Organic Fertility: Are Green Manures Enough?

By Brenda Frick, Ph.D., P.Ag.

Every crop carries a package of nutrients that had been removed from the soil. When the crop is sold off farm, these nutrients are lost. To maintain soil fertility and to be sustainable in the long term, these nutrients must be replaced. Finding acceptable methods of replacing lost nutrients is crucial to the long term success of organic farming.

Lack of available soil nutrients can limit plant growth. Dr. Malhi, at Agriculture and Agri-Food Canada in Melfort, explains that “in the Prairie Provinces, most soils are deficient in available nitrogen (N) for optimum yield. There are many soils low in available phosphorus (P), and some soils contain insufficient amounts of sulfur (S) and potassium (K) for high crop yields.”

Replacing N is relatively simple using legume green manures. Nitrogen is abundant in the air, including the air within the soil. Microbes associated with legumes fix N, taking it from the air and converting it into a form that the legumes can use. When the legumes decompose, they release the N to the soil in a form that is available to other plants.

What about the rest of the nutrients removed with the crops? Unfortunately, P, K and S limitations are not so easily addressed. Each of these nutrients comes from the weathering of minerals in the soil. This is a slow process. Some practices may speed the process. Phosphorus, for instance, is abundant in prairie soils, but much of it is in a form that is not readily available to plants. Some plants such as mustards, buckwheat and lupines may be able to improve the availability of P somewhat, by excreting acids into the region around their roots, making P more soluble. Other plants, especially legumes, form associations with mycorrhizae that increase the volume of soil that a plant can access. This is important for P, which is quite immobile in the soil. Plant roots or mycorrhizae must grow to the P, rather than wait for the P to come to them. These methods increase the proportion of P that is available to plants, but they do not increase the total amount of P in the soil.

Manure (and composted manure) is an excellent source of N, P, K and S. Manure use is common for organic gardening, for high value organic crops or for building soil in targeted areas. For livestock producers, using manure from their own animals on crop land is an excellent way to cycle nutrients, but as long as products are sold off farm, some nutrients are lost. For grain farmers, adding manure from neighboring livestock can help to cycle nutrients regionally. Unfortunately, the supply of suitable local manure is often limited, and the material is heavy to transport. Alfalfa pellets, another N option, has similar drawbacks.

Rock phosphate and elemental sulfur are allowed under organic regulations. Studies involving rock P have found that the short-term benefits are often limited. Rock P is literally ground rock, and it is largely unavailable to plants in that form. Other concerns with rock P include the potential for heavy metal contamination and the cost, economic and environmental, of trucking rocks long distances.

Additional amendments are being suggested to organic producers. These include minerals, new microbial inoculants, microbial stimulants such as sugars, the products of manure bio-digestion and other biological byproducts such as wood ash. Wood ash, a byproduct of the forestry industry, is a rich source of P, K and S.

Plants require nutrients beyond N, P, K and S. These four elements, as well as calcium (Ca) and magnesium (Mg) are considered macronutrients, because plants need relatively large amounts of them. Plants also need smaller amounts of several micronutrients, such as copper, manganese, boron, iron and zinc. Fortunately, our soils are generally well supplied with Ca, Mg and the micronutrients and deficiencies of these are rare.

If producers suspect that their soils are deficient in a particular nutrient, they can send soil and tissue samples for testing, to properly identify which mineral is lacking. The organic amendment industry is relatively new in Canada, and testing of most products is only beginning. Producers are well advised to test new products on small areas before making major and costly changes in their fertility program.

Brenda Frick, Ph.D., P.Ag., is the Senior Research and Extension Associate for OACC at the College of Agriculture and Bioresources, University of Saskatchewan. She welcomes your comments at 306-966-4975 or via email at organic@usask.ca.

 

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Posted October 2007