Study showed 75 percent of cropland has high or excessively high levels and adding more has little impact on yields
HARROW, Ont. — Far less phosphorus is tied up in Ontario soil than previously thought, says an Agriculture Canada researcher.
“It was not a correct theory. Most of it is available,” TieQuan Zhang told the recent Innovative Farmers Association of Ontario conference.
The implications are enormous because phosphorus loading of the Great Lakes system is a major concern, especially in Lake Erie. The nutrient encourages the growth of blue-green algae, which produces cyanobacterial toxins and starves the water of oxygen.
Zhang said farmers and the environment would benefit if agricultural phosphorus applications were reduced.
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He said the International Plant Nutrition Institute has found that 75 percent of Ontario cropland has either high or excessively high levels of phosphorus. In those situations, the addition of phosphorus has little if any impact on corn and soybean yields.
“We’re thinking it will take about 25 to 30 years to go to a normal level in which phosphorus might need to be added and would not cause an environmental problem.”
Zhang said he and other researchers have learned that much of the phosphorus in soil is only loosely bound to soil particulates, which allows it to move in and out of solution.
Those findings have been confirmed in Brookston clay soil in a corn-soybean rotation for the past seven years at the Eugene Whelan Experimental Farm near Windsor, Ont., where they were able to maintain corn yields in plots in which no phosphorus was added. There have even been instances in which yields in the non-phosphorus plots exceeded those in plots that received phosphorus.
There was only a marginal yield effect on soybeans.
Zhang said adding a small amount of seed-applied phosphorus would be a workable compromise to address what may be a problem related to a shallow rooting system.
Researchers have not conducted similar on-the-ground experiments on sand and loam soil, but Zhang said the results would be similar.
However, he said farmers may be reluctant to break with past practices. Phosphorus is typically applied every year to fields where corn and soybeans are to be grown, although some have reduced rates.
“The frequency of agricultural soils highly enriched in phosphorus is substantially higher in (the) northeast than in the rest of the continent,” soil scientist Tom Bruulsema wrote in a International Plant Nutrition Institute document.
Manure use, inherent soil fertility and the cultivation of specialty crops with high demand for phosphorus have contributed to these numbers.
Zhang said the phosphorus situation varies across Canada. Quebec, with 50 percent of its agricultural soil falling into the high to excessively high range, is similar to Ontario. Levels are often high in the Maritimes and British Columbia.
High phosphorus soil is much less of a concern on the Prairies.
Zhang said another misconception relates to the way phosphorus is lost.
Surface runoff was once blamed for most of the loss, but Zhang said re-searchers now know that tile drain runoff of soluble phosphorus and phosphorus attached to particulates is a far greater concern.
“Tile loss is a major pathway, up to 95 percent,” he said. “It’s huge. Surface loss is a less important pathway.”
Zhang said phosphorus should be the limiting factor when applying manure rather than adjusting rates according to nitrogen requirements.
“You can always top up your nitrogen needs with chemical fertilizer.”
Fifty to 60 percent of phosphorus in manure is inorganic, which is free to be taken up by plants.
Overall levels vary according to manure type with liquid pig manure having the most when compared to liquid dairy, solid beef and chicken manure.
Management is another consideration.
Less tillage has positive implications, but it also allows for the development of macropores, which are voids created by roots and earthworms. They tend to concentrate phosphorus at the soil surface.
The two factors can increase the level of phosphorus leaching to tile drains, Zhang said. Banding required phosphorus or occasional soil cultivation may improve the situation.
