Soil Genesis and Classification, Sixth Edition. S. W. Buol, R. J. Southard, R. C. Graham and P. A. McDaniel. 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
Morphology and Composition of Soils
Soil morphology deals with the form, structure, and organization of the soil material. Soil morphology is ordinarily first observed, described, and studied in the field, but investigation can be continued in the laboratory with optical and electron microscopes. Field observations with the unaided eye or with a hand lens are considered macromorphology, whereas observations utilizing a microscope are considered micromorphology. Morphology describes and measures a wide range of soil properties, and includes as assessment of soil particles and aggregates that pro-vide estimates of soil void characteristics and hydraulic properties (Lin et al. 1999a, 1999b).
Soil composition includes chemical, physical, and mineralogical measurements of soil material removed from defined positions (horizons or layers) within pedons. Soil moisture and temperature dynamics are important components of soils that vary from year to year but can be characterized and classified within ranges that permit identification of many potential soil uses.
New methods for analyzing soil are constantly being developed and tested. As stated in Chapter 1, soil classification follows science. A comprehensive soil classification system must provide a basis for comparing all soils and therefore must utilize comparable methods to determine soil composition. We confine discussions to the methods most commonly used to characterize soil for classification in Soil Taxonomy. Researchers should not be limited to the methods presented here, but only after an analytical technique has been successfully used on a large number of soils and a substantial amount of data collected can it be evaluated and utilized to classify soils.
Soil Macromorphology: Examination and DescriptionMacromorphology is best evaluated from the in situ examination of the soil profile. Arecently dug pit large enough for observation of a pedon is desirable. Old exposures such as road cuts and ditches are acceptable only for preliminary examination because morphological features often become altered after prolonged exposure. The exposed profile should be probed by hand, with the aid of a knife or small pick to remove any alterations resulting from digging equipment and to expose the natural condition of the soil. Following this cleanup is a good time to photograph the profile, including a tape measure or some other suitable reference for scale. The profile examination
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begins by marking the depths in the profile where differences are observed in any number of soil properties including color, structure, texture, root distribution, rock fragment content, cementation, and/or carbonate content, as shown in Figure 2.1. This is the first approximation of soil horizon boundaries. The soil horizon boundaries may be adjusted as examination progresses, but it is desirable to establish an approximate boundary before beginning detailed examination of each horizon.
Recognizing Soil Horizons and Describing Their Properties in the Field. Soil horizons are described according to several properties including color, texture, consistence, structure, nodules or concretions, ped coatings, pH (by field method), car-bonate content, root and pore distribution, and boundary characteristics, and horizon continuity. The following definitions of soil properties are based on the current practices in the United States (Soil Survey Division Staff 1993; Schoeneberger et al. 2002).
Elementary soil body (homogeneous)
Soil bodies on the upland
WaterA soil body(Polypedon)
Figure 2.1. Profile, pedon, and solum. This pedon is from one of three elementary soil bodies that are inclusions in a large body.
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Soil Color. Soil color is probably the most obvious feature of the soil and is easily seen by laypersons. There are a number of methods used to describe color, but the standard method for soils is the Munsell system (Munsell 1912). The Munsell system identifies color based on three measurable variables: hue, value, and chroma. Hue is the dominant spectral color and is related to wavelengths of light. Value is a measure of degree of darkness or lightness of the color and is related to the total amount of light reflected. Chroma is a measure of the purity or strength of spectral color. These three variables have been combined into reference charts that cover the range of colors found in soils and bound in book format. In the Munsell soil color book, the various hues are arranged by page, one hue to a page. The units of value are arranged vertically, and the units of chroma are arranged horizontally on each page. Opposite each page of color chips is a page of color symbols and corresponding English names. An example of a color notation made for a soil horizon color is 10YR 6/3. The interpretation of the notation is 10YR (10 yellow-red) hue, a value of 6, and a chroma of 3. The proper name for this color is pale brown.
Soil color is moisture dependent, especially with respect to color value. The moisture status of the soil, moist or dry, is noted when color is described. Moist is thewater content at which added water does not change the color; dry is air dry. In some soils the color of the ped exterior differs from the color of the ped interior, and the colors of both parts of the ped are described. In some cases, the color of crushed and smoothed soil material is described (e.g., spodic materials, defined later in this chapter), but prolonged rubbing of the soil material prior to measuringsoil color should be avoided. The visual appearance of soil color is also affected by the source of light. Early morning or late evening light tends to make soil colors appear redder. Heavy overcast or deep shade under forest canopy makes measurement of soil color in the field difficult. If soil color must be determined in a laboratory or office after field sampling, the sample should beexamined in skylight by a window or out-of-doors. Fluorescent lighting must be avoided.
Many soil horizons have intimately mixed color patterns. The dominant color of the horizon (the matrix color) is recorded and the mottling (from motley: variegate in color) is described in terms of quantity, size, and contrast of the other colors. The spatial pattern of mottling should be noted. Quantity is indicated as a percentage of the surface area examined that each color represents:
Few = less than 2% of the areaCommon = 2 to 20% of the areaMany = 20% or more of the area
Size refers to the length of the mottle if the length is at least two times the width, and to the width of the mottle if the length is less than two times the width. Five size classes are used as follows:
Fine = less than 2 mmMedium = 2 to 5 mm
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Coarse = 5 to 20 mmVery Coarse = 20 to 76 mmExtremely Coarse = 76 mm or more
Contrast is a notation of the visual distinction between the colors described as combinations of differences in hue, value, and chroma of the matrix and the mottle. (See Schoeneberger et al. 2002 for details.)
Faint = indistinct contrast that is evident and recognizable only upon close examination
Distinct = color differences between the matrix and the mottle are more apparentProminent = mottles are obvious and mottling is one of the outstanding features
of the horizon
Standard practice is to list the matrix color first, followed by a description of the quantity, size, and contrast of the other colors in the mottled pattern. In some cases, the soil colors are so intricately mixed that a dominant matrix color is not obvious. Under these circumstances, the proportion of each of the colors is estimated and each is described using Munsell notation. Careful observation and notation of the spatial arrangement of the various red, red-yellow, and gray colors (redoximorphic features) with respect tostructural aggregates, root channels, or other large voids are useful in interpreting seasonal patterns of saturation and reduction in the pedon (Vepraskas 1992).
Soil Texture. Soil texture is defined as the relative proportions of the various soil separates in a soil material (Soil Science Society of America 1996). Percentages of sand, silt, and clay are reported on a mass basis. The continuum of soil texture has been divided into 12 textural classes (Figure 2.2A) used to describe soil hori-zons. The textural classes sand, loamy sand, and sandy loam are subdivided into additional classes based on the proportions of the various classes of sand-sized particles (e.g., very fine sand, loamy coarse sand, fine sandy loam). The lower part of Figure 2.2A shows how solid mineral particles are grouped by size in three sys-tems. For most soil purposes the USDA scale is used. The family particle size groups of Soil Taxonomy are shown in Figure 2.2B for comparison but are not used when describing soil profiles. The textural groups diagramed in Figure 2.2A are determined using only the mineral particles less than 2 mm in diameter. Wetting a sample, working it between the thumb and fingers and comparing the feel to samples of known texture is an art developed by soil scientists to estimate texture in the field.
A rock fragment modifier prefaces the textural name if particles larger than2 mm compose 15% or more of the volume of the bulk soil material. Classes of rock fragment modifiers for round and angular particles larger than 2 mm in diameter are:
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SiltSilt loam 90
Comparison of particle size scalesSieve openings in inches
Gravel or stone
100 50 10 5 2 1 0.5Grain size in millimeters
0.42 0.25 0.1 0.05 0.02 0.01 0.0050.074
3 2 1 433 10 20U. S. Standard sieve numbers
Silt or clay
40 60 2001
40 30 20 10 0
Figure 2.2A. Guide for textural classification of the fine earth fraction (< 2 mm).
Size Name Diameter Modifier Name
Gravel 275 mm GravellyCobble 75250 mm CobblyStone 250600 mm StonyBoulder >600 mm Bouldery
For rock fragments that are flat the following names, based on length of their longest axis, are used:
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Size Name Longest Dimension Modifier Name
Channer 2150 mm ChanneryFlagstone 150380 mm FlaggyStone 380600 mm StonyBoulder >600 mm Bouldery
If the fine earth fraction texture is loam and the soil material contains between 15 and 35% by volume rock fragments and gravel is the dominant size fraction, then the textural class of the material is gravelly loam. If the soil contains from 35 to 60% gravel, the textural class is very gravelly loam, and if it contains from 60 to 90% gravel it is classified as extremely gravelly loam. The same conventions are used for the other rock fragment modifiers of texture (for example, channery sandy loam, very flaggy clay loam). If the soil material contains 90% or more rock fragments, the textural class is named for the dominant rock fragment size class, for example, cobbles. In all cases, the term describing the quantity of rock fragments is based on the total volume of rock fragments. The rock fragment size modifier is based on the largest dominant fragment size. If there is a mixture of sizes, a smaller size class is named only if the volume percent of the smaller class is more than two times the
40 30Very fine sand (0.050.1) is treated as silt for family groupings:coarse fragments are considered the equivalent of coarse sand inthe boundary between the silty and loamy classes.
20 10 0
Figure 2.2B. Guide for soil family groupings on the basis of particle size, i.e., particle-size distribution of the whole soil.
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volume percent of a larger class. For example, a texture with 21% gravel and 10% cobbles is gravelly, but a texture with 20% gravel and 11% cobbles is cobbly.
Organic soil materials use terms in place of textural classes based on the degree of organic matter decomposition. Peat is organic material that is relatively undecomposed plant tissues; muck is highly decomposed organic material, and few plant tissuefibers are observed; mucky peat is of intermediate decomposition. To estimate undecomposed tissue or rubbed fiber content wet samples are rubbed with the hands and carefully examined to observe undecomposed tissue.
Soil Structure. Structure refers to the aggregation of individual soil particles into larger units with planes of weakness between them. Individual aggregates are known as peds. There is no technique for observing structure that is applicable to all soils. Carefully probing the exposed soil profile with a knife and prying out volumes of soil while observing how the material crumbles into peds is most often applicable.
Horizons that do not have aggregates with naturally preserved boundaries (peds) are considered to be structureless. Two forms of a structureless condition are recog-nized: single grain when the soil material is not cohesive and the individual particles are pried or fall from the profile face, or massive when the material is cohesive and fails to show any naturally occurring planes of weakness when pried from the profile face.
In horizons where peds are observed, three features of structure are usually described.
Type refers to the shape of peds. Granular peds are more or less spherical with little or no accommodation of adjoining peds. Platy peds are flat and plate-like, usually oriented horizontally. Prismatic peds are elongated in the vertical axis, with flat tops, and horizontally bounded by rather flat ped faces. Columnar peds are similar to prismatic peds, but have r...