Soil organic matter (SOM) is in a constant state of flux, with additions and losses simultaneously occurring. To maintain SOM levels, one must ensure that losses do not exceed additions.
SOM can be lost from soil through both wind and water erosion. However, the primary mechanism for SOM loss is microbial decomposition. Soil microbes use SOM as a source of energy and nutrition, converting SOM into carbon dioxide and its constituent mineral elements. The rate of SOM decomposition is controlled by a number of factors including soil temperature, moisture, aeration (oxygen), and the quality or characteristics of the SOM. In agricultural soils, these factors are all greatly influenced by soil management. For example, aggressive tillage and cultivation increases aeration and breaks apart soil aggregates that protect SOM from microbial decomposition. Reducing tillage and cultivation is an effective management strategy to maintain, or even increase, SOM content (see Reduced Tillage).
There are a number of ways to increase or maintain SOM. Increasing the quantity of plant residues returned to the soil is one of the most sustainable strategies for maintaining SOM. Most vegetables leave little residue in the field, and SOM will usually decrease if these are the only residues provided to the system. Although cover crops can add enough biomass to maintain SOM, it can be difficult to increase SOM with some cover crop species or mixes. Including sod-forming crops in the rotation, however, can increase SOM. A more rapid and direct method of increasing SOM is by adding organic amendments, such as organic mulches and compost. While the application of organic amendments can rapidly increase SOM, extractable soil phosphorous (P) concentrations must be monitored to avoid excessive applications (see Fertilizers and Soil Amendments).