Soil is the thin outer zone of the earth's crust that supports rooted plants and is the product of climate and living organisms acting on rock. A true soil is a mixture of air, water, mineral, and organic components. The relative mix of these components determines both the value of the soil for agricultural and other human uses and the extent to which chemicals or biological organisms added to soil will be transported and/or transformed within the soil. Soils are characteristically heterogeneous. A trench dug into the soil zone typically reveals several horizontal layers having different colors and textures. These layers and their generic structure are illustrated in Figure 1. These multiple layers are often divided into three major horizons-(1) the A horizon, which encompasses the root zone and contains a high concentration of organic matter; (2) the B horizon, which is unsaturated, is below the roots of most plants, and contains a much lower organic carbon content; and (3) the C horizon, which is the unsaturated zone of weathered parent rock consisting of bedrock, alluvial material, glacial material, and/or soil of an earlier geological period.