Erosion is the removal by water, wind, glaciers, or gravity of weathered rocks from the place where they formed. As soil particles become smaller, chemical processes take on added importance because of the larger surface area available for reactions to take place. Chemical breakdown occurs toward progressively more stable products. Generally, the more quartz a rock has, the more resistant it is to weathering into soil. Colors of soils are almost entirely related to iron compounds and humus content.
Pure sand is too porous to hold water.
Pure clay is too impervious to drain. Loam is a fertile mixture of sand, clay, silt, and humus. Downward movement of dissolved material is known as leaching. Precipitation of the leachate occurs in a zone of accumulation. Between 200 and 1000 years are required to make an inch of topsoil.