Amount of Organic Matter in Soils

The depletion of the soil humus supply is apt to be a fundamental cause of lowered crop yields.

—J.H. HILLS, C.H. JONES, AND C. CUTLER, 1908

The amount of organic matter in any particular soil is the result of a wide variety of environmental, soil, and agronomic influences. Some of these, such as climate and soil texture, are naturally occurring. Agricultural practices also influence soil organic matter levels. Tillage, crop rotation, and manuring practices all can have profound effects on the amount of soil organic matter. Hans Jenny carried out pioneering work on the effect of natural influences on soil organic matter levels in the U.S. more than sixty years ago.

The amount of organic matter in a soil is the result of all the additions and losses of organic matter that have occurred over the years (figure 3.1). In this chapter, we will look at why different soils have different organic matter levels. While we will be looking mainly at the total amount of organic matter, keep in mind that all three “types” of organic matter—the living, dead, and very dead—serve critical roles and the amount of each of these may be affected differently by natural factors and agricultural practices.

If additions are greater than losses, organic matter increases. When additions are less than losses, there is a depletion of soil organic matter. When the system is in balance and additions equal losses, the quantity of soil organic matter doesn’t change over the years.Anything that adds large amounts of organic residues to a soil may increase organic matter. On the other hand, anything that causes soil organic matter to decompose more rapidly or be lost through erosion may deplete organic matter.