Spotlight on Fertility

October 2, 2009 06:26 AM

Adequate fertility is a crucial building block for high soybean yields. "Exceptionally good soybean yields come from fields that are optimum in fertility,” says Farm Journal Field Agronomist Ken Ferrie. "If a field is low in fertility, soybeans will struggle more than corn.”

When you plan your fertilizer program for high-yielding soybeans, forget what you learned from growing corn. Fertility-wise, the two crops are as different as cats and dogs.

"Corn responds more to applied fertility,” Ferrie says. "Soybeans, on the other hand, respond to soil fertility. That means you can't fix problems in one year, as you can with corn. It also means you need a balanced soil fertility profile, from pH to micronutrients.”

Because the soybean plant is a legume, soil pH is important. "We rely on rhizobia bacteria to make nitrogen [N] for soybeans,” Ferrie says. "But in an acid soil, the bacteria can't produce enough N. So low pH gets growers in trouble. You need to keep it in the sweet spot—the 6.3 to 6.5 range.”

In that pH span, soil microbes thrive, letting the plant take up phosphorus (P), potassium (K) and micronutrients.

Potassium pitfalls. Soybeans differ from corn in their ability to take up P and K. "With soybeans, it's more important to have soil phosphorus and potassium levels in the optimum to high range,” Ferrie says.

Soybeans are efficient feeders on P. The plant creates an acidic area around its taproot, which lets it solubolize P from the soil.

But they are less efficient with K. "Potassium is the element we watch most closely in the soil,” Ferrie says. "With potassium, two problems can occur, both related to the clay content of the soil. If the soil has a high clay content, it can fix potassium to soil particles and make it unavailable to plants. In sands, on the other hand, potassium can leach out of the soil.

"In these situations, consider applying potassium every year, rather than applying a two-year supply ahead of corn. Work with an agronomist to identify fixation and leaching problems. ”

Manganese misery. Alkaline soils can hamper the uptake of micronutrients, such as manganese and iron. In soils low in manganese, availability to soybean plants is further reduced when soil pH exceeds 6.5 and especially if it is near 7.0, says Purdue University Extension cropping specialist Tony Vyn. "Plant-available manganese is also much lower if these soils experience droughty periods,” he adds.

Foliar applications of iron fertilizer can correct an iron deficiency, but more research is needed on fixing manganese problems. Research to date suggests soil-applying manganese fertilizer is not effective. At Purdue and the University of Illinois, banding manganese fertilizer 2"x2" off the row at planting time has not produced much yield response.

"Once symptoms appear in a soybean crop, the best solution is to apply foliar manganese fertilizer at a rate of 1 lb. to 2 lb. of manganese per acre, in the form of manganese sulfate,” says University of Illinois soil fertility specialist Fabián Fernández. "Make the application as soon as symptoms appear, or seven to 10 days after a glyphosate application. Reapply at the R1 and R3 stages, if necessary.”
But foliar-applying manganese doesn't guarantee a yield response. In two replicated studies in Van Wert County, Ohio, by Ohio State University (OSU) Extension educator Andy Kleinschmidt and OSU soil fertility specialist Robert Mullen, a foliar manganese application increased manganese levels in plants but not yield.

A related issue is whether glyphosate herbicide causes problems with manganese utilization, even though tissue tests show adequate levels. "The jury is still out,” Fernández says. "But some growers apply chelated manganese with their glyphosate herbicide to prevent or correct the problem.”

Tank-mixing the two products is controversial. "There is some evidence that if you tank-mix manganese fertilizer with glyphosate herbicide, the efficacy of both may be reduced,”
Fernández cautions. "If you apply manganese with glyphosate, it's very important to follow the mixing instructions on the herbicide label.”

Based on research sponsored by the Indiana Soybean Alliance, Purdue's Vyn advises against tank-mixing most formulations of manganese and glyphosate unless you have proof that herbicide efficacy and manganese availability will not be compromised.

You can trigger manganese deficiency by overliming soils and raising the pH too high. "Overliming often results when growers routinely apply a certain amount, rather than basing the rate on soil tests,” Ferrie says. "It also happens when growers try to build up a low soil pH too quickly, by applying 3 tons to 5 tons of lime per acre. It took time for the soil to become acid, so you need to bring it back over time with two or three small lime applications.”

If you have both high- and low-pH soils, Ferrie and Vyn recommend going to variable-rate lime application.

Magnesium management. Soybeans like high magnesium levels more than corn, Ferrie says. Soybeans will struggle if the soil lacks that nutrient. You can diagnose a magnesium deficiency problem from physical symptoms and confirm it with a tissue test.

"Low soil magnesium levels usually occur because magnesium leaches out of lighter soils or because there is no magnesium in the lime source,” Ferrie says. "Also, magnesium can become unavailable to plants if phosphorus levels get too high. That usually results from overapplying manure.”

If areas of your soil are magnesium-deficient, break them out into individual management zones. Then fix them by applying dolomitic lime.

Dolomitic limestone usually has more neutralizing power than calcitic lime, so it may be more cost-effective, Ferrie points out. However, it is not available everywhere. If shipping cost makes it too expensive or if the field doesn't need lime, you can apply a product such as K-Mag, which contains sulfur and magnesium.

You needn't apply magnesium every year, Ferrie notes. "Monitor your soil's magnesium level, and keep it in the optimum range by applying dolomitic limestone when levels fall,” he says. "You may not need dolomitic lime every time you apply. If the only lime available in your area is dolomitic, soil levels will stay optimal and you probably won't have deficiency problems.”

Although it's not exactly a fertility issue, remember to inoculate soybean seed with rhizobia bacteria if soybeans are being grown in a field for the first time—or if the field is coming out of the Conservation Reserve Program.

"Just because a field is optimum in fertility doesn't guarantee high-yielding soybeans,” Ferrie concludes. "Yield can be cut by disease, insects, drought, etc. If fertility isn't there, you have lost the potential for maximum yield.”


You can e-mail Darrell Smith at


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