Soil Organic Matter

Soil organic matter (SOM) is critical for soil health because of the beneficial chemical, physical and biological properties it imparts. SOM increases soil nutrient availability, being a source of nutrients itself as well as by increasing cation exchange capacity. SOM also benefits the physical properties of soil by contributing to soil structure and increasing water holding capacity. Finally, SOM supports soil biology as a substrate for the growth of soil microbial communities. In turn, these soil microbes mineralize SOM into plant-available nutrients, develop symbiotic mycorrhizal relationships between soil fungi and plant roots, and produce sticky substances responsible for binding soil aggregates.

SOM is composed of materials containing carbon that came from living organisms, including plant and animal residues, organic-based amendments, and soil bacteria and fungi, all in various stages of decomposition. SOM is often divided into two categories. The more stable component is humus, which is primarily comprised of long-dead material that is highly decomposed and beneficial for soil structure and carbon storage. Active SOM, on the other hand, is relatively undecomposed and accessible to soil microbes for mineralization. It is important to maintain both types of SOM (see 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. 

Native total SOM content in soils used for vegetable production in New England is almost always less than 10% and typically in the 2-6% range. SOM accumulation is generally limited by several abiotic factors, including temperature, moisture, and soil texture. Well-drained, coarsely 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, loamy soils often have 3-6% SOM. 

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, well-aerated, and have a pH between 6 and 7 (also ideal conditions for most vegetable crops). Mineralization of nutrients will proceed rapidly under these conditions, provided there is an adequate supply of SOM and an abundant soil microbial community.

SOM directly influences water holding capacity through its capacity to absorb large amounts of water. It also indirectly boosts water holding capacity by improving soil structure, creating more pore space for water storage and larger pores for air. Soil structure is enhanced by SOM because, as it decomposes, sticky compounds like gums, carbohydrates, and resins are produced by microorganisms. These gums bind soil particles together into secondary aggregates. This in turn bolsters cation exchange capacity, which is influenced by both clay and SOM content (both supply negatively charged sites that hold cations). In most New England soils, the stable humus portion of SOM accounts for the vast majority of the cation exchange capacity, as these soils are typically low in clay content. See also Cation Exchange Capacity and Base Saturation