Organic matter is the basis of soil productivity. Plants can be grown in the greenhouse in quartz sand by constantly supplying them with nutrients in a solution. But, under field conditions, the soil itself must contain plant food; it must hold water and yet be well drained; it must contain bacteria to cause decomposition of organic material and release plant foods; it must be well aerated. Not only is organic matter a source of nitrogen, but it provides a means by which undue loss of nitrogen through leaching and volatilization is prevented. It lightens heavy soils and holds lighter soils together.
Sources of organic matter. Plants are the original source of organic matter. Organic matter in wooded areas has been built up from decaying leaves and the rotting roots and tops of trees and other vegetation that has fallen back on the soil. In treeless regions the prairie grasses have built up an accumulation of organic material. On cropped land the crop residues in the form of roots, straw, stalks, and leaves not otherwise used are returned to the land to provide its organic content. Commonly, many of these crop residues are used for feed and bedding, and considerable organic material is eventually returned to the soil in the form of farm manures.
Coarse sands and gravel soils may contain less than 1 percent of organic matter, a good mineral soil 3 to 6 per cent, and peat 90 percent or more.
C. rather, howard, et.al,. 1951. Field Crops. McGraw-Hill book company: New York.