Amount of Organic Matter in Soils
Amount of Organic Matter in Soils 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 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. Anything that adds large amounts of organic residues to 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.
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. NATURAL FACTORS Temperature In the United States, it is easy to see how temperature affects soil organic matter levels. Traveling from north to south, higher average temperatures lead to less soil organic matter. As the climate gets warmer, two things tend to happen (as long as rainfall is sufficient): More vegetation is produced because the growing season is longer, and the rate of decomposition of organic materials in soils increases because soil organisms work more rapidly and are active for longer periods of the year at higher temperatures. Faster decomposition with warmer temperatures becomes the dominant influence of deter-mining soil organic matter levels. Rainfall Soils in arid climates usually have low amounts of organic matter. In a very dry climate, such as a desert, there is little growth of vegetation. Decomposition is also low because of low amounts of organic inputs and low microorganism activity when the soil is dry. When it finally rains, a very rapid burst of decomposition of soil organic matter occurs. Soil organic matter levels generally increase as average annual precipitation increases. With more rainfall, more water is available to plants, and more plant growth results. As rainfall increases, more residues return to the soil from grasses or trees. At the same time, soils in high rainfall areas may have less organic matter decomposition than well-aerated soils— decomposition is slowed by restricted aeration. Soil Texture Fine-textured soils, containing high percentages of clay and silt, tend to have naturally higher amounts of soil organic matter than coarse-textured sands or sandy loams. The organic matter content of sands may be less than 1%; loams may have 2% to 3%, and clays from 4% to more than 5%. The strong chemical bonds that develop between organic matter and clay and fine silt protect organic molecules from attack and decomposition by microorganisms and their enzymes. Also, clay and fine silt combine with organic matter to form very small aggregates that in turn protect the organic matter inside from organisms and their enzymes. In addition, fine-textured soils tend to have smaller pores and less oxygen than coarser soils. This also limits decomposition rates, one of the reasons that organic matter levels in fine-textured soils are higher than in sands and loams.
