Or, What You See from Space (and What You Don’t)
Part 2 of 2In 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 FarmingMineralization 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.Original caption: Decrease of organic-matter content in a fallow, untreated soil in contrast to the gain in soil treated with 2 1/2 tons of red clover annually, representing over 500 pounds annual increase in organic matter per acre. If you have the time and interest, the entire Albrecht article is very interesting as it gives an historic perspective from one of the most important soil scientists of the 20th century. Many of his views and insights are still valid today. SOM values thus have very practical applications, including planning for fertilizer needs. The linked chart (from an agronomy handbook sent to me by soil testing company Midwest Laboratories, Inc.) shows the relationship between SOM and nitrogen availability. Here’s the math behind that chart. Stabilized SOM has a ratio of carbon to nitrogen atoms of 12:1. As explained by Albrecht, this nitrogen becomes available to plants as soil organic molecules are broken into parts. The top 6 inches of soil is typically considered to have a mass of 2 million pounds, and so 5% SOM is 100,000 lbs. The nitrogen is usually about 5-6% of the SOM mass (other parts include C, P, K, S, etc.) and 6% of 100,000 lbs is 6000 lbs of N. If 2% of this N mineralizes, then 120 lbs is available for plant growth, which is a good amount for most crop types. The nitrogen has an economic value in that it is a form of fertilizer that farmers don’t need to purchase, giving us an additional source of value from the diverse pasture aside from just the revenue from grazing ruminants. What Farmland LP does during the conversion of conventional farmland to certified organic farmland is place most of our acres into perennial pasture and/or forage crops that are rich in legumes. These perennial crops are growing year-round, not just for a season or two like an annual plant, and become very effective givers of organic matter to the soil. As I’ve explained before and touched upon in the first post in this series, the livestock actually increase plant productivity by grazing enough to foster renewed plant growth 5-7 times per year while depositing manures that activate the soil microbes and speed the nutrient cycle. Rotational grazing not only leads to 25-50% greater plant productivity than set stocking, but results in perhaps 10 tons of dead organic matter added yearly: on the surface as manure, trampled leaves and stems, and via the sloughing of roots. Some fraction of this, about 15-20%, is converted to SOM and thereby enhancing the value of the farmland we manage.