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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
Soils normally are composed of air, water, mineral matter, and organic matter. The composition of an average textbook soil is: 5 percent organic matter; 25 percent air; 25 percent water; and 45 percent mineral matter. Utah soils contain less than 1 percent organic matter.
Mineral matter is the weathered rock in the soil. Organic matter is the decomposed animal and plant materials.
Soil organic matter is dynamic and changes continually through further decomposition. You maintain the stability in quantity and in quality through adding new raw materials. Organic matter is a temporary product or stage in a natural cycle.
Organic matter is formed by the biological decomposition of plant and animal residues. Factors that regulate decomposition are temperature, moisture, aeration, acidity, supply of plant nutrients, tillage, and the kind and amount of crop residues and manures returned to the soil. In the decomposition process, some plant substances are converted rapidly to carbon dioxide, water, and minerals (mineralization). Other substances are only slightly altered at first.
Decomposition is rapid when fresh plant residues begin to decay. As the organisms consume the more easily decomposable materials, the level of activity is reduced.
The kind of plant or animal residue from which soil organic matter is formed also influences the activity of the organisms. Some plant materials resist decomposition, others decompose quickly.
Adding organic matter to soils is essential. Composting is our attempt to duplicate and speed the natural decaying process of organic matter. The compost pile lets gardeners return garden refuse to the soil in a usable form. As a soil conditioner, organic matter can improve aeration, water holding capacity, structure and tilth (workability of the soil), and nutrient storage.
Organic matter added directly to the soil provides a source of food for the soil micro organisms responsible for the decomposition process. Composting facilitates the initial stages of decomposition of organic matter before it is added to the soil. Composting is done because the organic material is inconvenient for incorporating it into the soil at a given time.
Micro organisms utilize carbon, in the form of sugars or starch, as an energy source for growth. They require nitrogen for making protein. When materials with too much carbon, such as straw or paper, are added to the soil a nitrogen deficiency may be created in growing plants. This is due to a rapid increase in the organism population because of a large food supply. This results in competition between organisms and crop plants for available nitrogen.
As decomposition begins in a compost pile, temperatures rise to about 140 F in the center. Large piles are more consistent in heating than small ones. In small piles or bins, interior temperature may never go much higher than about 105 F. Decomposition is more rapid at the higher temperatures.
Moisture content of the pile also affects rates of decomposition. The pile should be kept damp but not wet. Aeration, increased by turning the pile, will also speed decomposition. No other additives or inoculants appear justified in most cases. Shredding or grinding materials for composting increases surface area and speeds decomposition.