GLENLEA, Man. – Standing in a small field of alfalfa, Martin Entz points out that the crop is not as lush as it could be, despite high levels of nitrogen, good soil structure and ample moisture.
The reason for the lacklustre crop is the low level of phosphorus in the soil. It was down to 17 pounds an acre in 1999 and has probably ebbed even lower since then.
“This is a very good picture of something that is becoming less sustainable,” said Entz, an agronomy professor at the University of Manitoba’s plant science department.
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“If we’re exporting the nutrients away, they have to be added back in.”
Phosphorus is important to crops for a variety of reasons. It helps crops establish healthy roots and also plays an important role when cereal crops go into head and start filling out.
Livestock manure, which is a natural source of phosphate, is a common way to replenish phosphorus in the soil.
Michel Vuignier, an organic grower from St. Claude, Man., takes that approach. He stockpiles manure accumulated from overwintering his cattle and lets it compost through the summer. He applies the manure in the fall and plows it into the soil to prevent nutrient loss.
Vuignier is now into his second year as a certified organic grower and does not dismiss the risk of diminished yields due to low phosphate levels.
“The thing is to get on top of it before it happens.”
Grazing alfalfa rather than haying it is another way to improve phosphate levels. Manure from the cattle foraging on the field will provide phosphate that gets returned to the soil naturally.
Rock phosphate also works in organic production, but Entz offered a caution about using rock phosphate where the soil pH levels are high, which is the case for much of Western Canada. High pH soils have a lot of calcium, which binds with the phosphorus, making it less available to the crop. That has led to debate about the merits of adding rock phosphate.
Growers might also look at growing crops that make organic soil phosphorus available. Green manure legumes such as sweet clover are among the options.
Growers choosing that option need to remember they are not adding phosphorus to the soil. They are only making it more available to plant roots.
Encouraging the development of fungi called mycorrhizas is another way to make phosphorus more available to crops.
Mycorrhizas survive in the roots of several western Canadian crops. Their presence helps with the early establishment and growth of those crops, which range from sunflowers, flax, corn and potatoes through to legumes, sorghum and tall grasses.
Jill Clapperton, a rhizosphere ecologist at Agriculture Canada’s research centre in Lethbridge, Alta., cites three things producers can do to promote mycorrhiza in their fields:
- Manage phosphorus levels carefully. “You definitely want to stay away from adding too much phosphorous,” Clapperton said.
Once the levels of available phosphate exceed 20 pounds per acre, they will inhibit the mycorrhizas.
- Reduce the amount of tillage so there is less disturbance of the fungi.
- Include crops in the rotation that are known to build mycorrhizas.
Inoculant manufacturer Philom Bios said the soil fungus Penicillium balaii, found in JumpStart, makes rock phosphate and residual soil phosphate immediately available for crop use.
Some organic certification groups approve the use of JumpStart in their organic production rules.
The alfalfa crop that Entz used to illustrate the importance of phosphorus is located at the University of Manitoba’s Glenlea research station south of Winnipeg.
The alfalfa was planted as part of a four-year rotation that includes two years of alfalfa followed by flax and wheat.
Since 1992, Entz has been studying that rotation, along with several others, to learn more about organic crop production, in terms of agronomics and economics.
Based on data collected between 1992 and 1999, Entz concluded the rotation using alfalfa, flax and wheat was the most profitable of all the rotations in the study.
Annual input costs averaged $36.06 an acre per year. Annual net return per acre was close to $94.
The net return was calculated by giving alfalfa a value of $50 per tonne. The cost of replenishing the phosphate was not included in the calculation.
“The benefit of the phosphate that we’ve mined out does need to be accounted for,” Entz said.
