Soil Organic Matter

Soil organic matter (SOM) supports soil health because of the beneficial chemical, physical and biological properties it imparts. SOM increases a soil's ability to hold nutrients by directly increasing cation exchange capacity. SOM also benefits the physical properties of soil by building soil structure and increasing water holding capacity. Finally, SOM supports many of the biological properties of soil health through microbial mineralization of nutrients, development of mycorrhizal relationships with crop roots, and production of sticky substances that hold soil aggregates together.

SOM is composed of materials containing carbon that came from living organisms, including plant and animal remains, bacteria and fungi, etc., in various stages of decomposition. SOM includes humus, which is mostly dead microbes and decomposed organic matter that has reached a relatively stable stage. Native SOM content in soils used for vegetable production in New England is almost always less than 10% and typically in the 2%-6% range. Several factors control the amount of SOM a soil may have. These factors include soil texture and drainage. Well drained, course textured soils tend to have lower levels of SOM, due in part to the rapid microbial decomposition rates favored by these soil conditions. In contrast, in loam soils it is reasonable to aim for 4%-6% organic matter (see discussion below on Building Soil Organic Matter). In any case, it is important to maintain SOM because it is typically the most important component of soil for nutrient supply, water holding capacity, cation exchange capacity, and soil structure. 

SOM supplies nutrients through the process of mineralization, which is the microbial decomposition of organic compounds into carbon dioxide and their mineral constituents. Soil microbes are most active in warm soils (over 70°F) that are moist, but well aerated, with a pH between 6 and 7 (also ideal conditions for most vegetable crops). Mineralization of nutrients will proceed rapidly under these conditions.

SOM has a large influence on water holding capacity directly by its ability to absorb large amounts of water and indirectly by improving soil structure, which creates more pore space for storage of water as well as larger pores for holding air. Soil structure is enhanced by SOM because, as it decomposes, sticky compounds like gums, carbohydrates, and resins are produced by microorganisms. These gums cement soil particles together into secondary aggregates. The cation exchange capacity of soils is controlled by both clay and SOM content, with both types of particles supplying negatively charged sites that hold cations. In most New England soils, the humus portion of SOM accounts for the vast majority of the cation exchange capacity because soils are typically low in clay content. See also Cation Exchange Capacity and Base Saturation