There is growing interest among South Dakota's soybean growers in foliar fertilization of soybeans - especially for micronutrients based on plant analysis, says Ron Gelderman, SDSU Extension Soils Specialist.

"In general, foliar fertilization has not proven effective unless there is severe deficiency of a particular nutrient," Gelderman said. "Soybean requires large amounts of the macronutrients nitrogen (N), phosphorus (P), and potassium (K) and smaller amounts of secondary and micronutrients to produce maximum yields."

He explains that 1 bushel of soybeans requires about 4.9 pounds of N, 1.1 pounds of P, 2.4 pounds of K and 0.4 pounds of sulfur.

"On the other hand, micronutrients such as zinc, iron, manganese, copper and boron are taken up as a fraction of a pound on an acre basis," Gelderman said. "The amount that is actually needed is less than what is taken up in most cases, but those levels are not well defined because micronutrient deficiencies in soybean are rare - especially in South Dakota with our relatively high fertility soils."

He says that because of the large need for macronutrients, foliar application is usually not considered, adding that macronutrients are better applied as a soil application.

"In high soybean yield situations, there is some evidence that N added in-season at about early pod, (20-30 pounds/acre) may be beneficial but the uptake of the applied N is mostly from the soil rather than foliar," he said.

Gelderman points to a review of foliar fertilization studies conducted by Iowa State which revealed that only about 15 percent of the fields had a yield increase and the average yield response over all fields was less than 1 bushel per acre.

"There was not a good method to predict which fields would respond to the foliar application. Low soil testing fields gave a higher probability of response than high testing soils, but tissue analysis for P and Kconcentrations was not a reliable diagnostic tool," he said.

In South Dakota, there has been foliar fertilization studies on 14 soybean sites, three corn and two small grain sites over the past 35 years. These studies had multiple treatments covering almost all the macro, secondary and micro nutrients. Of these studies, Gelderman says there was only one positive response.

"That was added foliar P to very P deficient corn. There were four treatments that caused yield declines, either from leaf burn or other unknown causes," he said.

Gelderman says there has also been an increased interest in adding manganese with glyphosate applications to soybean.

He says most of these positive responses took place in Indiana on very high organic matter, high pH soils.

"These are soils on which we would anticipate a micronutrient deficiency. These soils are not found in South Dakota," Gelderman said.

The bottom line?
According to research, Gelderman says foliar nutrient applications seldom pay on soybean and are more likely to cause yield declines.

"In addition, plant analysis is not a good predictor of fertilizer response to added nutrients," he said. "Soil testing is a much better indicator of nutrient need. Unless a rescue operation is needed, most nutrient applications should be done prior to or at planting."