How much organic matter is needed in the soil and what factors slow the decomposition of organic matter in the soil?
The amount of organic matter needed in the soil depends on the location and type of soil. Usually, soils with 3-6 % OM are quite productive if the are not affected by other limiting factors (depth, wetness, salinity, etc.) The main factors that slow the decomposition of OM are temperature and moisture and possibly toxic soil constitutents.
Think of environmental conditions under which humans are comfortable with regard to temperature and moisture. Soil microorganisms are most active under conditions similar to those where humans are comfortable. These are conditions at which most biological entities function at their best.
Soil organic matter is the fraction of the soil that consists of plant or animal tissue in various stages of breakdown. Most of our productive agricultural soils have between 3 and 6% organic matter. The amount of organic matter in mineral (sand, loam or clay) soils ranges from very low being 1% by weight, to average being 2 to 4%, and high being greater than 5%. There are also “muck” or organic or peat based soils that are 30 to 40% organic matter. The general consensus is the more soil organic matter the better. The basic components of soil are minerals, organic matter, water and air. The typical soil consists of approximately 45% mineral, 5% organic matter, 20-30% water, and 20-30% air.Organic matter is important for better soil fertility and structure, and overall soil health. To increase organic matter levels you can: add organic materials like solid or liquid manures, plants or crop residues and reduce drainage or cultivation of soil to slow the decay of organic matter. Organic matter improves soil structure, which results in increased water infiltration following rains and increased water-holding capacity of the soil; it also enhances root growth into more permeable soil. This results in better plant health and allows more movement of mobile nutrients to the root. Soil organic matter significantly improves the soil's capacity to store and supply essential nutrients, and to retain toxic elements. It allows the soil to cope with changes in soil acidity, and helps soil minerals to decompose faster. Organic matter plays a significant role in crop production and soil health by improving physical, chemical and biological functions in the soil. Increasing levels of organic matter aid in soil structure, water-holding capacity, nutrient mineralization, biological activity, and water and air infiltration rates. Climatic conditions, such as rainfall and temperature, and soil moisture and aeration (oxygen levels) affect the rate of organic matter decomposition. Organic matter decomposes faster in warm, humid climates and slower in cool, dry climates. Salinity, toxicity and extremes in soil pH (acid or alkaline) result in poor biomass production and, thus in reduced additions of organic matter to the soil. Inherent factors affecting soil organic matter include climate and soil texture and clay mineralogy. Climatic conditions, such as rainfall and temperature, and soil moisture and aeration (oxygen levels) affect the rate of organic matter decomposition. Organic matter breakdown is regulated by many factors, including soil moisture, thermal regimes, soil texture, bedrock type, nutrient status, water capacity, illuviation and bioturbation rates, root penetration resistance, and the availability of oxygen to support aerobic microbial activities. A multitude of factors can affect the decomposition process, increasing or decreasing its rate. Some of the most frequently observed variables are temperature, moisture, insect activity, and sun or shade exposure. Coverings can impact the decomposition process, and are found frequently in forensic cases.
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