n. The relative distribution of different sizes of mineral particles in a soil.
Soil texture refers to the mineral particles of a soil and does not refer to organic matter. Mineral particles vary in size from those easily seen with the unaided eye to those below the range of a high-powered microscope. These mineral particles are grouped into size classes, called separates. Sizes of the separates used in the USDA’s system of nomenclature for soil texture range from 2.0 to 0.002 mm.
The various sizes of mineral particles have quite different physical characteristics, so the nature of a mineral soil is determined largely by the particular separate that is present in larger amounts. Thus, a soil possessing a large amount of clay-sized particles has quite different physical properties from one made up mostly of sand- and/or silt-sized particles. The analytical procedure by which the percentages of the various soil separates are obtained is referred to as a “mechanical analysis.”
A mineral soil (that is, one consisting mainly of rock and mineral fragments rather than plant remains and other accumulated organic materials) is a mixture of soil separates, and it is on the basis of the proportion of these various separates that the textural class names of a soil are determined. See Brown (1990, available at http://edis.ifas.ufl.edu/SS169) and the Soil Science Society of America’s website (http://www.soils.org/?check) for further information.