Addition of animal manure and phosphorus resulted in elevated organic matter levels. A rotation of continuous cereal grains resulted in a decrease in soil organic matter levels. An example of the potential role cropping system and rotation can have on soil organic matter. The following information illustrates the amount of fresh organic residue left following certain crops. Estimated addition of organic matter to soils by crops grown and residues left on the field.
Tillage also affects organic matter concentrations.
The greater the amount of tillage, the less the amount of humus that accumulates. Uncultivated soils are higher in total soil organic matter than they are after cultivation. Tillage reduces organic matter levels by accelerating the oxidation process. This happens when the organic matter is broken into finer pieces and mixed with air in moist soil. Cultivation of grassland soils leads to depletion of soil organic matter.