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Potassium Deficiency Symptoms in Soybeans Reported

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Potassium deficiency symptoms in soybeans near Hunter, N.D. (August, 2009) Potassium deficiency symptoms in soybeans near Hunter, N.D. (August, 2009)
It was just a matter of time before deficiencies began to show up somewhere in the state.

North Dakota State University has received reports of potassium (potash) deficiency symptoms in soybeans, according to R. Jay Goos, professor of soil science.

"We've been expecting this for many years," Goos says. "North Dakota soils have been farmed for more than 100 years with little or no input of potassium (K) fertilizer. It was just a matter of time before deficiencies began to show up somewhere in the state."

Goos says that the vast majority of North Dakota soils still are well-supplied with potassium and do not need K fertilization at this time.

"The problems are showing up on fields with sandier textures and a longer history of producing soybeans,” Goos says. “I ran into potassium deficiency symptoms in soybeans on a field near Colfax in 2008 and by Hunter in 2009. In both cases, the soil was a fine-sandy loam."

The large increase in soybean acres is partially responsible for the problem. A 120-bushel-per-acre corn crop removes about 20 pounds per acre of K2O equivalent. A 50-bushel-per-acre wheat crop removes about 15 pounds per acre. However, a 40-bushel-per-acre soybean crop removes almost 60 pounds per acre of K2O equivalent. Also, during the past 10 years, some farmers grew soybeans after soybeans, which led to a drain of available K in the soil far greater than one would have with corn or wheat.

Symptoms may be more severe this year because of dry weather.

"When the topsoil dries out, the water films between the soil and the roots become very thin, so the diffusion of K to the roots slows greatly,” Goos says. “This probably was the factor that led to these deficiencies being more obvious this summer on soils marginal in available K."

Potassium deficiency symptoms appear as a yellowing (chlorosis) or death (necrosis) along the edges of the leaves. The youngest leaves usually appear to be OK. The symptoms occur on mature leaves.

Potassium deficiencies are difficult to correct in a standing crop, so prevention is the best management practice.

“We have a good soil test for potassium and have established recommendations,” Goos says. “Potassium does not leach away like nitrogen or get tied up strongly with the soil like phosphorus. A broadcast application of potash fertilizer followed by tillage is the traditional method of application. Potassium also can be placed in a fertilizer band a safe distance away from the seed. Band applications are quite effective, but the amount that can be applied is limited."

Landowners who rent to others also need to assure that soil tests are taken and a K fertilization program is instituted, especially if soil tests drop below 120 parts per million. Soils can become depleted by high-removal crops such as soybeans, alfalfa and potatoes.


NDSU Agriculture Communication

Source:R J Goos, (701) 231-8581, rj.goos@ndsu.edu
Editor:Rich Mattern, (701) 231-6136, richard.mattern@ndsu.edu
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