The solid portion of soil is composed of minerals and organic matter. The organic matter includes plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by the soil biota. Each of these contains many types of compounds including proteins, sugars, polysaccharides such as cellulose, hemicellulose, and starch, and complex fats, waxes, and lignins. All the lifeessential elements are contained in the collection of organic compounds found in soils.
The total amount of organic matter in soil and the nutrients it contains vary with the climate, nature of the parent material, soil pH, and the kind and amount of vegetation produced on or applied to the soil.
The process of soil organic matter decomposition and humus formation may be represented as a partial oxidation process as follows:
organic substances + O2 + microorganisms + suitable environment ------> CO2 + H2O + Energy (both as heat and for microbial growth) + humus + new microbial cells + inorganic plant nutrients (Ca2+, Cl-, Co2+, Cu2+, Fe2+, H3BO3, H2PO4-, K+, Mg2+, Mn2+, MoO42-, NH4+, Ni2+, S2-, and Zn2+)
Under natural soil conditions, organic residues undergo an initial rapid decomposition, which liberates large amounts of carbon dioxide, water, energy, and releases small quantities of inorganic nitrogen, phosphorus, sulfur and other plant nutrients. Probably the most important function soil microorganisms serve is to recycle organic carbon to carbon dioxide, thereby maintaining photosynthesis on the earth.
The microbial oxidation process slowly decomposes plant and animal residues. Nutrients contained in the plant and animal tissue are transformed from unavailable organic forms to plant-available inorganic forms during the microbial oxidation. Hence, organic matter acts similar to a very slow-release fertilizer and buffers the soil-plant environment against drastic fluctuations in available plant nutrients. The complete cycling of carbon through plants, animals, organic residue, humus, and CO2 eventually occurs.
A soil containing one percent organic matter contains up to 45 million kilocalories of potential energy per acre-furrow-slice. This energy is equivalent in heating value to about 6 tons of coal, or 31 barrels of crude oil or 168,000 cubic feet of natural gas. Soil microorganisms use only a small amount of the energy from the decomposition of organic residues for building new cells. The production of a single pound of soil organic matter from plant residues that have been amended with fertilizer nitrogen requires the total destruction of perhaps 10 to 20 pounds of plant residues to provide the energy for the process. Furthermore, whenever fresh residues are applied to soil, the decomposition of "humified" organic materials already present in the soil is accelerated.
The principal reasons for adding organic residues to soils are to modify the tilth of the soil, making seed bed preparation easier, to add plant nutrients, and to dispose of unusable or unwanted organic waste. Other benefits of adding organic matter to soils include improved soil structure, increased cation exchange capacity, increased water holding capacity, increased aeration, and a reduction in soil erosion.