All that glitters may not be gold regarding foliar fertilizer. Macronutrients and tissue testing are gospel in many parts of agriculture, but receiving a consistent, positive yield return on foliar-applied nutrients isn’t backed by replicated research, according to University of Arkansas (UA) Extension personnel.
As producers battle consistently anemic commodity prices, any avenue to push yields is all the more alluring, but Nathan Slaton, a soil scientist with UA Extension, says most mid-season to late-season foliar feeding is unwarranted. He has conducted multiple years of soybean trials with commercial foliar products and found no yield bumps, yet has consistently noted yield increases in controlled plots using pre-plant fertilization with phosphorus and potassium.
“We don’t have a vested interest except to find out what works best in a farmer’s field,” Slaton says. “In this tough economic climate, we don’t want farmers to spend on inputs that don’t bring an unbiased, research-supported return.”
UA Extension soybean specialist Jeremy Ross echoes Slaton’s stance, and says many commercial foliar claims regarding yield boosts don’t stand up to testing: “We’ve gone through a lot of these products in many states and we don’t see yield impacted as a result of foliar feeding. A lot of the products have a low concentration of nutrients and if you pencil it out, the numbers don’t work.”
Ross advocates soil testing with a reputable lab, and nutrient application according to fertility recommendations. He also sticks to a strip trial mantra for any input or management product, including foliar feeds. Strip trials require extra time and effort in the midst of a harvest swirl, but Ross says the data is invaluable: “Feel-good applications do happen and they cost money. Instead, compare the strip trial numbers at season’s end and the facts will make the work worthwhile.”
Despite the unprecedented market promotion of foliar feeding products, Slaton believes the issue hinges on two bottom-line questions. What is the frequency of crop response and what is the average yield increase? He says unbiased, research-based data is difficult to obtain, but over multiple years and tests, university research results show lackluster yield boosts through foliar feeding. “Do your own testing because of the high amount of products available coupled with the low amount of funding for university testing,” he notes. “If I was going to spend thousands for a treatment, I’d demand the data out of my own fields.”
Macro v Micro
Darren Goebel, director of Global Commercial Crop Care for AGCO, says his corn research shows a dim yield response to foliar fertilizer: “It’s highly improbable to see much crop yield benefit from macronutrients delivered by foliar feeding.”
Goebel also points toward the potential damage to the plant factory which functions when leaves capture sunlight and convert it to sugars pumped into the ear. Damage to the factory causes lesions. Essentially, foliar products can produce leaf burn, he says.
However, Goebel notes the legitimacy of foliar applications to deliver micronutrients to increase yield. “If a field is deficient, I believe soil-applied micronutrients still bring a better response,” he adds. “Overall, from a cost and yield perspective, I look at foliar feeding as a Band-Aid for a problem that occurred in the field, not as a practice to gain high yielding crops.”
Some micronutrients are a foliar feed exception, effective and backed with research, Slaton notes. A handful of boron dissolved in water and sprayed onto soybean foliage can prevent substantial yield loss when applied at the right time in fields that are prone to boron deficiency. However, Slaton stops short of stretching particular micronutrient successes to the wider range of nutrients: “Growers must question the nutrient type and application point. Is there research to support an application of a specific nutrient at the specific window to bring a positive yield response?”
Tissue Samples
Tissue analysis has the potential to be a powerful tool, Slaton says, but the current level of knowledge surrounding the practice isn’t as solid as commonly believed. Primarily, tissue analysis in most row crops is used as a trouble-shooting tool. In soybeans, there are critical concentrations of elements published, but the data tends to focus on a single growth stage, according to Slaton.
“I’m all for tissue analysis,” he explains, “but the way it’s used is wrong. We’re not able to interpret tissue analysis across a wide range of growth stages and elements. We’re just not at that level yet.”
Pared down, tissue sampling followed by product recommendations is a minefield of interpretation. In addition, Slaton says even when tissue analysis shows a deficiency of a particular element, there is no assurance of a yield response to subsequent fertilization. He encourages producers to compare tissue samples with fertilization programs, soil samples, yield maps and other available tools. “Take tissue samples, but don’t take them to trigger nutrient applications,” he advises. “Use them as a part of the overall data collection process.”
| “We’re just going where the evidence leads after years of testing across a multiplicity of locations,” says UA Extension soybean specialist Jeremy Ross.
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Ross backs tissue sampling as a solid tool, but warns overreliance can lead to extra applications of products that “may not add anything, could arrive too late, or simply be incorrect.”
During Goebel’s first years as a crop consultant, he relied exclusively on tissue samples that often indicated an absence of growth problems or issues. As he began pulling soil samples in tandem, the soil data was often telltale. Goebel typically looks for visual symptoms of nutrient deficiency, and then collects soil and tissue samples. On soil tests, he makes a careful check of pH levels which can make a big impact on micronutrient uptake. “Sometimes people don’t realize tissue sampling leaves a lot of room for error because it’s a matter of incredibly minute nutrient levels,” Goebel says. “Even dust on leaves can skew results. I’ve sent multiple samples from the same field and the same error has produced wildly divergent results.”
Chasing Evidence
Critical concentrations are generated from tissue surveys or replicated research trials from a diverse group of crop fields at a particular growth stage. (For some nutrients such as potassium there is published research that defines critical concentrations. Slaton and Ross are working on defining soybean yield response according to the time of K application.) The data forms a basic bell curve, but the relationship between yield boosts and nutrient concentrations is ill-defined. “In the absence of hard evidence, survey information has to be taken with a grain of salt,” Slaton says.
“Nutrient deficiencies are merely a range on the bell curve and at a specific point during the season, there’s going to be all sorts of different concentrations of nutrients for many legitimate reasons,” Ross explains.
In summation, Goebel advises growers to consider the cost of foliar macronutrient application at minute levels: “Calculate it out on a pound per acre basis and translate it to a soil applied nutrient; the numbers don’t make sense.”
“We’re just going where the evidence leads after years of testing across a multiplicity of locations,” Ross says. “In the end, we want producers to have the most economical recommendations to produce the best crop possible. If a farmer is about to pay out thousands on a foliar treatment, he wants evidence,” Slaton adds. “He expects hard proof.”
