Is it time to rethink your phosphorus management?

Farmers may need to rethink their phosphorus management due to the dramatic shift in Manitoba acres towards canola and soybeans at the expense of cereals, an Agriculture and Agri-Food Canada researcher says.

Cynthia Grant, a soil management and fertility specialist with the Brandon Research Centre told the Manitoba Agronomy Conference farmers are growing more crops that deplete phosphorus (P) in their rotations. They need to consider how to replace it to avoid depleting soil reserves.

“Historically our P fertilizer inputs and our P fertilizer removal was pretty well balanced in Manitoba and that was because people would put on more P with a cereal crop and maybe put on less with the canola and through the rotation things balanced off quite well,” Grant said. “But things have changed over the past few years, because if you look at our crop rotations now we are seeing a big increase in canola and soybeans and there is a decrease in cereals. So we are growing more of those crops that deplete the P and moving away from crops that were building P in the rotation.”

Phosphorus is critical for the energy and growth functions of plants from rooting, tillering and early flowering through to seed production and uniform ripening. Unlike nitrogen (N) fertilizer, P fertilizer isn’t very mobile in the soil. N is highly soluble and moves through the soil solution to plant roots via transpiration and mass flow. P on the other hand moves primarily by diffusion and the concentration of P remains very close to the fertilizer granule. This means that P has to be placed in the seed row or side banded to make sure that it contacts the plant roots early in the season, when it is critical to help plants establish.

Canola and soybeans are both heavy users of P, but the seedlings can’t survive heavy applications in the seed row.

Grant presented a chart showing the P removal rates of different crops and the recommended safe limits for seed-placed P fertilizer. If the removal rate is higher than the amount applied, a deficit is created.

In the case of wheat, barley and oats, the safe rates of seed-placed P were high enough to create P surpluses in the soil. However, with canola, soybeans and peas, recommended P fertilizer rates created a significant deficit and was not sufficient to meet the needs of the plant.

She said it’s important to balance the P needs of different crops throughout the entire rotation. If only sensitive crops are grown in rotation, soil P reserves can be seriously depleted over time. The addition of cereal crops to the rotation allows the opportunity to increase the amount of P applied for those crops and help make up the deficit created by the sensitive crops.

It’s generally accepted that P becomes less available over time and that less than 30 per cent of the actual P is used in the year of application. As long as the remaining 70 per cent isn’t lost through run-off or erosion, the P does become available to subsequent crops.

“The P in the soil solution will react with calcium or magnesium or iron and aluminum and will very quickly become less soluble,” said Grant. “It’s a natural reaction that shifts the equilibrium towards the formation of these very insoluble compounds. But during the growing season the plant removes P from the soil solution and reshifts the equilibrium the other way and replenishes the solution to supply P to the growing plant during the growing season. So the previous years’ P applications can remain in that labile pool and are still plant available.”

Several long-term studies of cropping systems have shown that 80 per cent to 100 per cent of the P applied can be recovered over time. That leftover P can play an important role in a P management strategy.

An eight-year study showed that after cropping with wheat and flax for eight years, soil available P was maintained at high levels after a single application rate of MAP anywhere from 200 kg/ha to 400 kg/ha. Although these are application rates that are higher than most farmers are comfortable with, a useful alternative is manure, which is an excellent source of P.

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