A soil is a body of weathered mineral and organic materials that has been altered through time into a series of discrete layers. The movement of water through the materials facilitates the development of layers (horizons) that are distinguished by whether they have lost or accumulated different mineral and organic constituents (Figure 1). A number of strongly interrelated variables contribute to the development of a soil; these are most succinctly summarized by Hans Jenny's clorpt equation, which emphasizes the interaction of the five soil-forming factors of climate, organisms, relief, parent material, and time.

Every soil starts from an accumulation of some type of parent material (fractured bedrock, river alluvium, glacial till, wind-blown loess, plant residue, etc.) that is then altered through time by weathering processes that ...