Invest in fertilizer | Soybeans suck up phosphorus and potash, which need to be replenished
MELITA, Man. — A soil fertility specialist with Manitoba Agriculture says farmers shouldn’t count on soybeans providing nitrogen for next year’s crop.
“Soybeans are pigs,” John Heard told the Westman Agricultural Diversification Organization field day July 30.
“They make nitrogen but they cart 95 percent of it off to the elevator. So don’t think you’re going to cut your nitrogen in the following crop.”
With the rapid expansion of soybean acres in Manitoba, a myth has developed that soybeans are self-sufficient when it comes to crop nutrients.
Heard said the crop does produce its own nitrogen but some soybean growers aren’t doing enough to maintain soil fertility.
“Growers are quite intrigued by this crop that survives on less than nothing,” Heard said.
“(But) you can’t grow this crop with just inoculant.”
For example, soybean plants re-move .9 to one pound of phosphorus per bushel, which is similar to the removal rate for canola.
Unlike canola, soybeans don’t respond to added phosphorus. Instead, the legume prefers to pull residual phosphorus out of the soil.
However, producers might assume it’s unnecessary to add the nutrient because soybeans don’t immediately react to a phosphorus application.
“Soybeans are good at sneaking nutrients out of the soil,” Heard said.
“They are supported by a mycorrhizal network that scavenges phosphorus out of the soil.”
Nonetheless, research by Gyles Randall at the University of Minnesota shows that soybean yields suffer if soil phosphorus levels are low.
Based on field experiments from the late 1990s, Randall found that soybean fields with high residual phosphorus yielded 54 bu. per acre on average. Sites with low-test phosphorus averaged 42 bu. per acre.
“There is a consequence (to yields), but it’s not readily apparent because this crop can tap into the reserves,” Heard said.
“(But) if you are going to continue farming in low (phosphorus) testing soils, be content with sub-optimal yields.”
Soybean yields also depend on potassium levels.
The crop will remove 1.4 lb. of potassium per bu., which means a 50 bu. per acre crop would pull out 70 lb. of potash per acre.
“There is no other crop that you grow … that has as much potash in the seeds as soybeans do,” Heard said.
Clay and clay-loam soils in Manitoba do have abundant stores of potassium, but sandier soils do not.
Bending down to inspect a soybean plant at the WADO plot site, Heard pointed to a lower leaf with yellowing around its edge, which is an indicator of potash deficiency.
“During seed fill, the seed removes between 1.2 and 1.4 lb. of potash per bushel,” he said.
“It moves potash from the leaves to the pods. Malnourished plants, the leaves will start showing potash deficiency in later August. That’s a wake-up call to the grower that he needs to do some soil tests.”
Dennis Lange, a Manitoba Agriculture crop production adviser in Altona, said a number of Manitoba producers are following a soybean-soybean rotation because prices are strong and the crop is easy to grow.
Heard said the beans will have a drastic effect on soil fertility if growers aren’t adding sufficient quantities of phosphorus or potash to the soil.
“If you grow soybeans back to back, that’s a rented land or retirement policy,” he said.
“If you supplied (the crop) with fertilizer, that’s called being a sustainable farmer…. Otherwise you are Fort McMurray, you are just depleting the soil reserves.”
Ten to 15 parts per million of phosphorus is a reasonable target for soybeans if a grower decides to replenish depleted soil and restore yield potential.
Heard said boosting residual phosphorus is a matter of mathematics.
“What you remove, at some point to be sustainable, you’ll have to replenish.”
Therefore, if a producer is growing wheat after a 40 bu. per acre soybean crop, he should add 35 lb. per acre of phosphorus to cover the amount that the beans pulled out and an additional 25 lb. per acre for the wheat.
Heard said this is the wrong time to scrimp on soil fertility. With new crop soybeans at around $12 per bu., growers can afford to spend on potash and phosphorus.
“You can buy more phosphorus with a bushel of soybeans now than when the price (of soybeans) comes down,” he said. “It is years like this when we should be investing in (fertility).”