Nitrogen, zinc, spraying, sidedressing, preplant, postharvest — so many words come into play when speaking the fertilizer language. Where does foliar fit in?

“I think foliar (fertilization) has its place, but I wouldn't use it as the primary method of applying large quantities of nutrients,” says Keith Kelling, retired soil scientist at the University of Wisconsin-Madison. “It can work for micronutrients and for supplemental applications of primary and secondary nutrients.”

Robert Mullen, soil fertility specialist at Ohio State, agrees. “The only time I would promote foliar fertilization is to correct a micronutrient deficiency that is either indicated or has been diagnosed before,” he says. “We would really prefer that the deficiency be determined using tissue testing, but if the field has a history of deficiency or some other soil property indicates a likelihood of deficiency, then application is warranted.”

That application, however, is for micronutrients. What about the macros?

“Their (foliar macronutrients) use is recommended sparingly,” says Mullen. “Essentially it is used to supplement the crop through a tough stretch.”

Small amounts of foliar fertilization could supplement inadequate preplant fertilization and could enhance growth if soil conditions limit uptake, according to Antonio Mallarino of Iowa State University. However, in replicated field trials conducted from 1994 to 2006, foliar fertilization evaluated at several growth stages increased yield in only 15-20% of the fields with an average yield response to the best treatment (3 gal./acre of 3-18-18) across all trials of 0.7 bu./acre.

Other trials in the 1970s and 1980s across the Midwest and in southern states spraying at various reproductive growth stages showed inconsistent results with increases of 5 bu./acre and decreases of up to 6 bu./acre. Mallarino says that recent work in the Midwest under rain-fed conditions showed similar results, along with yield decreases when nitrogen sources were sprayed at high rates alone or in a mixture.

“UNFORTUNATELY, THERE IS no simple yardstick that can be used to identify conditions that increase chances of response to foliar fertilization,” says Mallarino. “Foliar fertilization of soybeans will not be cost-effective in Iowa when applied across all fields because the average response is less than 1 bu./acre.”

He also adds that unless fields are targeted for spraying, the probability of an economic response to foliar fertilization of beans is about 15-20%.

“The probability of a yield response that offset costs will be increased by targeting fields for spraying,” advises Mallarino. “These include fields with low soil nutrient levels due to insufficient preplant fertilization and conditions where soil or climate factors limit nutrient uptake in late spring and early summer.”

If foliar fertilization is used, whether for micro or macronutrients, application should happen at a stage when more will fall on the leaves, not the soil.

“The timing of the application is dictated by the timing of the onset of deficiency,” says Mullen. “Intuitively, with a foliar spray, application should be made when there is adequate foliage in the field to intercept the spray. Thus, early applications are not generally recommended, and typically, micro/macro deficiencies are not evident early.”

Though a deficiency may be realized, it's important to remember that foliar fertilization is not a replacement for a “sound soil fertility program,” says Mullen. “In fact, a sound fertility program should limit the need for foliar nutrients.”

The need for foliar nutrients should be determined through soil testing. Knowing what exists in your soils gives you the upper hand in determining what nutrients are needed and how to apply them. With products being marketed to entice farmers to use foliar fertilization, it's even more important to know what's necessary for your fields.

“There are N products being touted as having much more N than analysis and as being very effective, but we see no data supporting that,” says Gyles Randall, soil scientist at the University of Minnesota Southern Research and Outreach Center. “The products being marketed are high-margin products — if prices are high, people sense that farmers will take a chance on them. The farmer has to make the decision of getting a return on investment below 50-50, and quite a bit less than that for corn.”

Randall says that the best return for the money spent comes from applying nutrients to the soil and letting the roots take it up. “Foliar fertilization of corn and soybeans isn't nearly as efficient as putting nutrients on the soil,” he says.

“Foliar applications of macronutrients are difficult to justify economically,” says Mullen. “A producer is much better off to correct these issues with soil fertilization.”

The bottom line is that foliar fertilization may be helpful, but mainly for adding micronutrients such as zinc, boron, etc. If used, it should be used as a supplement for deficiencies and applied at a growth stage where it will fall mostly on leaves and not just the soil. Only necessary nutrients should be applied, so beware of products that may have more than what your crop is deficient in — too much of some nutrients applied through foliage may cause burning.

“It's important to recognize foliar fertilizers for what they are,” Mullen points out. “They're a way to provide a little nutrient — primarily micronutrients — to get the crop through a tough time and are not a replacement for a sound soil fertility program.”