SPADA Draft Documents

( l ) Humus .—The more or less stable fraction of the soil organic matter remaining 88 after the major portions of added plant and animal residues have decomposed. Usually it 89 is dark in color. 90 ( m ) Silt .—Comprised of particles in the size diameter range of 0.02-0.002 mm. 91 Smaller than sand and more difficult to drain. 92 ( n ) Soil horizon .—A layer of soil, approximately parallel to the soil surface, differing 93 in properties and characteristics from adjacent layers below or above it. 94 ( o ) Soil profile .—A vertical section of the soil through all its horizons and extending 95 into the parent material. 96 ( p ) Solum .—Comprised of surface and subsoil layers that have undergone the same 97 soil-forming conditions. 101 testing with soils difficult to scope. The study of soil is interdisciplinary involving 102 chemistry, biology, physics, genesis and taxonomy, in addition to agricultural and 103 conservation practices. Soils are an important natural resource. They are a medium for 104 plant growth, a regulator for water supply, a recycler of raw materials, a habitat for soil 105 organisms, an engineering medium and an environmental interface. Overall, soils are a 106 very complex matrix including physical and living components that lead to ever-changing 107 compositions. 108 Variability in soils can be problematic. The physical and living components of soil 109 change with depth of the soil, leading to soil horizons in a single soil profile that have 110 98 99 100 6.0 Background Information on Soil There are over 19,000 identified soils in the United States alone, making experimental

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