Soil Organic Matter
Functions of Organic Matter
While the relative importance of any given fraction of organic matter will vary from one soil to another and depend on factors such as climate and cropping history, organic matter influences plant growth primarily through its effect on the physical, chemical, and biological properties of the soil. For example, fresh crop residues which are readily broken down provide energy for key soil biological processes such as nutrient cycling. Humus is particularly important in the provision of nutrients, cation exchange, soil structure, water-holding capacity, and in supporting biological processes.
Influence on Soil Physical Properties
Organic matter performs many including enhancement of aggregation, aeration, water movement, and reduction of evaporation and thermal conductivity. Organic matter causes soil particles to clump together to form soil aggregates, and gummy substances produced by soil organisms bind the soil clumps. Humus particles that coat mineral particles also bind those particles together. Aggregated soils will allow water to enter and drain into the soil by creating a network of pores and channels throughout the soil profile. Because water infiltrates soil high in organic matter more quickly during rainstorms, less water runs off—water that can remove soil from the field. A well-aggregated soil has a variety of pore sizes and shapes. After a wetting event, some pores will drain completely and fill with fresh air, while others are small enough to hold onto the water, which will be available for plant uptake.
Influence on Soil Chemical Properties
Chemically, soil organic matter affects the cation exchange capacity and the capacity for buffering changes in soil pH. The capacity of a soil to retain positively charged nutrients such as calcium (Ca2⁺), magnesium (Mg2⁺), zinc (Zn2⁺), potassium (K⁺), and ammonium (NH4⁺) is known as cation exchange capacity (CEC). Soil organic matter increases CEC, which is a measure of the soil’s ability to retain cations or anions, i.e., soil nutrients. The CEC is determined by the texture, type of clay, organic matter content, and pH. In general, the higher the organic matter content in soil, the higher the CEC, and the more likely the soil will retain nutrients. Soil organic matter may provide 40 percent to 50 percent of the soil’s total CEC. Humus generally accounts for 50 to 90 percent of the cation-adsorbing power of mineral surface soils.
Influence on Microbial Community
Soil organic matter and soil microbial community are inextricably connected. There are many benefits to organic matter, most of which begin with enhancing the biological diversity and activity in the soil. As organic matter increases, microbial activity tends to increase. Organic matter consists of 58 percent carbon, which is required in combination with other nutrients for microbial activity. Microorganisms excrete compounds that also act as a binding agent for soil particles, which can increase aggregate stability, water infiltration, and water holding capacity.
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