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Figure 5. Standard soil-texture triangle, The Natural Environment, Department of Geography, University of Texas at Austin

• Brown, R.B. Soil Texture, University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS)
Calculate Soil Texture from Soil Triangle, Claes
American Association of State Highway and Transportation Officials (AASHTO)

Soil Texture

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Separates

A soil separate is a group of mineral particles that fit within definite size limits expressed as diameter in millimeters.

Sand, silt, and clay particles, called soil separates, are defined by their size. According to the USDA: 

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

Sand Separates

Soil texture is a term commonly used to designate the proportionate distribution of the different sizes of mineral particles in a soil. It does not include any organic matter. These mineral particles vary in size from those easily seen with the unaided eye to those below the range of a high-powered microscope. According to their size, these mineral particles are grouped into "separates." Size limits on sand separates used by the USDA.

Sand Separate — Particle Diameter (mm)

Very Coarse sand — 2.0-1.0
Coarse sand — 1.0-0.5
Medium sand — 0.5-0.25
Fine sand — 0.25-0.10
Very fine sand — 0.10-0.05

Particles

Soil texture describes the proportions of soil particles less than 2 mm in diameter and larger than 2 mm in diameter. The separates larger than 2 mm in diameter, or course fragments, are described using terms such as stony, cobbly, gravelly, slaty, cherty, and flaggy. Each term has a precise meaning reflecting the size, shape, and composition of the coarse fragments. These fragments are part of the soil mass and they influence moisture storage, infiltration, and runoff. Fine particles are protected from blowing and washing by these coarse fragments.

Nature of Mineral Soils

Since various sizes of particle have quite different physical characteristics, the nature of mineral soils is determined to a remarkable degree by the particular separate that is present in larger amounts. Thus, a soil possessing a large amount of clay has quite different physical properties from one made up mostly of sand and/or silt. The analytical procedure by which the percentages of the various soil separates are obtained is called a mechanical analysis.

Mineral soils (that is, those soils consisting mainly of rock and mineral fragments, rather than plant remains and other accumulated organic materials) are a mixture of soil separates, and it is on the basis of the proportion of these various separates that the textural class names of soils are determined.

Soil Textural Classes

Rarely do soils consist entirely of a single separate, but instead are a mixture. Textural classes are based on different combinations of sand, silt, and clay. The twelve basic textural classes in order of increasing proportions of the fine separates and with appropriate abbreviations are:

  1. Sand (s)           
  2. Loamy sand (ls)
  3. Sandy loam (sl)
  4. Loam (l)
  5. Silt loam (sil)
  6. Silt (si)
  7. Sandy clay loam (scl)
  8. Clay loam (cl)
  9. Silty clay loam (sicl)
  10. Sandy clay (sc)
  11. Silty clay (sic)
  12. Clay (c)

Comparison of USDA and Engineering Classification Systems

The system for determining USDA texture is significantly different from the Unified and the American Association of State Highway and Transportation Officials (AASHTO) systems that are traditionally used by engineers. One major difference is that the cutoffs between particle sizes are different among the three systems.

Another important difference is that USDA texture depends entirely on particle size; the Unified and AASHTO designations depend not only on particle sizes but also on other properties such as Atterberg limits (liquid limit and plasticity index).

There is, unfortunately, no way to translate directly from the USDA system to the other systems and back. Sandy clay loam in the USDA system, for example, may be either SC or CL in the Unified system, depending on the percentages of different sizes of particles; similarly, it may be A-6 or A-2-6 in the AASHTO system. Conversely, CL from the Unified system may be clay, silty clay, silty clay loam, clay loam, loam, silt loam, sandy clay, or sandy clay loam in the USDA system, depending on the results of a mechanical analysis done using USDA standards; a soil designated A-6 in the AASHTO system may be clay loam, loam, silt loam, or sandy clay loam in the USDA system.

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