Clay Percentage

For the Region IV SASES[1] Collegiate Soil Judging Contest Clay Percentage

(See Chapter 3, Soil Survey Manual)

Clay content:
as a weight percentage of the soil fines should be recorded in the “Clay%” column.

Actual %Allowed Deviation %
< 20+/- 2
20 to 40+/- 3
> 40+/- 4

The textural class and % clay for each horizon will be determined by the judges and supported by laboratory data. Soil textural classes as defined in the Soil Survey Manual Chapter 3 (1993) and their official abbreviations (supplied to contestants as part of Attachments 1 and 2) are to be used. Deviation from standard nomenclature will be incorrect (i.e., sandy silt or silty loam instead of the proper silt loam).

Credit for sand, loamy sand, and sandy loam textures will NOT be given if sand modifiers are required and not provided (i.e. very fine, fine, coarse, or very coarse).

Textural class:
record in the “Class” column.

 

TextureAbreviationTextureAbreviation
Coarse sandCOSFine sandy loamFSL
SandSVery fine sandy loamVFSL
Fine sandFSLoamL
Very fine sandyVFSClay loamCL
Loamy coarse sandLCOSSiltSI
Loamy sandLSSilt loamSIL
Loamy fine sandLFSSilty clay loamSICL
Loamy very fine sandLVFSSilty claySIC
Coarse sandy loamCOSLSandy clay loamSCL
Sandy loamSLSandy claySC
ClayC

 


The following is a quote from:

Soil Survey Division Staff. 1993. Chapter 3 – Examination and Description of Soils. In Soil Survey Manual, United States Department of Agriculture Handbook No. 18 pp136-143. [last visited May 8, 2016]

Particle Size Distribution

This section discusses particle distribution. The finer sizes are called fine earth (smaller than 2 mm diameter) as distinct from rock fragments (pebbles, cobbles, stones, and boulders). Particle-size distribution of fine earth or less than 2 mm is determined in the field mainly by feel. The content of rock fragments is determined by estimating the proportion of the soil volume that they occupy.

Soil Separates

The United States Department of Agriculture uses the following size separates for the <2 mm mineral material:

Name Size (mm)
Very coarse sand: 2.0-1.0 mm
Coarse sand: 1.0-0.5 mm
Medium sand: 0.5-0.25 mm
Fine sand: 0.25-0.10 mm
Very fine sand: 0.10-0.05 mm
Silt: 0.05-0.002 mm
Clay: < 0.002 mm

Figure 3-14 compares the USDA system with others.

fig3-14_large

Figure 3-15 illustrates classes of soil particles larger than silt.

fig3-15_large

Soil Texture

Soil texture refers to the weight proportion of the separates for particles less than 2 mm as determined from a laboratory particle-size distribution. Field estimates should be checked against laboratory determinations and the field criteria should be adjusted as necessary. Some soils are not dispersed completely in the standard particle size analysis. For these, the field texture is referred to as apparent because it is not an estimate of the results of a laboratory operation. Apparent field texture is a tactile evaluation only with no inference as to laboratory test results. Field criteria for estimating soil texture must be chosen to fit the soils of the area. Sand particles feel gritty and can be seen individually with the naked eye. Silt particles cannot be seen individually without magnification; they have a smooth feel to the fingers when dry or wet. In some places, clay soils are sticky; in others they are not. Soils dominated by montmorillonite clays, for example, feel different from soils that contain similar amounts of micaceous or kaolintic clay. Even locally, the relationships that are useful for judging texture of one kind of soil may not apply as well to another kind.

The texture classes (fig. 3-16) are sand, loamy sands, sandy loams, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay. Subclasses of sand are subdivided into 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.

fig3-16_large

Definitions of the soil texture classes follow:

Sands.—More than 85 percent sand, the percentage of silt plus 1.5 times the percentage of clay is less than 15.

Coarse sand. A total of 25 percent or more very coarse and coarse sand and less than 50 percent any other single grade of sand.

Sand. A total of 25 percent or more very coarse, coarse, and medium sand, a total of less than 25 percent very coarse and coarse sand, and less than 50 percent fine sand and less than 50 percent very fine sand.

Fine sand. 50 percent or more fine sand; or a total of less than 25 percent very coarse, coarse, and medium sand and less than 50 percent very fine sand.

Very fine sand. 50 percent or more very fine sand.

Loamy sands.—Between 70 and 91 percent sand and the percentage of silt plus 1.5 times the percentage of clay is 15 or more; and the percentage of silt plus twice the percentage of clay is less than 30.

Loamy coarse sand. A total of 25 percent or more very coarse and coarse sand and less than 50 percent any other single grade of sand.

Loamy sand. A total of 25 percent or more very coarse, coarse, and medium sand and a total of less than 25 percent very coarse and coarse sand, and less than 50 percent fine sand and less than 50 percent very fine sand.

Loamy fine sand. 50 percent or more fine sand; or less than 50 percent very fine sand and a total of less than 25 percent very coarse, coarse, and medium sand.

Loamy very fine sand. 50 percent or more very fine sand.

Sandy loams.—7 to 20 percent clay, more than 52 percent sand, and the percentage of silt plus twice the percentage of clay is 30 or more; or less than 7 percent clay, less than 50 percent silt, and more than 43 percent sand.

Coarse sandy loam. A total of 25 percent or more very coarse and coarse sand and less than 50 percent any other single grade of sand.

Sandy loam. A total of 30 percent or more very coarse, coarse, and medium sand, but a total of less than 25 percent very coarse and coarse sand and less than 30 percent fine sand and less than 30 percent very fine sand; or a total of 15 percent or less very coarse, coarse, and medium sand, less than 30 percent fine sand and less than 30 percent very fine sand with a total of 40 percent or less fine and very fine sand.

Fine sandy loam. 30 percent or more fine sand and less than 30 percent very fine sand; or a total of 15 to 30 percent very coarse, coarse, and medium sand; or a total of more than 40 percent fine and very fine sand, one half or more of which is fine sand, and a total of 15 percent or less very coarse, coarse, and medium sand.

Very fine sandy loam. 30 percent or more very fine sand and a total of less than 15 percent very coarse, coarse, and medium sand; or more than 40 percent fine and very fine sand, more than one half of which is very fine sand, and total of less than 15 percent very coarse, coarse, and medium sand.

Loam.—7 to 27 percent clay, 28 to 50 percent silt, and 52 percent or less sand.

Silt loam. 50 percent or more silt and 12 to 27 percent clay, or 50 to 80 percent silt and less than 12 percent clay.

Silt. 80 percent or more silt and less than 12 percent clay.

Sandy clay loam. 20 to 35 percent clay, less than 28 percent silt, and more than 45 percent sand.

Clay loam. 27 to 40 percent clay and more than 20 to 46 percent sand.

Silty clay loam. 27 to 40 percent clay and 20 percent or less sand.

Sandy clay. 35 percent or more clay and 45 percent or more sand.

Silty clay. 40 percent or more clay and 40 percent or more silt.

Clay. 40 percent or more clay, 45 percent or less sand, and less than 40 percent silt.

The texture triangle (fig. 3-16) is used to resolve problems related to word definitions, which are somewhat complicated. The eight distinctions in the sand and loamy sand groups provide refinement 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.

Groupings of soil texture classes.—The need for fine distinctions in the texture of the soil layers results in a large number of classes of soil texture. Often it is convenient to speak generally of broad groups or classes of texture. An outline of soil texture groups, in three classes and in five, follows. In some areas where soils are high in silt, a fourth general class, silty soils, may be used for silt and silt loam.

General Termsa Texture Classes
Sandy soil materials:
Coarse-textured Sands (coarse sand, sand, fine sand, very fine sand) Loamy sands (loamy coarse sand, loamy sand, loamy fine sand, loamy very fine sand)
Loamy soil materials:
Moderately coarse-textured Coarse sandy loam, sandy loam, fine sandy loam
Medium-textured Very fine sandy loam, loam, silt loam, silt
Moderately fine-textured Clay loam, sandy clay loam, silty clay loam
Clayey soils:
Fine-textured Sandy clay, silty clay, clay
a. These are loamy, and clayey texture groups, not the sandy, loamy, and clayey particle-size classes defined in Soil Taxonomy.

Organic Soils

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 a proportional amount of organic carbon, between 12 and 18 percent, if the clay content is between 0 and 60 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. A special effort is made to identify and estimate the volume occupied by sphagnum fibers, which have extraordinary high water retention characteristics. 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 2 cm 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.

Muck (sapric) is well-decomposed, organic soil material. Peat (fibric) is relatively undecomposed, organic material in which the original fibers constitute almost all of the material. Mucky peat (hemic) is material intermediate between muck and peat.


Date last modified: May 7, 2016.
Donald G. McGahan, Ph.D.
Work of Authorship


[1]Students of Agronomy, Soils, and Environmental Sciences

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