Soil Components Soils are composed of mineral particles of various sizes, and humus , which is decaying organic matter .
Soils contain differing proportions of air , water , and living organisms , such as earthworms.
The mineral particles are the products of weathering of parent material, and usually contain two or more elements , most commonly silica, aluminum, magnesium and iron . The mineral particles are classified by size from largest to smallest as sand, silt, and clay . The diameter of sand particles ranges from 0.02 mm to 2 mm , and soils dominated by sand have good aeration , but poor water-holding capacity . Clay particles, the smallest, average less than 0.002 mm in diameter, and have very good water-holding capacity , but poor aeration . Silt particles range in average diameter from 0.002 to 0.02 mm .