Soil chemistry can be considered as the natural chemical composition of a given soil. This natural chemical composition of a soil is a function of that soil’s parent material. In many areas of the world soil is formed in place and derived directly from the weathering and degradation of rocks. When soil is derived from rocks, its soil chemistry is a direct reflection of the rocks’ chemistry, including the minerals found in the rocks. There are different forms of rocks, including the major classifications of sedimentary, metamorphic, and igneous (Fig. 5.2). Within each of these major separations there are different categories of rocks. Thus, rocks are a collection of different minerals. Soils derived from these different rocks will have different chemical complexes. For example, soils derived from sandstone will have a limited array of different chemicals, whereas soils derived from igneous rock might have a greater diversity of minerals and chemicals. However, not all soils are formed in situ from the rocks that exist in place.