Recent farm surveys in Saskatchewan show that soil on organic farms is deficient in available phosphorus.
Long-term rotation studies like those in Scott, Sask., and Glenlea, Man., also indicate that phosphorus becomes a concern in organically managed plots. Should organic producers be worried?
Phosphorus is an essential plant nutrient, which plants need to harvest energy from the sun. Plants and animals, including humans, need phosphorus for energy transfer and as part of cell membranes and genetic material.
When crops are harvested and removed from the farm, significant amounts of phosphorus are lost.
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The Canadian Fertilizer Institute estimates that a 40 bushel per acre wheat crop will remove about 25 pounds per acre of phosphorus and a 50 bu. per acre pea crop will remove about 35 lb. per acre.
Unlike nitrogen, which can be literally pulled out of the air with the help of appropriate microbes, phosphorus comes ultimately from rock. If phosphorus is continuously removed through exports of grain and meat, eventually off-farm sources may be needed to replace them. Some rock phosphates are approved for organic use.
Phosphorus is abundant in our soil but only a small portion of it exists in soluble forms that plants and microbes can use, which is the form that is measured in standard tests of available soil phosphorus.
The largest portion of soil phosphorus is rock or mineral based. In prairie soil, it is generally bound into chemical complexes with calcium. These complexes, called apatites, can be chemically broken down torelease phosphate ions, but this natural process is slow under prairie conditions.
The deficiency of available phosphorus in organically managed soil indicates crops are removing the phosphates as quickly as they are being released from the rock. It also suggests that crop growth may be limited by the rate at which the phosphates become available.
Of course, soil is much more than weathering rocks. A crucial portion of soil phosphorus is bound into organic molecules, both in dead material and the living fraction of the soil. Legumes are effective at scavenging phosphorus as it becomes available, so their residues can be phosphorus rich.
Some weeds, such as kochia, are good phosphorus scavengers and make good phosphorus green manures. Animal manures are especially rich in phosphorus.
A biological cycle moves phosphorus among the three phosphorus pools Ð apatite, phosphate ions and organic compounds. Phosphate ions are released from rocks and taken up by plants, transferred to animals as they eat plants and returned to the soil in animal wastes and in the bodies of plants and animals as they die. Soil organisms cycle these organic materials among themselves and back into phosphates that plants can use.
Microbes play several vital roles in cycling phosphorus. Some microbes, especially the type of fungi known as arbuscular mycorrhizae, partner with plants in a way that allows the plant increased access to phosphorus. Some microbes break down organic matter, releasing and recycling phosphates.
Others produce chemicals that react with the apatite, releasing phosphates. Some of these latter microbes add acids to the soil solution, making phosphorus more soluble. Others act on the calcium, forming complexes that free the phosphorus.
Our understanding of soil biology is incomplete, but we do know that microbial activity in soil can be encouraged by reducing toxic chemical inputs, reducing tillage and increasing organic matter in the soil, which feeds the microbes.
When synthetic phosphate fertilizers are used, the biological cycling of phosphorus may be of lesser importance. But biological cycling may be pivotal to the sustainability of organic farming. Available phosphorus in the spring may be less critical in organic systems if there is adequate organic matter to allow strong microbial activity.
Microbes can make additional phosphorus available throughout the growing season and the timing of active plant growth may coincide well with times of greatest microbial activity. The challenge comes in finding ways and systems that encourage phosphorus cycling. Further research is required.
Soil ecology and fertility are important areas of research that will be addressed by experts from Alberta, Saskatchewan and Manitoba at the Organic Connections conference Nov. 14-16. I encourage you to find out more by attending the conference. For more information, including registration, see www.organicconnections.ca or call 306-226-4501.
I thank Jeff Schoenau and Diane Knight, soil scientists at the University of Saskatchewan, for sharing their expertise with me as I explored this topic.
Frick is the prairie co-ordinator for the Organic Agriculture Centre of Canada located at the University of Saskatchewan. Frick can be reached at 306-966-4975, at brenda.frick@usask.ca, or www.organicagcentre.ca.The opinions expressed in this column are not necessarily those of The Western Producer.
