Higher soybean yields are mining soil phosphorus and potassium, Manitoba agronomists warn

As soybean yields rise, the crop is drawing down soil reserves of phosphorus and potassium, and growers need to manage those nutrients across the whole rotation, Manitoba Agriculture soil fertility specialist Marla Riekman said at a variety trial farm tour, Manitoba Co-operator reports.

Using the Prairie Nutrient Removal Calculator, Riekman said a 40-bushel soybean crop removes about 35 pounds per acre of P2O5 and 46 pounds of K2O. Growers do not need to apply those amounts to the soybean crop each year, because the soil supplies much of what the crop needs, but when removal keeps exceeding what is added, the gap comes out of soil reserves. "Both potassium and phosphorus need to be thought about as nutrient in equals nutrient out," she said.

Prairie soils, especially heavier clays, have historically held large potassium reserves, but higher-yielding crops are drawing them down. At the tour site, soil phosphorus was about 11 parts per million, inside Riekman's target range of roughly 10 to 15, which leaves room to cut applications when fertiliser is expensive; potassium, at about 180 ppm, was "getting low". Once soil tests fall too far, rebuilding takes years of adding more than crops remove.

Riekman also said potassium deficiency symptoms, yellowing around the margins of older leaves, can appear in dry conditions even when soil potassium is adequate, because drought limits uptake. She advised soil and tissue tests before assuming more fertiliser will fix the problem.

"Soybeans like fertile soil, not fertilizer," said Terry Buss, production agronomist with Manitoba Pulse and Soybean Growers. Because soybeans are effective at drawing nutrients from the soil and often respond little to fertiliser applied in the year they are grown, he said, growers should build fertility before the soybean crop and add phosphorus and potassium elsewhere in the rotation when prices and crop returns make it economical. The report is by Don Norman.

Photo: United Soybean Board / Wikimedia Commons (CC BY 2.0)

Source: Manitoba Co-operator