Building Soil Organic Matter

Soil Organic Matter (SOM) is in a constant state of flux – additions and losses are simultaneously occurring. To maintain SOM levels, one must reduce the rate of SOM losses while increasing the rate of SOM additions.

SOM can be lost from soil by 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 mineral elements. The rate of microbial decomposition of SOM is controlled by a number of factors including: soil temperature, moisture, aeration (oxygen), and the quality or characteristics of the SOM. The rate of decomposition can be greatly influenced by management. For example, aggressive tillage and cultivation increases aeration and destroys the 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, page 20).

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 methods of maintaining or increasing SOM additions. Most vegetables leave little residue in the field and SOM will usually decrease if only vegetable residues remain in fields. However, significant amounts of biomass can be added to the soil by including cover crops in a rotation. Although cover crops can add enough biomass to maintain SOM, it is difficult to increase SOM with most cover crops. Sod forming crops included in the rotation, however, can increase SOM. Another more rapid and direct method of increasing SOM is by adding organic amendments such as organic mulch 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, page 13).