Soil organic matter is non-uniform in nature and does not have any defined physical or chemical structure. It can be present in soils under various stages of decomposition. Crop and plant residue, tree litter, livestock manure (Figure 1), animals and different types of soil organisms, their by-products and to a lesser extent human waste contribute to form organic matter. The active fraction (roots and residue) is the food source of soil microorganisms. The stable fraction (humus), stores carbon and reduces loss of organic carbon to decomposition.

Decomposition Process and Functions

Earthworms, mites and beetles shred organic material into small pieces. Nematodes consume carbonaceous living material such as plants roots, microorganisms and other nematodes. Bacteria and fungi break down and decompose organic matter while feeding on it, releasing essential nutrients: nitrogen, phosphorous, potassium, sulfur and others. Microorganisms also release enzymes that increase the rate of decay (Fortuna, 2012). As organisms decompose organic matter, they use carbon as a source of energy and nitrogen to produce protein. On average microorganisms require 25 to 30 parts of carbon for every part of nitrogen they consume. Therefore, high C:N ratios above 30:1 will result in slower decomposition rates, whereas, low C:N ratios below 20:1 will result in nitrogen losses.

Though small in proportion by weight, organic matter is linked to important functions in the soils and is critical to maximizing biological activity within the soil. Humus along with clay particles provide cation exchange sites in soils, reducing nutrient leaching losses. Microorganisms using organic matter as a food source promote aggregation of soil particles, resulting in improved structure (Figure 2), soil water infiltration, and holding capacity. It reduces the potential for runoff and erosion as well. Higher organic matter levels also result in favorable soil temperatures, improved plant root growth, healthy microbial populations, and moderate pH ranges.

Practices That Can Increase Soil Organic Matter Levels

• Rotate crops to include:
  • cover crops to provide soil cover to reduce evaporation, erosion, and runoff;
  • perennial grasses to add aboveground as well as belowground biomass;
  • legumes for green manure purposes; and
  • crops/plants that produce greater biomass.
• Incorporate straw/crop residues.
• Reduce tillage to minimize soil carbon losses and to slow organic matter decomposition processes.
• Apply fertilizers on cover crops and legumes to produce more biomass.
• Apply manure, plant material, or other carbon-rich waste.
• Graze, rather than harvest forage.