CALGARY – High phosphorus prices are prompting producers to take a close look at the fields on which they have continually applied the pricey nutrient.
While some producers rely on soil tests to ensure they meet projected crop needs, many apply fertilizer based on targeted crop yields and expected crop removal of nutrients.
Either way, crops often fail to use every pound of phosphate that goes into the soil, and it can build up over time. At least that is what appears to happen in some cases.
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Soil tests can indicate that sufficient phosphorus is present in forms that will support a crop, but the results are not always as expected when more advanced testing procedures are applied and a crop is grown.
Rigas Karamanos of Western Co-operative Fertilizers in Calgary said an experiment was established on an Alberta research farm in 1981to look at long-term effects of phosphorus fertilizer application methods.
At the end of the study, researchers found that some of the plots had residual phosphorus that plants had not removed during the trials and these appeared sufficient to produce targeted barley yields.
In three subsequent barley crops on which no additional phosphorus was applied, yields failed to meet expectations.
Soil samples didn’t tell the whole story about the location of phosphorus. While overall supplies were generous, nutrient probes showed that phosphorus was not always where plants needed it and roots often failed to reach the supplies when it was needed.
“We found that you can’t take a holiday from P,” Karamanos said.
“Residual P from long-term P fertilization was not sufficient to alone provide all the P requirements of barley grown after P fertilization was discontinued …. Fertilizing with phosphorus serves a dual purpose. It supplies the crop’s phosphorus requirement for the year of application and supplies residual P to maintain soil P levels for future crops.”
Karamanos said producers can calculate anticipated returns from their fertilizer with the following formula: yield increase (bushels per acre) x price per bu. – fertilizer rate (pound per acre) x price per lb.
