Making withdrawals without deposits can lead to shortfalls in crops

When farmers begin harvesting corn and soybeans in the weeks ahead, they typically won’t be thinking about how much fertilizer they’re hauling to the elevator in that grain.

That can be unfortunate because they also won’t be considering those numbers when they have to make a decision about applying phosphorus, potassium, sulfur and other nutrients this fall or next spring.

“You need to think about that you’re auguring out fertilizer at the same time you’re auguring out corn,” says Bobby Golden, Extension soil scientist with the Delta Research and Extension Center at Stoneville, Miss.

“Corn is going to take .35 pound of P2O5, 0.25 pound of K2O and 0.08 pound of sulfur in ever bushel,” says Dr. Golden. “If you put that on a per-acre basis on 210-bushel corn, which most of your guys can make, that’s 74 pounds of P2O5, 53 pounds of K2O and 17 pounds of sulfur. How many of your guys are putting out 17 pounds of sulfur on their corn?”

Golden, who was speaking at a Grow Smart Field Day sponsored by BASF for agronomists for ag retailers and distributors in Stoneville, Miss., said not very many growers are putting out 17 pounds of sulfur on corn or even less (11 pounds) that is needed for soybeans.

“Soybeans are nutrient hogs,” he says. “Besides .18 pound of sulfur, a bushel also contains .73 pound of P2O5  and 1.2 pounds of K2O. On a 60-bushel soybean crop, you will remove 44 pounds of P2O5 and 72 pounds of K2O and 11 pounds of sulfur. I bet not a one of your growers is putting 11 pounds of sulfur per acre on their soybeans.

No free sulfur

“That’s important because we used to get sulfur for free. Then when EPA cleaned up the environment, we didn’t get sulfur for free any more. All that sulfur is caught in the coal smokestacks so in the last 30 years Mississippi has gone from a contributor to sulfur in the soils to a net deficit.”

Dr. Golden says soils testing high and optimum for nutrient levels in Mississippi have been declining over time while soils testing medium have remain fairly steady. Soils testing low and very low, on the other hand, have been on the rise.

“Over time, when we’ve been pulling out these big yields in our corn and soybean rotations, we’ve seen low and very low samples occurring at a much more frequent pace,” he notes. “In the medium level, that’s when our first recommendation kicks in. That’s the 0-40-60 – the one that people can afford.

“That’s not the 250 pounds of Triple Super and the 280 pounds of muriate of potash that will be required for the low or very low – the ones they balk at. I’ll tell you that with most soil-testing philosophies it takes eight years to move from one category to the next. So if you ever get behind and fall out of that medium category, it’s very, very hard to catch up.”

Calibration studies that are being conducted across Mississippi (30 sites for corn and 29 sites for soybeans) indicate the costs for not maintaining crop nutrients at medium levels can be significant.

Significant yield losses

“What this shows us is that we have a soil test that it’s the medium or lower category, one that would require an application and we do not put that application out, this is what we’re costing ourselves – 11.5 percent for phosphorus on corn and 13 percent on soybeans. The yield loss for potassium on corn can average 20 percent on corn and 10 percent on soybeans.

“If we’re low on K in corn that means we have the potential to lose 41 bushels of yield. Forty-one bushels at current market price is $157 an acre,” Dr. Golden said. “Tell me now you can’t afford that fertilizer bill in October.”

Over two years in a corn-soybean rotation, the two crops can remove as much as $200 an acre in fertilizer, he notes, a development that can lead to lower yields in subsequent years if not corrected.

The sulfur shortage has become apparent as farmers have increased their acres of corn and soybeans, Golden notes. Burning crop stubble such as the heavy residue left by high-yielding corn and rice crops also isn’t helping.

“When we burn corn stubble, we lose 72 percent of the sulfur that was contained in the biomass,” he said. “So if you’re burning you corn stubble in that corn and soybean rotation, you’re going to have to apply sulfur to make up for that loss.”

Members of the audience at the BASF event said most growers are applying ammonium sulfate or AMS on corn and eliminating sulfur deficiencies. But they don’t apply AMS on soybeans because they don’t need the nitrogen. Agronomists attending the event seemed to agree 50 pounds of AMS on soybeans would not interfere with nodulation of the soybean roots.

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