ABSTRACT
As discussed in Chapters 2 and 3, the solid constituents of soils consist of a variety of inorganic and organic compounds. Most of the coarse materials, such as sand, silt, and undecomposed organic matter are chemically inert. They are important constituents for building up the soil and affect many soil properties. Since they are coarse in size, they have low specific surface area and do not exhibit colloidal properties. They may participate in a number of soil reactions and exhibit some adsorption capacities, but they are not really chemically active. However, sand and silt may weather to form clay, whereas the undecomposed organic matter will be broken down by decomposition processes into humus. Clay and humus are the smallest constituents of soils and exhibit colloidal properties. They have large specific surface areas and display a surface chemistry different from the coarse materials. The surface chemistry of clay and humus is attributed to the presence of electrical charges in their molecules. Because of these charges they are chemically very active and are considered the seat of soil activity (Brady, 1990), causing many soil
chemical reactions, such as cation exchange, anion exchange, and complex and chelation reactions.
