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Soil Survey Manual - Chapter Three (Part 7 of 9)

Examination and Description of Soils

Table of Contents

Chapter 3 Full Table of Contents

Soil Structure
    Compound Structure
    Extra-structure Cracks
    Internal Surface Features

Soil Structure

Soil structure refers to units composed of primary particles. The cohesion within these units is greater than the adhesion among units. As a consequence, under stress, the soil mass tends to rupture along predetermined planes or zones. These planes or zones, in turn, form the boundary. Compositional differences of the fabric matrix appear to exert weak or no control over where the bounding surfaces occur. If compositional differences control the bounding surfaces of the body, then the term "concentration" is employed. The term "structural unit" is used for any repetitive soil body that is commonly bounded by planes or zones of weakness that are not an apparent consequence of compositional differences. A structural unit that is the consequence of soil development is called a ped. The surfaces of peds persist through cycles of wetting and drying in place. Commonly, the surface of the ped and its interior differ as to composition or organization, or both, because of soil development. Earthy clods and fragments stand in contrast to peds, for which soil forming processes exert weak or no control on the boundaries. Some clods, adjacent to the surface of the body, exhibit some rearrangement of primary particles to a denser configuration through mechanical means. The same terms and criteria used to describe structured soils should be used to describe the shape, grade, and size of clods. Structure is not inferred by using the terms interchangeably. A size sufficient to affect tilth adversely must be considered. The distinction between clods and fragments rests on the degree of consolidation by mechanical means. Soil fragments include (1) units of undisturbed soil with bounding planes of weakness that are formed on drying without application of external force and which do not appear to have predetermined bounding planes, (2) units of soil disturbed by mechanical means but without significant rearrangement to a denser configuration, and (3) pieces of soil bounded by planes of weakness caused by pressure exerted during examination with size and shape highly dependent on the manner of manipulation.

Some soils lack structure and are referred to as structureless. In structureless layers or horizons, no units are observable in place or after the soil has been gently disturbed, such as by tapping a spade containing a slice of soil against a hard surface or dropping a large fragment on the ground. When structureless soils are ruptured, soil fragments, single grains, or both result. Structureless soil material may be either single grain or massive. Soil material of single grains lacks structure. In addition, it is loose. On rupture, more than 50 percent of the mass consists of discrete mineral particles.

Some soils have simple structure, each unit being an entity without component smaller units. Others have compound structure, in which large units are composed of smaller units separated by persistent planes of weakness.

In soils that have structure, the shape, size, and grade (distinctness) of the units are described. Field terminology for soil structure consists of separate sets of terms designating each of the three properties, which by combination form the names for structure.

Shape.—Several basic shapes of structural units are recognized in soils. Supplemental statements about the variations in shape of individual peds are needed in detailed descriptions of some soils. The following terms describe the basic shapes and related arrangements:

platy: The units are flat and platelike. They are generally oriented horizontally. Platy structure is illustrated in figure 3-26. A special form, lenticular platy structure, is recognized for plates that are thickest in the middle and thin toward the edges.

prismatic: The individual units are bounded by flat to rounded vertical faces. Units are distinctly longer vertically, and the faces are typically casts or molds of adjoining units. Vertices are angular or subrounded; the tops of the prisms are somewhat indistinct and normally flat. Prismatic structure is illustrated in figure 3-27.

columnar: The units are similar to prisms and are bounded by flat or slightly rounded vertical faces. The tops of columns, in contrast to those of prisms, are very distinct and normally rounded, as illustrated in figure 3-28.

blocky: The units are blocklike or polyhedral. They are bounded by flat or slightly rounded surfaces that are casts of the faces of surrounding peds. Typically, blocky structural units are nearly equidimensional but grade to prisms and to plates. The structure is described as angular blocky if the faces intersect at relatively sharp angles; as subangular blocky if the faces are a mixture of rounded and plane faces and the corners are mostly rounded. Figure 3-29 illustrates angular blocky units.

granular: The units are approximately spherical or polyhedral and are bounded by curved or very irregular faces that are not casts of adjoining peds. Granular units are illustrated in figure 3-30.

Figure 3-26 (Click here or on picture for high resolution 198 KB image)

Strong thin platy structured soil.

Strong thin platy structure

Figure 3-27 (Click here or on picture for high resolution 195 KB image)

Strong medium platy structured soil.

Strong medium platy structure. The prisms are 35 to 45 mm across

Figure 3-28 (Click here or on picture for high resolution 137 KB image)

A cluster of strong medium columnar pads.

A cluster of strong medium columnar pads. The cluster is about 135 mm across.

Figure 3-29 (Click here or on picture for high resolution 142 KB image)

Strong medium and coarse blocky peds

Strong medium and coarse blocky peds.

Figure 3-30 (Click here or on picture for high resolution 195 KB image)

Strong fine and medium granular peds

Strong fine and medium granular peds.

Size.—Five classes are employed: very fine, fine, medium, coarse, and very coarse. The size limits of the classes differ according to the shape of the units. The size limit classes are given in table 3-13. The size limits refer to the smallest dimension of plates, prisms, and columns. If the units are more than twice the minimum size of "very coarse," the actual size is given: "prisms 30 to 40 cm across."

Grade.—Grade describes the distinctness of units. Criteria are the ease of separation into discrete units and the proportion of units that hold together when the soil is handled. Three classes are used:

Weak. The units are barely observable in place. When gently disturbed, the soil material parts into a mixture of whole and broken units and much material that exhibits no planes of weakness. Faces that indicate persistence through wet-dry-wet cycles are evident if the soil is handled carefully. Distinguishing structurelessness from weak structure is sometimes difficult. Weakly expressed structural units in virtually all soil materials have surfaces that differ in some way from the interiors.

Moderate. The units are well formed and evident in undisturbed soil. When disturbed, the soil material parts into a mixture of mostly whole units, some broken units, and material that is not in units. Peds part from adjoining peds to reveal nearly entire faces that have properties distinct from those of fractured surfaces.

Strong. The units are distinct in undisturbed soil. They separate cleanly when the soil is disturbed. When removed, the soil material separates mainly into whole units. Peds have distinctive surface properties.

Table 3-13. Size classes of soil structure

Shape of structure
Size Classes Platy1
mm
Prismatic and Columnar
mm
Blocky
mm
Granular
mm
1 < 1 < 10 < 5 < 1
2 1 - 2 10 - 20 5 - 10 1 - 2
3 2 - 5 20 - 50 10 - 20 2 - 5
4 5 - 10 50 - 100 20 - 50 5 - 10
5 > 10 > 100 > 50 > 10

1. In describing plates, "thin" is used instead of "fine" and "thick" instead of "coarse."

The distinctness of individual structural units and the relationship of cohesion within units to adhesion between units determine grade of structure. Cohesion alone is not specified. For example, individual structural units in a sandy loam A horizon may have strong structure, yet they may be less durable than individual units in a silty clay loam B horizon of weak structure. The degree of disturbance required to determine structure grade depends largely on moisture content and percentage and kind of clay. Only slight disturbance may be necessary to separate the units of a moist sandy loam having strong granular structure, while considerable disturbance may be required to separate units of a moist clay loam having strong blocky structure.

The three terms for soil structure are combined in the order (1) grade, (2) size, (3) shape. "Strong fine granular structure" is used to describe a soil that separates almost entirely into discrete units that are loosely packed, roughly spherical, and mostly between 1 and 2 mm in diameter.

The designation of structure by grade, size, and shape can be modified with other appropriate terms when necessary to describe other characteristics. Surface characteristics of units are described separately. Special structural units, such as the wedge-shaped units of Vertisols, are described in appropriate terms.

Compound Structure

Smaller structural units may be held together to form larger units. Grade, size, and shape are given for both and the relationship of one set to the other is indicated: "strong medium blocks within moderate coarse prisms," or "moderate coarse prismatic structure parting to strong medium blocky."

Extra-Structural Cracks

Cracks are macroscopic vertical planar voids with a width much smaller than length and depth. A crack represents the release of strain that is a consequence of drying. In many soils, cracks bound individual structural units. These cracks are repetitive and usually quite narrow. Their presence is part of the concept of the structure. The cracks to be discussed are the result of localized stress release which forms planar voids that are wider than the repetitive planar voids between structural units or which occur in massive or weakly structured material at relatively wide intervals. These cracks may be coextensive with crack space between structural units. If they are coextensive, the width exceeds that of the associated structural cracks. The areal percentage of such cracks, either on a vertical exposure or on the ground surface, may be measured by line-intercept methods. For taxonomic purposes, the width and depth of cracks has importance. Four kinds of extra-structural cracks may be recognized:

Surface-initiated reversible cracks form as a result of drying from the surface downward. They close after relatively slight surficial wetting and have little influence on ponded infiltration rates.

Surface-initiated irreversible cracks form on near-surface water reduction from exceptionally high water content related to freeze-thaw action and other processes. The cracks do not close completely when rewet and extend through the crust formed by frost action. They act to increase ponded infiltration rates.

Subsurface-initiated reversible cracks form as a result of appreciable reduction in water content from "field capacity" in horizons or layers with considerable extensibility. They close in a matter of days if the horizon is brought to the moderately moist or wetter state. They extend upward to the soil surface unless there is a relatively thick overlying horizon that is very weakly compacted (loose or very friable) and does not permit the propagation of cracks (mechanically bulked subzones (fig. 3-13, for example)). Such cracks importantly influence ponded infiltration rates and evaporation directly from the soil.

Subsurface-initiated irreversible cracks are the "permanent" cracks of the USDA soil taxonomy system (see figure 3-31). They have a similar origin to surface-initiated irreversible cracks, although quite different agencies are involved.

The foregoing genetic definition of cracks does not directly relate to prediction of infiltration. For such predictions, the surface connectiveness of the cracks and their depth must be specified. Surface-connected cracks occur at the ground surface or are covered by up to 10-15 cm of soil material that would permit the accumulation of free water at the plane that marks the top of the crack under conditions that may occur in most years. If the antecedent water state of the overlying zone were very moist, free water from 25 mm of rainfall in one hour should reach the top of the cracks. Usually the zone would have very high or high saturated hydraulic conductivity. Such subzones may exhibit structure or be single grain. The structure units range widely in size. The common characteristic is that the consistent units of the mass as a whole are highly discrete and the porosity of the interstices among the structural units is high. If not too thick, the mechanically bulked subzone of tilled surface horizons would be such a zone.

Figure 3-31 (Click here or on picture for high resolution 155 KB image)

Picture of cracked, dry soil.

Natural fragments formed by cracking of a massive soil as it contracted upon drying.

A crack depth index value may be obtained by insertion of a blunt wire, approximately 2 mm in diameter1. Penetrant cracks are 15 cm or more in depth as measured by wire insertion. Cracks that are both penetrant and surface connected are described as penetrant surface connected. Penetrant surface-connected cracks act to increase transient ponded infiltration. Prominence of the penetrant surface-connected cracks would depend on the linear distance of such cracks per unit area of ground surface. The linear distance may be allowed to decrease as crack depths increase. No classes are provided.

Internal Surface Features

Surface features include (1) coats of a variety of substances unlike the adjacent soil material and covering part or all of surfaces, (2) material concentrated on surfaces by the removal of other material, and (3) stress formations in which thin layers at the surfaces have undergone reorientation or packing by stress or shear. All differ from the adjacent material in composition, orientation, or packing.

Descriptions of surface features may include kind, location, amount, continuity, distinctness, and thickness of the features. In addition, color, texture, and other characteristics that apply may be described, especially if they contrast with the characteristics of the adjacent material.

Kinds.—Surface features are distinguished by differences in texture, color, packing, orientation of particles, or reaction to various tests. If a feature is distinctly different from the adjacent material but kind cannot be determined, it is still described.

Clay films (synonymous with clay skins) are thin layers of oriented, translocated clay.

Clay bridges link together adjacent mineral grains.

Sand or silt coats are sand or silt grains adhering to a surface. Some sand and silt coats are concentrations of the sand and silt originally in the horizon from which finer particles have been removed. Some sand and silt coats are material that has been moved from horizons above and deposited on surfaces. In some coats the grains are almost free of finer material; in others, the grains themselves are coated. If known, the composition of the coat is noted.

Other coats are described by properties that can be observed in the field. The coats are composed variously of iron, aluminum or manganese oxides, organic matter, salts, or carbonates. Laboratory analyses may be needed for a positive identification.

Stress surfaces (pressure faces) are smoothed or smeared surfaces. They are formed through rearrangement as a result of shear forces. They may persist through successive drying and wetting cycles.

Slickensides (fig. 3-32) are stress surfaces that are polished and striated and usually have dimensions exceeding 5 cm. They are produced by relatively large volumes of soil sliding over another. They are common below 50 cm in swelling clays which are subject to large changes in water state.

Figure 3-32 (Click here or on picture for high resolution 173 KB image)

Intersecting slickenslides in a soil.

Intersecting slickensides in the Bss horizon of a Udic Haplustert. The cracking is due to drying after exposure

Location.—The various surface features may be on some or all structural units, channels, pores, primary particles or grains, soil fragments, rock fragments, nodules, or concretions. The kind and orientation of surface on which features are observed is always given. For example, if clay films are on vertical but not horizontal faces of peds, this fact should be recorded.

Amount.—The percentage of the total surface area of the kind of surface considered occupied by a particular surface feature over the extent of the horizon or layer is described. Amount can be characterized by the following classes:

very few: Occupies < 5 percent.

few: Occupies 5 to 25 percent.

common: Occupies 25 to 50 percent.

many: Occupies > 50 percent.

The same classes are used to describe the amount of "bridges" connecting particles. The amount is judged on the basis of the percentage of particles of the size designated that are joined to adjacent particles of similar size by bridges at contact points.

Distinctness.—Distinctness refers to the ease and degree of certainty with which a surface feature can be identified. Distinctness is related to thickness, color contrast with the adjacent material, and other properties. It is, however, not itself a measure of any one of them. Some thick coats, for example, are faint; some thin ones are prominent. The distinctness of some surface features changes markedly as water state changes. Three classes are used.

Faint. Evident only on close examination with 10X magnification and cannot be identified positively in all places without greater magnification. The contrast with the adjacent material in color, texture, and other properties is small.

Distinct. Can be detected without magnification, although magnification or tests may be needed for positive identification. The feature contrasts enough with the adjacent material to make a difference in color, texture, or other properties evident.

Prominent. Conspicuous without magnification when compared with a surface broken through the soil. Color, texture, or some other property or combination of properties contrasts sharply with properties of the adjacent material or the feature is thick enough to be conspicuous.

The order of description is usually amount, distinctness, color, texture, kind, and location. Two examples: "few distinct grayish brown (10YR 5/2) clay films on vertical faces of peds"; "many distinct brown clay bridges between mineral grains." Only properties are listed that add to the understanding of the soil. If texture of the surface feature is obvious, as in most stress surfaces, repeating texture adds nothing. Kind and location are essential if the feature is mentioned at all. The conventions do not characterize the volume of the surface features. If volume is important, it is estimated separately.

Footnote

  1. Crack depth by wire insertion may yield shallower depths than is measured by pouring loose sand into the crack and excavating after wetting and the crack has closed. The latter method, however, is only suitable for detailed examination; whereas, wire insertion is relatively rapid and can be completed in a single observation.
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