in chapter 3. Phases of the smaller rock fragments accommodate most of the detailed phase distinctions that generally can be made accurately by field methods. The term "gravelly" is used in the examples of the names that follow, but the adjectives for each of the other kinds of rock fragments, such as cobbly or channery, are substituted as appropriate. The effect of 20 percent fine gravel on the use of arable soil, for example, is quite different from the effect of 20 percent flagstones; therefore, the phase limits may differ for larger fragments. The following definitions are applicable to arable soils. For other uses—such as forestry, range, or recreation—the sizes, shapes, amounts, and mixtures of rock fragments have different significance. Pebbles, cobbles, and stones influence forestry much less than they do cultivation, although they could affect access and reforestation.
The rock fragments in the surface layer commonly span two or more size classes. The included fragments may be of more than one shape. The name of the kind of fragment that is judged most important in limiting the management of the soil is used in the phase designation. Generally, the largest fragments that are present in significant amounts are the most restrictive on soil use. Classes of stoniness and boulderiness (chapter 3) are also used to define phases. The following phases of the larger rock fragments represent about the maximum detail that can be mapped consistently in most soil surveys. "Bouldery" is substituted for "stony" as appropriate.
Slope gradient, complexity, shape, length, and aspect are all potential bases for phase distinctions. By far the most commonly used is gradient. Complexity is also used in many surveys. Slope length can often be appraised directly from delineations on the map. In many cases, the significance to use and management of slope length depends on the kind of landscape in which the soil occurs. Shape is seldom used as a phase distinction; differences in shape are commonly related to differences in internal properties that distinguish taxa. Slope aspect is used mainly in high latitudes. Phases defined on the basis of slope should fit the landscape. They should be so distinct that they can be identified and mapped consistently without adding useless complexity to the map. In addition, such phases should separate areas that have significant differences in suitability or management needs. A uniform system of slope classes should not be used indiscriminately as the basis for differentiating phases. Slope phases that have narrow ranges in gradient may be needed for soils that can be used intensively. For other soils having additional limitations, such as stoniness, these narrow ranges may be useless. The limits of slope phases should be based on data or experience in order to indicate the most useful distinctions for each kind of soil. Range in the slope of a phase of one series may encompass the ranges of two or more phases of another series. A single set of slope classes that would serve as phase distinctions for all soils is impractical because of the varied relationships of slope to mappable landscapes and to the use and management of different kinds of soils. Table 3-1 defines slope classes in terms of flexible limits for both simple and complex slopes. The flexible limits permit use of terms to identify most distinctions of slope that may be needed. Names may use either numerical gradient limits, with or without designations of complexity, or descriptive terms. Slope terms for map units for taxa above the series are generally given in descriptive terms. The word "slopes" is used if gradient is specified as a percentage, but it is omitted if descriptive terms are used. Depth.—Soil depth phases are used where variations in depth to a contrasting layer are significant to soil use, management, or behavior. Terms for depth classes in Chapter 3 are generally used in naming phases, but modifications are needed in some areas. For instance, the class "moderately deep," ranging from 50 to 100 cm, may be too broad to satisfy the objective of a particular survey. This range can be divided, with perhaps one class ranging in depth from 50 to 75 cm and the other from 75 to 100 cm, if the more narrowly defined phases occur in a consistent pattern within the survey area and can be mapped. Generally, the phase that covers the least acreage is given the depth designation. If the deeper phase is more extensive, "moderately shallow" is used to designate the shallower phase. If the shallower phase is more extensive, "moderately deep" is used to designate the deeper phase. In some surveys, using the standard class terms may be misleading. For example, if a series that is normally more than 175 cm deep to bedrock has a phase that is 150 to 175 cm deep, calling the less extensive phase "deep" could be construed to mean that the phase is deeper than normal. In such cases, depth limits can be specified in the phase name, or substratum phase terminology can be used instead. Substratum.—Where underlying material contrasts sharply with the material above and interpretations are affected, substratum phases are used. The kind of contrasting material is indicated in the name of the map unit. Some examples of commonly used substratum phases are: gravelly substratum, sandy substratum, silty substratum, shale substratum, and till substratum. These terms are descriptive and not mutually exclusive. Where there is a choice between using a depth phase or a substratum phase to identify a map unit, a depth phase is generally used if the contrasting layer is bedrock. Soil water.—Phases are used to distinguish differences in soil-water state, water table level, drainage, and the like where the range of the taxon in one or more of these properties needs to be divided for purposes of the survey. Significant differences in these factors are commonly reflected in differences in soil morphology and are distinguished at the series level. In some soils, however, evidence of wetness, such as redoximorphic features, does not fully reflect the natural drainage or wetness of the soil. These soils may not be differentiated at the series level with the refinement needed for the purposes of the survey. Examples of soil water phases commonly used are: high water table, poorly drained, slightly wet, and drained. Some soils have properties that reflect former wetness, but they have been drained artificially; "drained" phases can be used to separate drained areas from undrained. In other soils, a water table fluctuates below the depth where properties are criteria for defining series; "water table" phases can be used to identify such soils. Salinity.—Saline phases are used to distinguish between degrees of salinity that are important for soil use or management. Electrical conductivity values and observations of plant growth are guides for recognizing phases. Designation of salinity phases depends on the various uses likely to be made of the soils and the effect of excessive amounts of salt on the uses. In farming areas, the crops most likely to be grown must be considered. Management induced salinity that fluctuates widely with management practices generally would not be a basis for phase distinctions. Vegetation, especially the native cover, often shows the location of saline soils and their boundaries. Using vegetation, landform, and other features as guides and correlating these field observations with laboratory or field analyses of soil samples, the surveyor can usually draw boundaries with reasonable accuracy. Plants, however, vary in their tolerance of salt by species, variety, age, and perhaps other factors. Some plants are not good indicators of salinity because they grow well in soils that may have excessive amounts. Other problems must also be considered in designating salinity phases. Different kinds of salts and combinations of salts have varied effects on soil behavior. In many soils, salts are transitory; in others, they are permanent. Excessive sodium may or may not be associated with excess salinity. The following classes of salinity, which are a general guide to naming phases, refer to the presence of salts in the soil. Salinity classes are defined in chapter 3.
Terms for saline phases follow terms for surface texture in phase names. Sodicity.—For some soils, recognizing a "sodic" phase is useful. The term "sodic" is used as a phase designation, if needed, generally without terms for degrees of sodicity. Physiography.—Landform or physiographic position may be used as a phase criterion to distinguish phases of a single taxon. A soil in a deposit of loess 3 meters thick on a terrace, for example, may be so much like a soil in a similar deposit on a till plain that the two are members of the same taxon. For some uses, however, the two soils need to be distinguished on the map. A physiographic phase can be used to identify the less extensive soil. Examples of terms that have been used to designate physiographic phases are: bench, depressional, fan, karst, ridge, and terrace. The terms generally identify phases that differ in position from what is typical for the soil. The typical physiography is not given in a phase name. The physiographic phase designation follows the term for surface texture and precedes any terms for slope or erosion. Erosion.—Differences in soil potential for use, management needs, or performance because of accelerated erosion are a basis for recognizing phases. Phases of eroded soil are identified on the basis of the properties of the soil that remains, although the amount of soil lost is estimated and noted. In some places, erosion has changed the taxonomic classification of a soil. Properties related to natural erosion are a part of the definition of a taxon, not bases for erosion phases. Erodibility, too, is an inherent quality of a soil and not itself a criterion for erosion phases. Eroded phases are defined so the boundaries on the soil maps separate soil areas of unlike suitabilities and soil areas of unlike management needs and responses. Guidelines for naming phases of soil that are eroded by water are as follows. Erosion classes are defined in chapter 3.
A "gullied" phase can be recognized if gullied land occupies less than about 10 percent of the map unit. Gullied phases are used for areas having gullies so deep that intensive measures, including reshaping, are required to reclaim the soil. Where the areas are more than 10 percent gullied land, the map units are named as complexes or associations of soil and gullied land. Guidelines for designating phases of soil eroded by wind are as follows:
Thickness.—The solum and the various horizons in soil have characteristic ranges in thickness for each taxon. Thickness phases may be used to divide the range of thickness of the solum or of the upper horizons. Phases are not used to differentiate thickness of the subsoil or the substratum. Four thickness phases are used:
A term is used for the less extensive of two thickness phases. For example, most delineations of a given soil may have an A horizon that is dominantly between 25 and 35 cm thick. If the A horizon is dominantly 35 to 40 cm thick in other delineations of the same soil and the difference is significant for purposes of the survey, a thick-surface phase can be recognized. The phase in which the A horizon is dominantly 25 to 35 cm thick is the norm; thickness of the A horizon is described for this phase but is not identified in the name. Climate.—In some places, especially in mountainous or hilly areas, precipitation or air temperature can differ greatly within short distances, yet these differences may not be reflected in internal properties of the soil. Air drainage can differ enough from one location to another to produce a difference in the dates of the last killing frost in the spring or the first in the fall, or one area may be frost free. Climatic phases are used for these situations. Only two climatic conditions are recognized for a given taxon: (1) the common climate, the climate that influences the greatest extent of the taxon, from which the climate designation is omitted, and (2) a departure from the common climate, for which a climatic designation is used. The departure may be in either of two directions from the norm: warm or cool, high precipitation or low precipitation. Each of the terms is connotative only in reference to the common climate of the taxon and must be described specifically for each phase to which it is applied. In many places, especially on plains, precipitation or temperature changes gradually over distance. A soil in a single survey area commonly includes only part of the range for the series. Climatic phases generally are not used if only part of the range is within a soil survey area. Climatic phases are local distinctions. They are used where temperature or precipitation differs markedly between parts of a survey area. Other.—A great variety of phase distinctions can be made. In addition to those already described, others may be needed to provide suitable map units; for example: frequently flooded, occasionally flooded, burned, calcareous, leached surface, dark surface. "Burned," for example, might be used for organic soils that have lost enough of their organic material by fire to alter their potential use or their management requirements. The phases designated by special terms are defined to fit special kinds of soils. Such phases are defined according to the common properties of the taxon of which they are members. Thus, the terms usually have different specific meanings when used for different taxa and in different survey areas. Footnotes
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