Increasing commodity prices increases farmers’ willingness to try new practices to potentially squeeze additional yield from their crops. Foliar fertilization, also known as foliar feeding, of soybean is an example of such a practice. Although soybean foliar feeding is not new to agriculture, it might be a new concept to some farmers.
In my experience, and in northcentral and northeastern Iowa where I work, soybean foliar feeding is an uncommon practice that seems to only gain in popularity when the soybean market prices reach high levels. As soybean market prices decline, so does the interest in soybean foliar feeding.
Researchers have conducted numerous studies during the past several decades trying to determine if foliar feeding might be a viable tool to help improve soybean yield. Many early studies focused on foliar feeding with nitrogen (N), phosphorus (P), and potassium (K); however, the researchers were unable to identify specific field or crop conditions where foliar fertilization provided a consistent benefit. Additionally, since N, P, and K are macronutrients that soybean plants require in high amounts during the growing season, researchers found that the soybean leaf tissue was unable to absorb sufficient nutrient amounts to be beneficial. Researchers also found that when N and/or K was over-applied, leaf burn occurred, which resulted in a yield loss.
Recent research has seemed to shift its focus away from macronutrient foliar feeding to micronutrient foliar feeding because of the small micronutrient amount required. Unfortunately, the research suggests foliar feeding with micronutrients is seldom worthwhile, probably because most soils contain adequate amounts of plant available micronutrients. However, there are instances where foliar fertilization has been used to manage micronutrient deficiencies of iron (Fe) and manganese (Mn) in soybean.
Research from North Dakota State University found that foliar application of Fe had a positive impact on the regreening of soybean leaves suffering from iron deficiency chlorosis (IDC) (Figure 1), but there were no significant improvements in yield.1 Iowa State University suggested that foliar applications of Fe could be beneficial for soybean plants suffering from IDC, but the Fe would need to be applied at the onset of the IDC symptoms and reapplied multiple times; however, the ensuing yield response would typically not be as good as that of a high-yielding IDC tolerant soybean product.2 In the end, researchers have concluded using multiple tactics such as soybean product selection, higher soybean seeding rates, planting an oat cover crop to reduce soil nitrates, planting into fields with low soil nitrate levels, and applying EDDHA-Fe chelates on soybean seed at planting are more likely to provide a positive economic yield response than foliar feeding with Fe.
Foliar fertilization with Mn has shown positive responses in states such as Indiana, Michigan, and Ohio where plant available Mn can be low due to, or a combination of, low manganese soil test values, dry soil conditions, or high soil pH. However, researchers from both Michigan State University and Purdue University concluded that the only times where soybean foliar feeding with Mn was profitable was when a Mn deficiency was identified either visually or through tissue testing (Figure 2).3,4
Farmers interested in learning more about foliar fertilization on their own farms should consider experimenting with it on a small scale using replicated strips to confirm that the potential benefits are economically profitable before making applications to a large amount of acreage.
1Chatterjee, A., Lova, S., Rasmussen, H., and Goos, R.J. 2017. Foliar application of iron fertilizers to control iron deficiency chlorosis of soybean. Crop Forage Turfgrass Manage. 3:1-7. doi:10.2134/cftm2017.05.0037
2Sawyer, J. 2009. Foliar fertilization of corn and soybean. Integrated Crop Management. Iowa State University, Ames. https://crops.extension.iastate.edu/.
3Douglas, J. 2009. Manganese deficiency in soybeans? Experts advise foliar application. News. Purdue University, West Lafayette. https://www.purdue.edu/.
4Staton, M. 2019. Foliar fertilizer applications to soybeans are rarely profitable. Michigan State University Extension, East Lansing. https://www.canr.msu.edu/.
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