Managing phosphorus, nitrogen and potassium is fairly straightforward: Base your rates on soil test levels and crop removal and plants respond accordingly. But micronutrients also are essential for top yield, points out Farm Journal Field Agronomist Ken Ferrie—and deficiencies can show up during the growing season.
Knowing your soils can tell you where deficiencies are most likely to occur so you can promptly react.
One micronutrient getting a lot of attention recently is manganese. “Manganese deficiencies usually occur in soils with high organic matter content, such as muck or peat, dark-colored sands with pH levels above 5.8 and lake-bed and outwash soils with pH above 6.5,” says Darryl Warncke, Michigan State University soil fertility specialist. “Because it’s difficult to increase soil manganese levels, you can expect problems in the same areas every year for manganese-sensitive crops,” he adds. “Soybeans are one of the most sensitive.”
Symptoms of manganese deficiency include yellowing between the veins on the newer leaves of soybean plants and, in severe cases, stunting. “Early detection is critical in order to prevent yield loss caused by reduced photo-synthesis and nitrogen fixation,” Warncke says.
In studies, Farm Journal Associate Field Agronomist Missy Bauer found foliar applications of manganese boosted soybean yield from 0.9 bu. to 3.4 bu. per acre on heavy lake-bed soils in northwestern Ohio. “We know we have a manganese deficiency in those soils,” she says.
Response by soil type. In sandy soils, the yield response has been smaller, and sometimes nonexistent, Bauer says. “In muck and peat soils—which, in my area of south-central Michigan, tend to be spots within fields—it makes it difficult to measure the yield response,” she says. “But tissue tests show we have improved manganese uptake by the plants.”
In mineral soils in central Illinois, Ferrie says, he has “really struggled to get a yield response to treatments.
Tissue tests show I get manganese into the plants, but I don’t always get a yield response at harvest. I also have seen one replication of a trial respond to treatment, a second replication show no response and a third replication actually go backward.”
Deficiencies occur because manganese gets tied up, or fixed, in soil, Ferrie explains. So—except for rare situations involving certain very light soils—foliar treatments, rather than soil applications, are required.
“Foliar application of 1 lb. of actual manganese in 30 gal. of water per acre will alleviate deficiency symptoms and improve growth,” Warncke says. “If you apply a lower rate, regardless of the source, you often will need a second application. In one manganese-deficient field where a low rate was applied, the soybeans greened up only where there was spray overlap.
“With severe deficiency symptoms, apply 1½ lb. to 2 lb. of manganese per acre, in one spray or cumulative in multiple sprays, or make a second application two weeks after the first,” Warncke adds.
Bauer advises her clients to take a proactive approach. “We don’t like to see deficiency symptoms appear,” she says. “Think about your field’s history. If you have had manganese deficiencies in the past, you probably will have them again the next time the field rotates back to soybeans.
“Use soil and tissue tests, especially in newly acquired fields. Tissue testing tells us a lot about what is going on inside a plant. It can reveal a nutrient deficiency before we see visual symptoms. We probably should be doing more tissue testing, especially if we have nutrient issues in a field.”
Fix deficiencies. To correct deficiencies in growing plants, Warncke recommends farmers spray foliar applications of manganese sulfate. “Spray-grade manganese sulfate is soluble and provides greater crop safety than chelated forms of manganese at comparable rates,” he says. “If you apply glyphosate herbicide to a field, wait at least two days before applying the manganese.”
Purdue University plant pathologist Don Huber, who studied glyphosate and manganese relationships before his recent retirement, suggests waiting seven to eight days between glyphosate and manganese applications.
Warncke warns against tank-mixing glyphosate and manganese. “If you don’t take care when tank-mixing, the performance of the products may be reduced,” he says. “If you must tank-mix manganese and glyphosate, avoid using manganese sulfate. Instead, use the chelated manganese carrier Mn-EDTA. Add ammonium sulfate to the tank before adding the manganese.”
To avoid injury, Mn-EDTA must be applied at rates of less than 1 lb. per acre, Warncke adds. So more than one application may be required.
The glyphosate issue. The recent spotlight on manganese and glyphosate stems from research by Huber at Purdue. There, studies suggested that glyphosate herbicide, as well as the glyphosate-resistance gene in soybeans, affect the uptake and utilization of manganese by soybean plants.
“I found glyphosate can immobilize manganese, and other nutrients, so they are no longer nutritionally functional,” Huber says.
It’s not surprising glyphosate has that effect, he continues. “Glyphosate kills weeds by tying up essential nutrients,” he explains. “It does that by inhibiting enzymes involved in micronutrient uptake. It also immobilizes manganese and other micronutrients in plant tissue. These activities cause weeds’ defense mechanisms to shut down so pathogens in the soil can mobilize and kill them. It also impacts defense mechanisms in crops and leads to increased incidence of disease.”
Huber also found that manganese uptake and efficiency in glyphosate-resistant soybean plants was reduced by 10% to 50%, even when the herbicide was not applied. His results suggest it is possible to reduce the effect by selecting for more manganese-efficient seed varieties.
“The greatest impact of glyphosate is on low-nutrient-available soils or under conditions that limit nutrient availability,” Huber says. “However, glyphosate affects certain groups of soil microorganisms that affect manganese availability for plant uptake, so it can induce a manganese deficiency even when a soil test might indicate adequate levels, or where a manganese deficiency has not been observed.”
A Kansas study. The interest in manganese and glyphosate was boosted by a study conducted by Kansas State University agronomist Barney Gordon prior to his retirement in March. In the study, a glyphosate-resistant soybean variety responded to manganese fertilization while a conventional variety did not. But Gordon cautions against reading too much into the results.
“This was a high-yield study, involving only two varieties, with all nutrients at adequate levels and irrigation available,” Gordon says. “The following year, both varieties responded to the addition of manganese, probably due to much different growing conditions. That suggests environmental conditions are an important factor in yield response.”
Iowa State University agronomist Bob Hartzler doesn’t consider the glyphosate and manganese inter-actions to be a serious threat as long as farmers understand the issue and
“Glyphosate is known to form complexes with manganese and other metal cations that may reduce the availability of the cation and glyphosate activity,” he says.
“However, most interactions between glyphosate-resistant soybeans and manganese have been found in areas with soils known to be deficient in manganese,” Hartzler continues. “We recommend managing glyphosate-resistant soybeans the same as conventional varieties in terms of fertility.”
Careless Liming Can Cause Manganese Problems
Soybean plants can suffer from manganese deficiency even in fertile soils if the soils are overlimed, points out Farm Journal Field Agronomist Ken Ferrie. “I’ve seen growers induce manganese deficiency by liming a low-organic matter, low-cation exchange capacity soil the same way they would lime a heavy soil, applying 3 tons to 4 tons of lime per acre every four or five years,” he says.
“Overliming also can occur if growers follow a lime recommendation that assumes conventional tillage in a strip-till or no-till environment,” Ferrie adds. “That recommendation assumes the lime will be mixed into the top 6" of soil. If you apply that amount in strip-till or no-till, it can cause the surface pH to increase.”
The bottom line, Ferrie says: “Always follow a soil test when implementing a liming program, and take your tillage practices into account. With strip-till or no-till, you may need to apply smaller amounts of lime more frequently.”
The Glyphosate and Manganese Debate
Whether glyphosate-resistant soybeans respond differently to manganese when compared with conventional varieties is still being debated in the scientific community.
Iowa State University agronomist Bob Hartzler states: “Although some research indicates glyphosate-resistant varieties may respond differently to manganese than conventional varieties, the majority of research does not support that observation.”
Retired Purdue University plant pathologist Don Huber responds: “There is now considerable research documenting this relationship. In fact, I am not aware of research designed to evaluate this relationship that has shown contrary results.”