
When soybean fields started looking like the one pictured here, Steve Gauck’s phone began buzzing. What was wrong with the soybeans?
“These symptoms likely appeared because of weather conditions,” says Gauck, a regional agronomy manager for Beck’s, based near Greensburg, Ind. “A dry spell was underway, and soybeans were stressed.”
It was also an indicator that potassium levels may have been trending low, he adds. When roots can’t function normally, they do not supply enough potassium.
Dorivar Ruiz Diaz, an Extension soil fertility specialist at Kansas State University, is talking to farmers about potassium, hoping to head off potential yield-robbing scenarios.
“With lower crop prices, people look to cut costs,” he says. “Cutting back or dropping potash application might seem practical. However, if you don’t know soil test levels everywhere, you could lose more than you gain. Where K soil test levels are low, cutting back would not be wise.”
Soil test trends
Soil test trends indicate potassium levels are decreasing in Kansas and Indiana and holding steady at best in Iowa, Ruiz Diaz reports. Based on work in Kansas and elsewhere, here are six key points growers should consider before lowering K rates:
1. Applying K fertilizer often increases soybean yield if tests are low to medium. Soybeans at 6 of 10 locations responded with increased yield in 2022 Kansas State trials. Soil test levels at responsive sites ranged from 60 to 120 parts per million. Test levels for K at nonresponsive sites were very high, from 305 to 409 ppm.
2. K fertilizer application paid, even at 2024 soybean prices. The optimum agronomic K level for the low-to-medium-testing responsive sites was 112 pounds of potash per acre. The optimum economic rate was 89 pounds per acre. Even at $9-per-bushel soybeans and K at 35 cents per pound, the average yield increase of 5 bushel per acre produced a net average return of over $27 per acre.
3. Adding P and K together in low-testing soils boosts yield increases. In a low-testing soil with phosphorus of 10 ppm and K at 95 ppm, adding P or K boosted yield about 5 to 6 bushels per acre, but adding both P and K at the same time boosted yield over 20 bushels per acre.
4. In-season early sidedress applications help yields. What if you spot deficiency symptoms during the season? Research indicates that adding K will boost yields, but perhaps not as much as if the K was applied preseason, Ruiz Diaz says.
5. In-season tissue testing may help. “Early-season tissue testing may suggest some corrective action even if you aren’t seeing symptoms,” Ruiz Diaz says. However, you may want to pay attention to potassium-to-magnesium ratios and critical values more than just K ranges, especially early in the season.
6. Proper soil test guidelines are key. Always sample at the same time of the year for potassium, Ruiz Diaz says. Even then, variations in results for K are common, because some common analysis methods for K are not the best predictors of K fertilizer response. Ruiz Diaz and others are currently evaluating the cation exchange resin method to determine if it is a better predictor of K response.
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