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Organic matter plays a key role in the productivity of soil. It improves soil‐water relations and slowly releases plant nutrients. The many cation exchange sites on organic matter affect nutrient movement and herbicide activity. The organic matter content of a soil changes very slowly under different cultivation practices.
Most people think of soil organic matter as decaying plant matter, like corn stalks or wheat stubble. However, the largest portion of organic matter in the soil is far more degraded than that.
It is largely in the humus form which is dark gray in color, resembling coal. This is what imparts the dark color to topsoil, soils with high organic matter tend to be darker.
As microbes decompose plant and animal matter, most of it goes into the atmosphere as carbon dioxide. The residue is decomposed again and again until stable compounds are farmed. These compounds – known as humus make up the bulk of soil organic matter. Each percent organic matter in an acre foot of soil represents 18 tons of soil.
Organic matter is less dense than soil minerals so as organic matter increases, soil density decreases. The measurement of this soil property is called bulk density. Soils of lower bulk density are usually more productive because they are porous and allow for good water and air movement.
Organic matter binds soil particles into aggregates creating a variety of large and small pores. Roots proliferate best in well aggregated soil because there is adequate aeration, water moves easily into the soil, and roots grow easily in the larger pores.
Cation exchange capacity (CEC) is a measure of the negative charge on a soil expressed as milliequivalents per 100 grams (meq/100g). In some soils half of the CEC is attributable to organic matter and the rest to clays.
The CEC of organic matter runs from 200 to 400 meq/100g whereas a clay like montmorillonite, runs from 80 to 150 meq/100g. Increasing CEC slows down leaching of potassium and dampens the effectiveness of many herbicides.
Since organic matter is made from plant material it contains all of the essential plant nutrients in a concentrated farm. These nutrients are released through the continuous cycle of microbial decomposition. For example, an acre foot of soil holds about 1800 lbs. of nitrogen in each percent organic matter, but the nitrogen is released sparingly.
Depending on the climatic zone, 20 to 50 lbs. of nitrogen per one percent organic matter are released annually. Phosphate, sulfur and zinc are released sparingly, whereas potassium is released immediately from decomposing vegetation.
At the beginning of decomposition wheat straw is about 60 parts carbon to one part of nitrogen. In the humus form, the carbon:nitrogen ratio is 10:1. Humus is about 58% carbon, 5% nitrogen, 0.5% phosphorus and 0.5% sulfur.
As decomposition continues, plant nutrients become more concentrated in the organic matter and carbon is lost as carbon dioxide. Soil organic matter content is largely influenced by climate.
Rainfall affects the amount of vegetation produced and temperature controls the rate at which it decomposes. Regions with high rainfall produce lush plant growth which leads to increased accumulation of soil organic matter. In cool climates decomposition is slow, but in tropical climates it is rapid.
Therefore a tropical climate may produce considerable vegetation that will be decomposed rapidly whereas cool humid environments also produce lush plant growth but it decomposes slowly. In most cases the organic matter content of the cooler soils will be higher due to decreased decomposition rate.
The temperature effect is evident as one travels from south to north on the Great Plains ‐ soils get darker or higher in organic matter as one moves north. The rainfall effect can be seen traveling from west to east as soils are much darker in Iowa and Missouri than in Colorado and Wyoming.
Organic matter accumulates and forms topsoil as plant roots and tops decay on the soil surface. This process takes thousands of years. The top soils of the Great Plains were formed under grass prairies and many of the agricultural soils of the eastern USA were farmed under forests.
Agriculture has decreased the organic matter content of some soils of the Great Plains. Plowing has been largely responsible for this because it aerates the soil and enhances microbial decomposition.
Cropping practices that remove all of the vegetative growth from the field also decrease organic matter content. Organic matter content can be slowly increased through high yields and proper straw management.
Each percent of organic matter represents about 18 tons per acre‐foot of soil, so to build the level by 0.1 percent, it will take 1.8 tons of humus. It may take 20 tons of plant dry matter to produce 1 ton of humus.
Manure is also useful in producing humus and supplies useful plant nutrients which in turn produce more vegetation. Commercial fertilizers also aid in increasing organic matter by promoting higher yields and more plant dry matter.