Every crop carries a package of nutrients that is removed from the soil. These nutrients are lost when the crop is sold off the farm and must be replaced if soil fertility is to be maintained.
Finding acceptable ways to replace lost nutrients is crucial to the long-term success of organic farming.
“In the prairie provinces, most soils are deficient in available nitrogen for optimum yield,” says Sukhdev Malhi of Agriculture Canada’s research centre in Melfort, Sask.
“There are many soils low in available phosphorus, and some soils contain insufficient amounts of sulfur and potassium for high crop yields.”
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Replacing nitrogen is simple using legume green manures. Nitrogen is abundant in the air, including the air within the soil. Microbes associated with legumes fix nitrogen, taking it from the air and converting it into a form legumes can use. When legumes decompose, they release nitrogen to the soil in a form that is available to other plants.
What about the rest of the nutrients removed with the crops? Unfortunately, phosphorus, potassium and sulfur limitations are not so easily addressed.
Each of these nutrients comes from the weathering of minerals in the soil. This is a slow process, but some practices may speed it up.
Phosphorus is abundant in prairie soil, but much of it is in a form not readily available to plants. Some plants such as mustard, buckwheat and lupins may be able to improve the availability of phosphorus by excreting acids around their roots, making phosphorus 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 phosphorus, which is immobile in the soil.
Plant roots or mycorrhizae must grow to the phosphorus, rather than wait for the phosphorus to come to them. These methods increase the proportion of phosphorus that is available to plants, but they do not increase the amount of phosphorus in the soil.
Manure is an excellent source of nitrogen, phosphorus, potassium and sulfur.
Manure use is common in organic gardening and high value organic crops and for building soil.
For livestock producers, using manure from their own animals on cropland is an excellent way to cycle nutrients, but nutrients will be lost if products are sold off the farm.
For grain farmers, adding manure from neighbouring livestock can help cycle nutrients. Unfortunately, the supply of suitable local manure is often limited, and the material is heavy to transport. Alfalfa pellets, which are another nitrogen option, have similar drawbacks.
Rock phosphate and elemental sulfur are allowed under organic regulations. Studies of rock phosphate have found that the short-term benefits are often limited.
Rock phosphate is ground rock and is largely unavailable to plants in that form. Other concerns with rock phosphorus are the potential for heavy metal contamination and the economic and environmental costs of trucking rocks long distances.
Organic producers have other options as well: minerals; new microbial inoculants; microbial stimulants such as sugar; the products of manure biodigestion and biological byproducts such as wood ash, which is a byproduct of the forestry industry and is a rich source of phosphorus, potassium and sulfur.
Plants require nutrients beyond nitrogen, phosphorus, potassium and sulfur. These four elements, as well as calcium and magnesium, are considered macronutrients because plants need relatively large amounts.
Plants also need smaller amounts of micronutrients, such as copper, manganese, boron, iron and zinc. Fortunately, our soil is generally well supplied with calcium, magnesium and the micronutrients and deficiencies of these are rare.
If producers suspect that their soil is deficient in a particular nutrient, they can send soil and tissue samples for testing.
The organic amendment industry is 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 programs.
Frick is the prairie co-ordinator for the
Organic Agriculture Centre of Canada
located at the University of Saskatchewan. She can be reached at 306-966-4975, at brenda.frick@usask.ca, or www.organicagcentre.ca.
