The remains of plants and animals, together with the products of their decay, make up the dead material. When this has become more or less decomposed it is called humus. It helps bind soil particles together and serves as food for plants but especially as food for the bacteria and other organisms which are essential to a healthy soil in good physical condition. Peat-like organic matter, which has not yet decayed much, helps to keep soil loose, porous, and able to contain needful amounts of air and water. The inorganic or mineral matter was derived from the decomposition of rocks by the chemical and mechanical processes of weathering, or from rocks ground by glaciers into gravel, sand, and that powder-like substance called clay.
Sand grains are plainly visible and feel gritty.
Clay particles are too fine to be distinguished by eye and a large proportion of them cannot be seen under an ordinary microscope. It is the clay content that makes a soil sticky when wet. Coarse and fine sands comprise more than 70 percent of a sandy soil. From 25 to 35 percent of a clay soil is clay. Loams range in clay content from 10 to 25 percent, less than 50 percent sand, and from 25 to 50 percent silt.