One particular regolith, soil, is of special importance to us because it provides a necessary substrate for plants. In turn we depend on plants, not only for food and oxygen, but also for lumber, drugs, and climate regulation, to mention but a few obvious benefits. Soils, defined as regolith capable of supporting cultivation, are one of our most precious natural resources. Plants depend on them for an anchoring base, oxygen, moisture and trace ions, all of which they use in their metabolic processes. To be fertile, a soil must have clays to hold ions, sand to aerate the soil and allow the passage of water and oxygen to the roots, and dissolved chemicals to provide the requisite nutrients. Any soil which lacks one or more of these vital ingredients will tend to be a poor soil. 

Soils are physical and chemical weathering products. Therefore, the same processes that create other weathering products will also lead to soil formation. As bare rock is exposed to the action of atmospheric chemicals, it begins to disintegrate and decompose. The uppermost layer of rock, will be broken apart and the materials will begin to chemically weather. This partially changed bed rock is called the C horizon. As time goes on this C horizon will continue to decompose into a mixture of clays and sand while the breakup and changes continue to affect the original rock at greater and greater depth. This undifferentiated surface layer of clays and sand will continue to react to the passage of water and slowly, over time, the fine clay particles will work their way deeper into the forming soil, leaving a concentration of sand at the surface. This zone where the clays and chemicals tend to accumulate is called the B horizon, while the upper sand richer zone is called the A horizon. A well developed mature soil will have all three of these horizons. 

At first glance, there seems to be a tremendous variety of soils. From a genetic and evolutionary standpoint, soils are weathering products. The dominant factor in weathering is climate, mainly the temperature and the amount and pattern of rainfall. Disregarding special conditions such as those where physical weathering processes predominate (such as those in arctic and mountain areas), and those prevalent in saturated materials, there are two main types of climates which control soil formation: dry and wet. 

Dry conditions will generally lead to accumulation of chemicals in soils, whereas wet conditions favor leaching, the selective removal of the more soluble ions. Temperature affects the speed at which chemical reactions proceed. For every 20 degrees Celcius rise in temperature, the speed of the reaction approximately doubles. Given the importance of water in the weathering processes, it is easy to see that weathering will be most active in hot and wet (tropical) climates.