from the definition of soil. The fine-earth fraction of soil particles ranges in size over four orders of magnitude: from 2.0 mm to smaller than 0.002 mm in diameter. Sand-sized particles are individually large enough (2.0 to 0.05 mm) to be seen by the naked eye and feel gritty when rubbed between the fingers in a moist state. Somewhat smaller, silt-sized particles (0.05 to 0.002 mm) are microscopic and feel smooth and slippery even when wet. Clay-sized particles are the smallest of the mineral particles (<0.002 mm), seen only with the aid of an electron microscope, and when wet form a sticky mass. The proportion of these different size classes in soil is referred to as soil texture, and terms such as sandy loam, silty clay, and clay loam are textural classes used to identify the soil's texture. When investigating a field site, considerable insights into the behavior and properties of the soil can be inferred from its texture, so it is often one of the first properties to be measured.
The surface of mineral soils contains an accumulation of living biomass, dead and decomposing organic material, and humus. This soil organic matter (SOM) typically accounts for 1-10% of the total soil mass, but because it is intimately associated with the mineral fraction it is difficult to isolate from the soil. The larger, recognizable remains of plant, animal, and soil organisms that can be separated from soils by hand picking and sieving techniques are referred to as parti-culate organic matter. These tissues undergo continuous decay, and over periods of years to decades, brown to black-colored, colloidal humus is synthesized and accumulates. Soil humic substances can account for 50-60% of the total SOM, and together with the nonhumic material provide a nutrient reservoir to sustain the soil microbial biomass.
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