Soybeans can handle more phosphorus than suspected

Soybeans are tough.

They can withstand heat, drought, drenched soils and many other stressors.

However, University of Manitoba researchers were surprised to learn that beans have another tough quality: they can tolerate a relatively high rate of phosphate fertilizer next to the seed with little impact on plant stand or yield.

The researchers applied 20, 40 and 80 pounds per acre of mono-ammonium phosphate (MAP), 11-52, to soybeans at eight sites in 2013 and 10 last year.

The rates were much higher than Manitoba guidelines, which recommend a maximum of 10 lb. of phosphorus per acre for beans.

“We saw no reductions (in plant stand) at 20, no reductions at 40 and I think in most of the trials, even no reductions at 80,” said Don Flaten, a U of M soil science professor known for his phosphorus expertise.

“I think soybeans are a lot tougher, in terms of reacting to seed row P than what we thought…. The probability of reducing stands because we’ve got more than 10 lb. (per acre) of seed row P seems to very low.”

There were four plots in the trial where 80 lb. per acre of phosphate reduced seedling stands, but only two cases where 80 lb. reduced yield.

Soybeans might be able to handle 20 to 40 lb. per acre of phosphate with the seed, but the practice is all risk and no reward because soybeans do not respond to added phosphorus.

“It didn’t matter what rate of phosphate we applied or what placement we used, broadcast, side band or seed placed, we got no increase in seed yield for soybeans in any of the 18 site years,” said Flaten, who presented his findings at Ag Days in Brandon last month.

Graduate student Gustavo Bardella said canola and wheat can use added phosphate, but soybeans and other legumes prefer to scavenge for phosphorus.

“If you take a look at fababeans, it’s very similar. They are very efficient at taking up phosphorus from the soil,” said Bardella, who is from Brazil and is leading the Manitoba research on soybeans and phosphate.

Beans may be indifferent to applied phosphate, but the crop pulls a substantial amount of phosphorus out of the soil.

For example, a 40 bushel soybean crop removes 34 lb. of phosphorus from the soil.

“It’s an absolute pig for soil phosphorus,” Flaten said.

“It is very, very aggressive at looking after its phosphorus needs.”

Manitoba data and anecdotal reports indicate that soybeans can deplete phosphorus, depriving other crops in the rotation of a necessary nutrient.

Many fields have become phosphorus deficient in eastern Manitoba, where some farmers have planted soybeans on tight rotations for nearly a decade.

“In some areas where we have intensive soybean production, like around Beausejour, the frequency of low testing phosphorus soils has doubled in the last five years,” Flaten said.

Terry Buss, a Manitoba Agriculture crop production adviser in Beausejour, said it’s difficult to recognize the symptoms of phosphate deficiency in beans.

“They (beans) seem to give up the ghost when we’ve sucked every last pound (of nutrients), ” Buss said.

“They hold their condition and then you flip a switch and things start to go south.”

Re-establishing phosphorus fertility can be costly if a grower allows soil phosphorus to drop to a low figure, such as five parts per million Olsen P.

Flaten said it takes 20 lb. of phosphate to raise Olsen P by one p.p.m.

Flaten said managing phosphate to satisfy all the crops in the rotation is preferable to applying phosphorus for the needs of an individual crop.

It may be possible to add more phosphorus during the soybean phase so that the nutrient is available for subsequent crops, but it’s safer to apply higher rates to cereal crops that can easily tolerate 50 lb. per acre of phosphate in the seed row.

“Seed row toxicity is a game of chance,” Flaten said.

“It happens once in a while, but when it does it’s absolutely devastating.”

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