“A lot of the [foliar] products have a low concentration of nutrients, and if you pencil it out, the numbers don’t work,” adds Jeremy Ross, UA Extension soybean specialist.
In addition to minimal yield response, Darren Goebel, director of Global Commercial Crop Care for AGCO, points to potential damage to the plant factory. Damage to the factory causes lesions. Essentially, foliar products can produce leaf burn, he says.
While some micronutrients can be delivered via foliar application, Goebel says the benefit is questionable when compared with soil treatments. “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,” he says.
Some micronutrients applied via a foliar feed are effective and backed with research, Slaton says. For example, 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 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,” he says. “Is there research to support an application of a specific nutrient at the specific window to bring a positive yield response?”
Tissue analysis can be helpful to trouble shoot mid- to late-season issues, but the data tends to focus on a single growth stage, Slaton notes.
“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 fertilizer 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 part of the overall data collection process.”
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 no growth problems or issues. As he began pulling soil samples in tandem, the data was telltale. Goebel typically looks for visual symptoms of nutrient deficiency, and then collects soil and tissue samples. On soil tests, he carefully checks pH levels, which can impact 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.”
Critical concentrations of nutrients 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 potassium 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. Ross encourages farmers to conduct their own strip trials for any input or management practice, including foliar applications. Strip trials require extra time and effort in the midst of planting and harvest, but 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,” Ross says.
Focus your on-farm research on two bottom-line questions, Slaton adds: What is the frequency of crop response and what is the average yield increase?
Consider the cost of foliar macronutrient application at minute levels, Goebel advises: “Calculate it out on a pound-per-acre basis and translate it to a soil-applied nutrient; the numbers don’t make sense.”