Chapter summary

Soils are the site of much geochemical and biological activity. Chemical weathering occurs when water, acids, and other substances react with minerals in rocks and soils. It occurs concurrent with physical weathering, which is the physical disintegration of rocks by various forces. Silicate clays and iron or aluminum oxide clays are the resistant end-products of chemical weathering. Chemical weathering releases elements into the soil solution for uptake by plants. In addition, nutrients contained in plant debris are mineralized when the organic material in soil decomposes. The rate of decomposition varies with temperature, soil water, and the chemical quality of litter. The weathering of rocks and decomposition of organic matter are part of the larger carbon cycle. The outcome of these processes is also seen in the soil profile and its development over time. There are 12 broad classes of soil, known as soil orders, that vary in relation to degree of weather, extent of soil development, climate, and associated vegetation. Soil formation is greatly influenced by parent material, time, topography, climate, and vegetation. Climate, particularly temperature and precipitation, determines the nature and rate of the weathering that occurs. Vegetation affects soil structure and fertility through the cycling of materials between plants and soil.

Weathering

The sand, silt, and clay particles that comprise mineral soil are derived from physical and chemical weathering that breaks rocks into smaller and smaller fragments until individual minerals are exposed or new minerals are created.