Wheat in study finds P where soil tests don’t

Can you spot the difference between the two sides of this wheat plot? Probably not, AAFC researchers say. (Environmental Science and Technology)

A long-term yield study in Saskatchewan shows wheat crops will use the phosphorus built up in soils over years of fertilizing — even where soil tests don’t show the nutrient is handy for use.

While the results still need to be tested elsewhere, with other crops, the study’s findings may help Canadian farmers rethink how much phosphorus (P) — if any — their long-fertilized fields actually need.

The study, results of which were recently published in the journal Environmental Science and Technology, is part of a broader federal project on “legacy” P — that is, the applied P crops don’t need or use, which can remain in the field and become more tightly bound to soil than recently-added fertilizer.

Barbara Cade-Menun, based at Agriculture and Agri-Food Canada’s Semiarid Prairie Agricultural Research Centre (SPARC) at Swift Current, Sask., led the study using a set of plots established in 1967 and fertilized every year with both P and nitrogen (N) until 1995.

After 1995, each plot on the set was split in two and P treatments were stopped on a half of each plot for a “long-term, controlled study” of legacy P, AAFC said in a recent release.

While soil tests showed “very low” concentrations of P on the no-added-P plots, there was no difference in yield, or in the concentration of P in the grain, as compared to the fertilized plot, AAFC said.

“Legacy P from a 28-year build-up was sufficient for 15 years of wheat cultivation, resulting in no significant differences in crop yield in 2010,” Cade-Menun’s team wrote in its ES+T paper.

In other words, the wheat plants that didn’t get the additional P fertilizer were able to use not only the legacy fertilizer P, but other pools of P that had built up in the soil such as organic P from decayed plant trash.

Because it’s more tightly bound, that legacy P may not show up in soil test-style analyses, which are meant to measure readily available soil inorganic phosphorus when deciding on a field’s fertilizer needs.


Having such plots available for longer-term research is key to studying new problems as they develop, such as the potential overapplication of P fertilizers, Cade-Menun said in AAFC’s release.

“While it’s well known that not all the fertilizer given to plants will be used, what we don’t know is how fertilizer compounds in the soil interact with the other soil elements, microbes and plants over time,” she said.

Cade-Menun’s team was able to observe P levels from the fields’ surface and subsurface layers with help from the Saskatchewan Structural Sciences Centre at the University of Saskatchewan, and from the Canadian Light Source Synchrotron in Saskatoon.

If farmers’ crops are consistently able to get at those P sources believed to be off-limits, AAFC said, that could translate to a cost saving for farmers and possibly cut down the risk of losing recently-added P into nearby bodies of water.

Oversupplies of P and N washed into creeks, rivers and lakes in turn are blamed for overgrowth of nuisance aquatic plants and algae, posing potential risks to lakes’ fish populations and the health of lakes’ human users.

“These results provide important information about legacy P to improve agricultural sustainability while mitigating water quality deterioration,” the team wrote in the ES+T paper.

The SPARC study, AAFC said, is part of a broader cross-Canada project on legacy P underway at “various locations” across Canada, led by Noura Ziadi at AAFC’s Soils and Crops Research and Development Centre (SCRDC) at Quebec City. — AGCanada.com Network

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