For farmers considering emergency or catch-up fertilization of their soybean fields yet this summer, Antonio Mallarino, an agronomy professor at Iowa State University (ISU) working in soil fertility and nutrient management research and extension, says in-season fertilizer application for soybeans will seldom be cost-effective in Iowa.

“Producers may be wondering about it, if fertilizer rates were inadequate of if late planting dates, replanting and cold or excessively wet conditions have altered the crop nutrient uptake and fertilization need,” Mallarino says. “Or they might wonder whether soil nutrient levels might limit high-yield potential when growing conditions are good.”

Is applying dry, granulated fertilizer a viable postemerge option? “The short answer to this question is probably not,” Mallarino says, noting two main reasons.

First, both P and K (but especially P) are needed at early growth stages to cell multiplication when the number of nodes, leaves and potential seed numbers are being largely determined. In addition, application of fertilizer to the soil surface or banding between rows will be of low efficiency.

“The only situation in which an in-season application of granulated P and K fertilizer might be considered is when soil tests are very low and the producer will certainly have to apply a high fertilizer rate for next year’s corn crop,” Mallarino says. “In this scenario, an in-season application may not be very efficient at increasing grain yield, but will increase soil P and K levels and decrease the rate needed before the next corn crop.”

Mallarino says, while foliar fertilization could help improve soybean growth and grain yield, the potential is quite low in fields that have been well fertilized or where factors other than nutrient supply limit growth.

In summary, Mallarino says, “With the possible exception being when soil samples confirm soil tests are very low or there was insufficient preplant fertilization, a large application of granulated fertilizer may result in some yield increase and will begin to build up soil test levels for the next crop, but the economic benefit for this year’s soybeans is very doubtful. Similarly, the probability of an economic response to foliar fertilization is small, but it may be justified when nutrient deficiency symptoms are obvious, with confirmed deficient-testing soil, or when soil or climatic factors limit nutrient uptake in late spring or early summer.”

Iowa Soybean Association Director of Production Research David Wright says, “With soybean prices declining, farmers are looking for new tactics to improve soybean yield. They should be cautious when considering applying foliar fertilizer to soybeans and should consult with an ISU Extension specialist or another trusted crop advisor.”