Or, What You See from Space (and What You Don’t)
Part 2 of 2
In the first post of this series, I explained how rotational grazing quickly builds soil organic matter (SOM). In this second part I discuss the importance of SOM for organic farming.Mineralization for Organic Farming
Mineralization is the process by which SOM, which consists of large and complex organic molecules, breaks down or decomposes into simpler, i.e., mineral, forms. These simpler forms are called Plant Available Nutrients, or PAN, and standard soil tests measure PAN levels of various minerals and nutrients. In conventional agriculture fertilizer is applied to compensate for any measured deficits. In organic farming systems we can make up for deficits with approved inputs, such as pulverized rocks high in certain minerals and animal manures, but we also rely more on the mineralization process for release of nutrients from the soil organic matter. In the agronomic system used by Farmland LP, deep-rooted, perennial plants and associated fungi are sown to both increase SOM and access minerals from well below the soil surface and bring them up to enrich the topsoil. To an organic farmer this means one of the most critical parts of a soil test is the level of organic matter. Prior to the wide-spread availability of synthetic nitrogen and other fertilizers, farmers essentially used accumulated stores of SOM to grow crops. The following is a quote and graph from the 1938 USDA Yearbook and was written by University of Missouri professor William Albrecht:In addition to carrying nitrogen, the nutrient demanded in largest amount by plants, soil organic matter either supplies a major portion of the mineral elements from its own composition, or it functions to move them out of their insoluble, useless forms in the rock minerals into active forms within the colloidal clay. Organic matter itself is predominantly of a colloidal form resembling that of clay, which is the main chemically active fraction of the soil. But it is about five times as effective as the clay in nutrient exchanges. Nitrogen, as the largest single item in plant growth, has been found to control crop-production levels, so that in the Corn Belt crop yields roughly parallel the content of organic matter in the soil (184). On a Missouri soil with less nitrogen than that corresponding to 2 percent of organic matter (40,000 pounds of organic matter per acre of plowed surface soil) an average yield of 20 bushels of corn per acre can hardly be expected. For yields approaching 40 bushels, roughly double the amount of organic matter is required. With declining organic matter go declining corn yields and therefore lower earnings on the farmers investment. Thus the stock of organic matter in the soil, particularly as measured by nitrogen, is a rough index of land value when applied to soils under comparable conditions. According to studies in Missouri, for example, the lower the content of organic matter of upland soil, the lower the average market value of the land.

Measuring SOM
We measure soil organic matter in two ways. The first is through standard agronomic sampling, which is done every 2-3 years and on a broad scale using “Smart Sampling” guidelines to help us plan for fertility inputs and to see how our soils are doing from field to field. This sampling method gives us a picture of SOM over a wide area, but a shallow depth (6-8”). For this kind of testing we may sample at a scale of one per every 2-6 acres at a cost of about $20 per sample. A farm service company manages this sampling, gathering the soil at specific locations that are predetermined with geo-coordinates and so are moderately replicable. Below is an image of the mapped SOM for our Wattenpaugh Farm as an example. The area shown is about 90 acres.