Which horizons constitute the solum

These horizons are described as follows. The O horizon is an organic layer made of wholly or partially decayed plant and animal debris. The O horizon generally occurs in undisturbed soil, since plowing mixes the organic material into the soil. In a forest, fallen leaves, branches, and other debris make up the O horizon.

The A horizon, called topsoil by most growers, is the surface mineral layer where organic matter accumulates. Over time, this layer loses clay, iron, and other materials to leaching.

This loss is called eluviation. Materials resistant to weathering, such as sand, tend to remain in the A horizon as other materials leach out. The A horizon provides the best environment for the growth of plant roots, microorganisms, and other life.

The E horizon, the zone of greatest eluviation, is very leached of clay, chemicals, and organic matter. Because the chemicals that color soil have been leached out, the E layer is very light in color. It usually occurs in sandy forest soils in high rainfall areas. The B horizon, or subsoil , is often called the "zone of accumulation" where chemicals leached out of the A and E horizon accumulate. The word for this accumulation is illuviation. The B horizon has a lower organic matter content than than the topsoil and often has more clay.

The A, E, and B horizons together are known as the solum. This part of the profile is where most plant roots grow. The C horizon lacks the properties of the A and B horizons.

It is the soil layer less touched by soil-forming processes and is usually the parent material of the soil. Variously sized chunks of the rock below are surrounded by smaller bits of rock and clay weathered from those chunks.

Some of the original rock is intact, but other parts have been chemically changed into new minerals. The R layer D horizon is the bedrock, or sometimes, the sediment from which the other horizons develop. Originally, this rock lay exposed at the surface where it weathered rapidly into soil. The depth from the surface to the R layer depends on the interrelationships between the climate, the age of the soil, the slope, and the number The layers of soil are called horizons.

Together they make up the soil profile. Photograph by Robert J. Field Mark Publications. Reproduced by permission. Most people do not consider the R layer as soil, but include it in the profile anyway, since the weathering of this bedrock usually produces the soil above it. In a perfect world, all soils demonstrate these horizons, making the lives of soil scientists and soil students blissful. In reality, however, some soils, like transported soils moved to their present locations by water, wind, or glaciers , lack horizons because of mixing while moving or because of youth.

In other soils, the A and B rest on bedrock, or erosion strips an A, or other complicated variations. If these are not available commonly because roots do not extend to the depth of concern then inferences may be made from morphology. A change in particle-size distribution alone e. Some guidelines for inferring physical restriction are given below. Very shallow This section discusses particle-size distribution of mineral soil separates.

Fine earth indicates particles smaller than 2 mm in diameter. Fragments 2 mm or larger consist of rock fragments , pieces of geologic or pedogenic material with a strongly cemented or more cemented rupture-resistance class; pararock fragments , pieces of geologic or pedogenic material with an extremely weakly cemented to moderately cemented rupture-resistance class; and discrete artifacts , pieces of human-manufactured material.

Particle-size distribution of fine earth is determined in the field mainly by feel. The content of rock fragments, pararock fragments, and discrete artifacts is an estimate of the proportion of the soil volume that they occupy. After pretreatment to remove organic matter, carbonates, soluble salts, and other cementing agents and after dispersion to physically separate individual soil particles, the U.

Department of Agriculture uses the following size separates for fine-earth fraction:. Very coarse sand Soil texture refers to the weight proportion of the separates for particles less than 2 mm in diameter as determined from a laboratory particle-size distribution. The pipette method is the preferred standard, but the hydrometer method also is used in field labs Soil Survey Staff, If used, the hydrometer method should be noted with the results.

Field estimates of soil texture class are based on qualitative criteria, such as how the soil feels gritty, smooth, sticky and how it responds to rubbing between the fingers to form a ribbon. Estimated field texture class should be checked against laboratory determinations, and the field criteria used to estimate texture class should be adjusted as necessary to reflect local conditions. Sand particles feel gritty and can be seen individually with the naked eye. Silt particles have a smooth feel to the fingers when dry or wet and cannot be seen individually without magnification.

Clay soils are sticky in some areas and not sticky in others. For example, soils dominated by smectitic clays feel different from soils that contain similar amounts of micaceous or kaolinitic clay. The relationships that are useful for judging texture of one kind of soil may not apply as well to another kind.

Some soils are not dispersed completely in the standard laboratory particle-size analysis. Examples include soils with andic soil properties high amounts of poorly crystalline, amorphous minerals and soils with high contents of gypsum more than about 25 percent. For soils like these, for which the estimated field texture class and the laboratory measured particle-size distribution differ markedly, the field texture is referred to as apparent because it is not an estimate that correlates well with the results of a laboratory test.

Apparent field texture is only a tactile evaluation and does not infer laboratory test results. The twelve texture classes fig. Subclasses of sand are coarse sand, sand, fine sand, and very fine sand. Subclasses of loamy sands and sandy loams that are based on sand size are named similarly. Coarse sand. Fine sand. Very fine sand. Loamy sands. Loamy coarse sand. Loamy sand. Loamy fine sand. Loamy very fine sand. Sandy loams. Coarse sandy loam.

Sandy loam. Fine sandy loam. Very fine sandy loam. Silt loam. Sandy clay loam. Clay loam. Silty clay loam. Sandy clay. Silty clay. The USDA textural triangle is shown in figure A soil sample is assigned to one of the twelve soil texture classes according to the values for the proportions of sand, silt, and clay, which are located along each of the three axes. The eight subclasses in the sand and loamy sand groups provide refinement that in some cases may be greater than can be consistently determined by field techniques.

Only those distinctions that are significant to use and management and that can be consistently made in the field should be applied when determinations of texture are based on field estimates alone.

The need for fine distinctions in the texture of the soil layers results in a large number of classes and subclasses of soil texture. It commonly is convenient to speak generally of broad groups or classes of texture. Table provides an outline of three general soil texture groups and five subgroups. In some areas where soils have a high content of silt, a fourth general class, silty soil materials, may be used for silt and silt loam. There are some horizons or layers for which soil texture class terms are not applicable.

These include bedrock and other cemented horizons such as petrocalcic horizons, duripans, etc. Other exceptions include layers composed of more than 90 percent rock fragments or artifacts and horizons or layers composed of 40 percent or more gypsum in the fine-earth fraction and that are not cemented. These exceptions are discussed below. For soil materials with 40 percent or more, by weight, gypsum in the fine-earth fraction, gypsum dominates the physical and chemical properties of the soil to the extent that particle-size classes are not meaningful.

Two terms in lieu of texture are used:. Coarse gypsum material. Fine gypsum material. These horizons or layers are described as bedrock or cemented material.

Additional information about the kind of rock, degree of cementation, and kind of cementing agent can also be provided. These layers are described as water or ice. They only refer to subsurface layers, such as in a floating bog. Figure shows a subsoil layer of ice. For soil materials with more than 90 percent rock or pararock fragments, there is not enough fine earth to determine the texture class. In these cases, the terms gravel, cobbles, stones, boulders, channers, and flagstones or their pararock fragment equivalents are used.

Layers that are not saturated with water for more than a few days at a time are organic if they have 20 percent or more organic carbon. Layers that are saturated for longer periods, or were saturated before being drained, are organic if they have 12 percent or more organic carbon and no clay, 18 percent or more organic carbon, and 60 percent or more clay or have a proportional amount of organic carbon, between 12 and 18 percent, if the clay content is between 0 and 60 percent.

Soils with more than 60 percent clay need an organic carbon content of at least 18 percent. The kind and amount of the mineral fraction, the kind of organisms from which the organic material was derived, and the state of decomposition affect the properties of the soil material. Descriptions include the percentage of undecomposed fibers and the solubility in sodium pyrophosphate of the humified material.

Attention should be given to identifying and estimating the volume occupied by sphagnum fibers, which have extraordinary high water retention. When squeezed firmly in the hand to remove as much water as possible, sphagnum fibers are lighter in color than fibers of hypnum and most other mosses. Fragments of wood more than 20 mm across and so undecomposed that they cannot be crushed by the fingers when moist or wet are called wood fragments. They are comparable to rock fragments in mineral soils and are described in a comparable manner.

Saturated organic soil materials. Mucky peat. Muck, peat, and mucky peat may be described in both organic and mineral soils provided the soils are saturated with water for 30 or more cumulative days in normal years or are artificially drained.

These materials only qualify for the diagnostic sapric, fibric, and hemic soil material of Soil Taxonomy when they occur in organic soils i. Non-saturated organic soil materials. Highly decomposed plant material. Moderately decomposed plant material. Slightly decomposed plant material. Modifiers may be needed to better describe the soil material making up the horizon or layer.

These include terms for significant amounts of particles 2. To describe soils with 15 percent or more, by volume, rock fragments, pararock fragments, or artifacts, the texture terms are modified with terms indicating the amount and kind of fragments. Examples include very gravelly loam, extremely paracobbly sand, and very artifactual sand. The conventions for use of these terms and the definitions of class terms are discussed in the following sections on rock fragments, pararock fragments, and artifacts.

Soil composition modifiers are used for some soils that have andic properties or formed in volcanic materials, soils that have a high content of gypsum, some organic soil materials, and mineral soil materials with a high content of organic matter. Terms are also provided for limnic soil materials and permanently frozen layers permafrost. The weathering processes of volcanic materials are evidenced by 30 percent or more particles 0.

For material that has 40 percent or more gypsum, a term in lieu of texture is used e. The following modifiers are used only for organic soil materials that are saturated with water for 30 or more cumulative days in normal years or are artificially drained. Highly organic. Excluding live roots, the horizon has organic carbon content by weight of one of the following:. Excluding live roots, the horizon has more than 10 percent organic matter and less than 17 percent fibers.

Excluding live roots, the horizon has more than 10 percent organic matter and 17 percent or more fibers. Limnic soil materials occur in layers underlying some soils of the soil order Histosols. By definition see Soil Taxonomy they are not recognized in mineral soils. They are mineral or organic soil materials originating from aquatic organisms or from aquatic plants that were later altered by aquatic organisms.

The following terms are used to describe the origin of the limnic materials:. Layers for which these terms are used may or may not also meet the definition for coprogenous earth, diatomaceous earth, or marl as defined in Soil Taxonomy. Layers of permafrost are described as permanently frozen e. Rock fragments are unattached pieces of geologic or pedogenic material 2 mm in diameter or larger that have a strongly cemented or more cemented rupture-resistance class.

Pararock fragments are unattached pieces of geologic or pedogenic material 2 mm in diameter or larger that are extremely weakly cemented through moderately cemented. Pararock fragments are not retained on sieves because they are crushed by grinding during the preparation of samples for particle-size analysis in the laboratory.

Rock fragments and pararock fragments include all sizes between 2. Thus, rock and pararock fragments may be discrete, cemented pieces of bedrock, bedrock-like material, durinodes, concretions, nodules, or pedogenic horizons e. Artifacts, however, are not included as rock or pararock fragments. They are described separately. Rock fragments and pararock fragments are described by size, shape, hardness, roundness, and kind of fragment. The classes are gravel, cobbles, channers, flagstones, stones, and boulders and their pararock counterparts i.

If a size or range of sizes predominates, the class is modified e. Gravel and paragravel are a collection of fragments that have diameters ranging from 2 to 76 mm. The upper size limit of gravel and paragravel is 76 mm 3 inches. This coincides with the upper limit used by many engineers for grain-size distribution computations. The 5-mm and mm divisions for the separation of fine, medium, and coarse gravel coincide with the sizes of openings in the number 4 screen 4.

The mm 3-inch limit separates gravel from cobbles, the mm inch limit separates cobbles from stones, and the mm inch limit separates stones from boulders.

The mm 6-inch and mm inch limits for thin, flat channers and flagstones, respectively, follow conventions used for many years to provide class limits for plate-shaped and crudely spherical rock fragments that have about the same soil use implications as the mm limit for spherical shapes.

Rock fragments in the soil can greatly influence use and management. It is important to not only consider the total amount of rock fragments, but also the proportions of the various size classes gravel, cobbles, stones, etc.

A soil with 10 percent stones is quite different from one with 10 percent gravel. When developing interpretive criteria, a distinction must be made between volume and weight percent of rock fragments.

Field descriptions generally record estimates of volume, while laboratory measurements of rock fragments are given as weight for the various size classes. Length of the transect or area of the exposure should be at least 50 times, and preferably times, the area or dimensions of the rock fragment size that encompasses about 90 percent of the rock fragment volume. This method is preferred because of the difficulty in visual evaluation of the 2 to 5 mm size separations.

The adjectival form of a class name of rock fragments or pararock fragments table is used as a modifier of the texture class name, e. Table provides rules for determining the proper texture modifier term for material with a mixture of rock fragment sizes.

This section also provides rules for assigning terms for soils with a mixture of rock and pararock fragments. Less than 15 percent. If there is too little fine earth to determine the texture class less than about 10 percent, by volume a term in lieu of texture i. The class limits apply to the volume of the layer occupied by all rock fragments 2 mm in diameter or larger. The soil generally contains fragments smaller or larger than those identified by the term.

The use of a term for larger pieces of rock, such as boulders, does not imply that the pieces are entirely within a given soil layer. A single boulder may extend through several layers.

Table can be used to determine the proper modifier if there is a mixture of rock fragment sizes. To use the table, first choose the row with the appropriate total rock fragments. Stop in the first column in which a criterion is met. More precise estimates of the amounts of rock fragments than are provided by the defined classes are needed for some purposes. For more precise information, estimates of percentages of each size class or a combination of size classes are included in the description, e.

Exposed bedrock is not soil and is identified separately in mapping as a kind of miscellaneous area i. The volume occupied by individual pieces of rock can be seen, and their aggregate volume percentage can be calculated.

For some purposes, volume percentage must be converted to weight percentage. The following rules are used to select texture modifiers if a horizon includes both rock and pararock fragments:.

Fragment hardness is equivalent to the rupture resistance class for a cemented fragment of specified size that has been air dried and then submerged in water. The hardness of a fragment is significant where the rupture resistance class is strongly cemented or greater. See the section on rupture resistance later in this chapter for details describing the fragment hardness classes and their test descriptions.

Fragment roundness is an expression of the sharpness of the edges and corners of rock fragments and pararock fragments. The roundness of fragments impacts water infiltration, root penetration, and macropore space. The following roundness classes are used:. Very angular Strongly developed faces and very sharp, broken edges Angular Strongly developed faces and sharp edges Subangular Detectable flat faces and slightly rounded corners Subrounded Detectable flat faces and well rounded corners Rounded Flat faces absent or nearly absent and all corners rounded Well rounded Flat faces absent and all corners rounded.

Fragment kind is the lithology or composition of the 2 mm or larger fraction of the soil. Kinds of fragments are varied based on whether their origin is from a geologic source or a pedogenic source. Examples of kinds of fragments are basalt fragments, durinodes, iron-manganese concretions, limestone fragments, petrocalcic fragments, tuff fragments, and wood fragments.

Artifacts are discrete water-stable objects or materials created, modified, or transported from their source by humans, usually for a practical purpose in habitation, manufacturing, excavation, agriculture, or construction activities. Examples are processed wood products, coal combustion by-products, bitumen asphalt , fibers and fabrics, bricks, cinder blocks, concrete, plastic, glass, rubber, paper, cardboard, iron and steel, altered metals and minerals, sanitary and medical waste, garbage, and landfill waste.

Artifacts also include natural materials which were mechanically abraded by human activities as evidenced by scrapes, gouges, tool marks, etc. Artifacts are generally categorized as either particulate or discrete.

The distinction is based on size: particulate artifacts have a diameter of less than 2 mm and discrete artifacts have a diameter of 2 mm or more. Discrete artifacts are easier to identify and are essentially fragments of human origin. Particulate artifacts are sometimes difficult to discern from naturally occurring fine-earth soil material. Artifacts are described if they are judged to be durable enough to persist in the soil resist weathering and leaching for a few decades or more.

Descriptions of artifacts generally include quantity, cohesion, persistence, size, and safety classes. They may also include shape, kind, penetrability by roots, and roundness. The conventions for describing artifacts are explained in the following paragraphs. Quantity refers to the estimated volume percent of a horizon or other specified unit occupied by discrete artifacts. If classes rather than quantitative estimates are given, they are the same as those described in this chapter for mottles.

Cohesion refers to the relative ability of the artifact to remain intact after significant disturbance. The cohesion classes are:. Noncohesive artifacts are similar to pararock fragments and will be incorporated into the fine-earth fraction of the soil during routine laboratory sample preparation.

Penetrability describes the relative ease with which roots can penetrate the artifact and potentially extract any stored moisture, nutrients, or toxic elements. The penetrability classes are:. Persistence describes the relative ability of solid artifacts to with-stand weathering and decay over time. Local conditions, such as temperature and moisture, significantly impact the persistence of artifacts in the soil.

The persistence classes are:. Loss of soil mass and eventually subsidence result. Roundness indicates the sharpness of edges and corners of natural objects, such as rock fragments, and human-manufactured objects, such as artifacts. The artifact roundness classes are the same as those used for fragment roundness above. Safety describes the degree of risk to humans from contact with soils that contain artifacts.

Physical contact with soils containing dangerous or harmful artifacts should be avoided unless proper training is provided and protective clothing is available. The safety classes are:. Examples include untreated wood products, iron, bricks, cinder blocks, concrete, plastic, glass, rubber, organic fibers, inorganic fibers, unprinted paper and cardboard, and some mineral and metal products. Sharp innocuous artifacts can cause injury, but the materials themselves are still considered innocuous.

The harm may be immediate or long-term and through direct or indirect contact. Examples include arsenic-treated wood products, batteries, waste and garbage, radioactive fallout, liquid petroleum products, asphalt, coal ash, paper printed with metallic ink, and some mineral and metal products. Size may be measured and reported directly or given as a class. The dimension to which size-class limits apply depends on the shape of the artifact described.

If the shape is nearly uniform, size is measured in the shortest dimension, such as the effective diameter of a cylinder or the thickness of a plate. For elongated or irregular bodies, size generally refers to the longest dimension but direct measurements for 2 or 3 dimensions can be given for clarification. The size classes for discrete artifacts are:. There are too many varieties of artifacts to provide a comprehensive list. The most common types include:. In some cases, the mineral soil may contain a combination of fragment or composition types for which the use of compound texture modifiers is useful.

For example, a soil horizon may contain both artifacts and other fragments, such as rock fragments and pararock fragments. In these cases, the rock fragments, pararock fragments, and artifacts are each described separately. Modifiers for both artifacts and rock or pararock fragments can be combined. The modifier for artifacts comes before the modifier for rock or pararock fragments, e. Modifiers for composition and rock fragments can also be combined. For example, a horizon of channery mucky clay or one of gravelly gypsiferous sandy loam contains rock fragments and also a content of high organic matter or gypsum.

There are many possible combinations. This section discusses the description of rock fragments especially stones and boulders that are on the soil as opposed to in the soil. This cover provides some protection from wind and water erosion. It may also interfere with seed placement and emergence after germination. For stones and boulders, the percent of cover is not of itself as important as the interference with mechanical manipulation of the soil.

For example, a very small areal percentage of large fragments, insignificant for erosion protection, may interfere with tillage, tree harvesting, and other operations involving machinery.

If the areal percentage equals or exceeds 80 percent, the top of the soil is considered to be the mean height of the top of the rock or pararock fragments. The volume proportions of the 2 to 5 mm, 5 to 75 mm, and 75 to mm fragments should be recorded.

This can be done from areal measurements in representative areas. The classes are given in terms of the approximate amount of rock fragments of stone and boulder size at the surface:. Class 1. The smallest stones are at least 8 meters apart; the smallest boulders are at least 20 meters apart fig. Class 2. The smallest stones are not less than 1 meter apart; the smallest boulders are not less than 3 meters apart fig. Class 3.

The smallest stones are as little as 0. Class 4. In most places it is possible to step from stone to stone or jump from boulder to boulder without touching the soil fig. Class 5. The smallest stones are less than 0. Classifiable soil is among the rock fragments, and plant growth is possible fig. These limits are intended only as guides to amounts that may mark critical limitations for major kinds of land use.

Table is a summary of the classes. Most soil survey organizations, including the National Cooperative Soil Survey in the United States, have adopted the Munsell soil color system for describing soil color using the elements of hue, value, and chroma.

The names associated with each standard color chip yellowish brown, light gray, etc. They were selected by the Soil Survey Staff to be used in conjunction with the Munsell color chips. The color chips included in the standard soil-color charts a subset of all colors in the system were selected so that soil scientists can describe the normal range of colors found in soils.

These chips have enough contrast between them for different individuals to match a soil sample to the same color chip consistently. Interpolating between chips is not recommended in standard soil survey operations because such visual determinations cannot be repeated with a high level of precision. Although digital soil color meters that can provide precise color readings consistently are available, they are not widely used in field operations. Therefore, the standard procedure adopted for soil survey work is visual comparison to the standard soil-color charts.

Elements of soil color descriptions are the color name, the Munsell notation, the water state moist or dry , and the physical state. If physical state is unspecified, a broken surface is implied. The color of the soil is normally recorded for a surface broken through a ped, if a ped can be broken as a unit. If ped surfaces are noticeably different in color from the ped interior, this should also be described.

The color value of most soil material is lower after moistening. Consequently, the water state of a sample is always given. Color in the moist state is determined on moderately moist or very moist soil material and should be made at the point where the color does not change with additional moistening. The soil should not be moistened to the extent that glistening takes place because the light reflection of water films may cause incorrect color determinations.

In a humid region, the moist state generally is standard; in an arid region, the dry state is standard. In detailed descriptions, colors of both dry and moist soil are recorded if feasible. The color for the regionally standard moisture state is typically described first.

Both moist and dry colors are valuable, particularly for the immediate surface and tilled horizons, in assessing reflectance. A Munsell notation is obtained by comparison with a Munsell soil-color chart. The most commonly used charts include only about one fifth of the entire range of hues. Figure illustrates the arrangements of color chips on a Munsell color card. The Munsell color system uses three elements of color— hue, value, and chroma.

Hue is a measure of the chromatic composition of light that reaches the eye. Five intermediate hues representing midpoints between each pair of principal hues complete the 10 major hue names used to describe the notation. The relationships among the 10 hues are shown in figure Each of the 10 major hues is divided into 4 segments of equal visual steps, which are designated by numerical values applied as prefixes to the symbol for the hue name. Four equally spaced steps of the adjacent yellow-red YR hue are identified as 2.

The standard chart for soil has separate hue cards, from 10R through 5Y. In addition, special charts for gley colors and for very light colors are available.

In practice, however, only the divisions on the color charts are used. Value indicates the degree of lightness or darkness of a color in relation to a neutral gray scale. On a neutral gray achromatic scale, value extends from pure black 0 to pure white The value notation is a measure of the amount of light that reaches the eye under standard lighting conditions.

Gray is perceived as about halfway between black and white and has a value notation of 5. The actual amount of light that reaches the eye is related logarithmically to color value. Lighter colors are indicated by numbers between 5 and 10; darker colors are indicated by numbers from 5 to 0.

These values may be designated for either achromatic i. Thus, a card of the color chart for soil has a series of chips arranged vertically to show equal steps from the lightest to the darkest shades of that hue. Figure shows this arrangement vertically on the card for the hue of 10YR. Note that the highest value shown on the standard color cards is 8. Color chips with value of 9 are included on special color cards for very light colors.

Chroma is the relative purity or strength of the spectral color. It indicates the degree of saturation of neutral gray by the spectral color. The scales of chroma for soils extend from 0 for neutral colors to 8 for colors with the strongest expression. The color chips are arranged horizontally by increasing chroma from left to right on the soil-color chart see fig.

On the soil-color chart for a specific hue e. The weakest expression of chroma the grayest color is at the left, and the strongest expression of chroma is at the right. These colors have zero chroma and are totally achromatic neutral. They have no hue and no chroma but range in value from black N 2.

The quality and intensity of the light source affect the amount and quality of the light reflected. The moisture content of the sample and the roughness of its surface affect the light reflected. The visual impression of color from the standard color chips is accurate only under standard conditions of light intensity and quality. Color determination may be inaccurate early in the morning or late in the evening. When the sun is low in the sky or the atmosphere is smoky, the light reaching the sample and the light reflected are redder.

Even though the same kind of light reaches the color standard and the sample, the reading of sample color at these times is commonly one or more intervals of hue redder than at midday. Colors also appear different in the subdued light of a cloudy day than in bright sunlight. If artificial light is used, as for color determinations in an office, the light source must be as near the white light of midday as possible.

With practice, compensation can be made for the differences. The intensity of incidental light is especially critical when matching soil to chips of low chroma and low value. Roughness of the reflecting surface affects the amount of reflected light, especially if the incidental light falls at an acute angle. The incidental light should be as near as possible at a right angle.

Under field conditions, measurements of color are reproducible by different individuals within 2. Notations are made to match the chips included on the color charts, typically the nearest whole unit of value and chroma. Soil color should be recorded to the closest color chip provided but not interpolated between chips.

For some hues, chips for value of 2. Determinations typically are not precise enough to justify inter-polation between chromas of 4 and 6 or between chromas of 6 and 8. Color should never be extrapolated beyond the highest chip. The soil-color charts for individual hues do not show value greater than 8. Observed colors are always rounded to the nearest chip. For many purposes, the differences between colors of some adjacent color chips have little significance.

For these, color notations have been grouped and named see fig. The dominant color is the one that occupies the greatest layer volume. It is always listed first among the colors of a multicolored layer.