Soil Test P Recommendations Best Guideline For Applications

When it comes to phosphorus management in potatoes, Blair Geisel, of Gaia Consulting, has a simple take-home message for growers – take the soil test lab’s recommendation seriously, even if they’re an imperfect art.

“Believe your soil test – or more correctly, believe the lab recommendation for phosphorus,” Geisel told the Manitoba Co-operator in a recent interview.

Although the soil test recommendations tend to be conservative in their P recommendations, they reliably add enough P to replace what’s removed by the crop, while preventing costly, unnecessary and potentially environmentally damaging over-application. A minority of growers make “insurance” applications which fail to boost yield.

Geisel was summing up the findings of the first two years of a multi-year study his company is undertaking for Manitoba potato growers and processors that looked at how to make phosphorus available most effectively to potato crops.

This macro-nutrient is particularly difficult to manage because of its reactivity with soil, which causes phosphorus to be present in the fields, but not necessarily in forms that are readily available to the plants. It takes time to break down into plant-available forms in the soil. By making regular applications, growers ensure that the pipeline remains full and the plantavailable nutrients are released during the growing season.

To make the grower’s job just a bit more challenging, potatoes are also noted for being inefficient at absorbing this particular nutrient from the soil. At the same time, farmers in all sectors are also being pressured to manage nutrients more effectively to minimize any environmental

impact – and no nutrient has been subject to more negative attention than phosphorus.

Geisel says there was a clear-cut winner in the study, however, that highlights the fact many growers are already following best management practices. The study looked at different application rates and at both broadcast and side-band applications. What they found was that yield response was exclusively noted in side-band applications, and in locations with the highest residual P in the soil. No yield response was seen at rates higher than 40 pounds per acre.

Where a yield response was observed, it was seen on areas with high residual levels of phosphorus, which likely means that the nutrients have been applied regularly over time, resulting in enough being present in plantavailable forms to boost yield. Past nutrient studies, have noted that when regular applications are withdrawn the yield loss can be “immediate.”

“Given the high levels of P that have been built up in Manitoba soils and the low correlation of yield response to soil test P, the best strategy is to apply maintenance amounts of P to replace what is removed by the crop,” the Gaia Consulting report says.

The study also found that key quality indicators – tuber size, specific gravity, fry colour or the incidence of hollow heart or sugar end defect – were unaffected by P levels.

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Gord Gilmour

Gord Gilmour is Editor of the Manitoba Co-operator.

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