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 contributes to soil structure and increases water holding capacity. Finally, SOM supports soil biology as a substrate for the growth of soil microbes. 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. The other component is active SOM, which is relatively undecomposed and is accessible to soil microbes for mineralization. It is important to maintain both types of SOM (see Building Soil Organic Matter).

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 (i.e. compounds that contain carbon) into carbon dioxide and their mineral constituents (including inorganic, plant-available forms of nutrients). Soil microbes are most active in warm soils (over 70°F) that are moist, well-aerated, and have a pH between 6 and 7, which are 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 abundant soil microbes. On average, there is a release of about 10-20 lb N per acre for each 1% soil organic matter over a full season. This is capped at about 4% or 40 lb per acre. Nitrogen contributions from SOM can be subtracted from the total amount of fertilizer-N recommended on soil tests, though because these releases of N vary with drainage and other soil conditions, they may not be well timed to crop needs, especially for early, short season crops. 

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. As microbes decompose SOM, they produce sticky compounds such as gums, carbohydrates, and resins, which bind soil particles together into secondary aggregates. These aggregates bolster 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.