Secondary Effects of Soil Organic Matter
The previous discussion covered some of the direct effects of soil organic matter, mainly on soil structure. There are other effects of soil organic matter which are not so visible, but which may be just as important as improved structure.
Organic matter is a reservoir of certain plant nutrients, especially nitrogen, phosphorus, and sulfur. Many soils have 90 percent or more of their total nitrogen present in organic materials. The figures are smaller for phosphorus and sulfur, but are still significant. These organic reserves act as a bank account of nitrogen, phosphorus, and sulfur for plants. Actually, a better analogy might be that organic materials are a nutrient variable rate annuity. The plant can’t use organic nitrogen any time it wants, just like you can’t access a variable rate annuity until it comes to term. Organic forms of nutrients must be mineralized, or converted to inorganic forms, to be useful to plants. This is largely carried out by microorganisms as a byproduct of their use of organic matter for themselves.
A second indirect benefit of organic matter in soils is in increasing a quantity called cation exchange capacity (CEC). Cation exchange capacity is the ability of a soil to hold certain elements which have a positive charge, called cations. Some plant nutrients, such as potassium and calcium are cations, so a soil with a higher CEC will generally be more fertile because of its higher content of these nutrients. While there are exceptions to this generalization, it provides one more reason to see higher organic matter content as a benefit to the soil and to the crop being grown on the soil.