By Kevin Fry, PennState

During the past several years, there has been considerable interest in tank mixing some foliar fertilizers with the Glyphosate application to soybeans in early June. Responses to foliar fertilizers in the literature have been inconsistent and mostly associated with crops that have a documented deficiency. Recently, there has been some concern that glyphosate could cause temporary manganese deficiency. A key question among soybean producers in our area is whether these applications are profitable. Potential foliar fertilizers that are marketed for soybeans include N-P-K blends, micronutrient blends and manganese foliar fertilizer treatments.

To address this question we initiated a study in Armstrong County in 2008. We designed a replicated strip test with four treatments which included an untreated check with only Glyphosate, and then three fertilizer treatments included with the Glyphosate: 3 gal/A of 5-15-15, 2 qt/A micro nutrient blend (Boron 0.20%, Iron 0.30%, Manganese 3.20%, Molybdenum 0.01%, and Zinc 2.10%), and 2 qt/A manganese (chelated Mn 5.0% and Sulfur 2.8%). In 2009 we expanded the study to three locations. Soil samples and tissue analysis were taken at each location just prior to application.

We did not observe any effect on weed control at any of the locations, there did not appear to be any antagonism with Glyphosate activity. We also did not find any significant differences in yield due to the application of the fertilizers at any of the four locations, see figure 1. The Armstrong 2008 site was conventional tilled and drilled in 7 inch rows. All the 2009 sites were no-till drilled in 7 inch rows. Visual evaluations at all four sites didn’t reveal any differences in leaf color or size, canopy height, and damage. Soil and tissue test results showed all nutrients for all four sites to be in the optimum or above range.

 Soybean Yield Responses to Foliar Fertilizers
Figure 1: Soybean yield response to foliar fertilizers in 2008 and 2009 in western PA.

Based on this study, foliar fertilizers are not a substitution for a sound soil fertility program and do not show a yield response when plant deficiencies do not exist. Soil and tissue test results showed no nutrient deficiencies at the time of application.

Our recommendations for foliar fertilizer applications in soybeans continue to be to use them to correct micro-nutrient deficiencies that may occur during the growing season. Continue soil testing and correct soil nutrient deficiencies with granular fertilizer or manure applications. Suspected nutrient deficiencies that occur during the growing season should be confirmed with tissue testing and could be corrected with a foliar fertilizer especially if dealing with a micronutrient deficiency.