Application method and timing may play a role in nutrient efficiency. McFarland said foliar fertilization may be a useful tool, under limited circumstances.

“We have two situations where we might consider use of foliar fertilizer,” he said. “If we have a deficiency in-season, we may need to apply foliar fertilizer to salvage the crop.”

The other situation where a foliar nutrient application might be justified is to take advantage of excellent growing conditions to achieve “a bonus yield.”

With a deficiency, the application may be justified, he says; for bonus yield, it’s unlikely to pay. Pushing yield requires significant amounts of the primary nutrients, like nitrogen, and foliar applications are not as efficient as soil-applied fertilizer. High rates also may burn leaves, so lower rates with multiple applications are often necessary to get enough nutrient into the plant, and that increases the cost.

“Plants are designed to take in nutrients through their roots,” McFarland said. “So do the math, and most often you will find that the (bonus) application provides no real economic advantage.”

With micronutrients, the situation might be different, however. A zinc deficiency, for instance, may be corrected with foliar applications. “But soil application is more effective,” McFarland said. “A farmer can get three years of soil applied zinc for what one foliar application will cost.”

He says no option except a foliar application is available to correct iron deficiencies. “But even with foliar application we get no translocation through the leaves, so we have to make multiple applications to be effective. That takes more time and more expense. The cost usually exceeds the advantage, even with iron chlorosis.”

He addressed other claims that purport to help a crop with additional fertility. Products with a combination of nutrients, for instance, claim to provide yield boosts. In numerous tests across Texas, however, yield responses with these additions “were flat versus basic fertility programs. There was no deficiency to fix,” McFarland said.

“Unless a deficiency exists, foliar nutrient applications provide little or no value and are much less effective than soil-applied fertilizer. It’s better to soil test and avoid the need to use foliar applications as a last resort.”

Potassium deficiencies, even in heavy Blackland soils that typically have adequate levels, have shown up more often in the past few years, McFarland said. “In many cases it’s drought related. Potassium uptake occurs through diffusion, so without adequate moisture, uptake slows or stops. When clay soils dry, they shrink, collapse and trap potassium in the clay layers. It can’t get to the roots.” Compaction also may play a role.

Liquid potassium offers a viable option. “With band injection, the yield response has been substantial,” McFarland said. “It is an advantage to place potassium in the active moisture zone.”

He recommends growers soil test to determine if potassium is adequate. “If it’s marginal, 125 to 150 parts per million, it may justify adding potassium. Deeper samples also may show available potassium. If it’s adequate at depth, yield response to an application is not likely.”

Soil pH also affects nutrient uptake and fertilizer efficiency. A pH below 6.0 can affect yield potential. “As pH drops, solubility of nutrients also drops.” A good range is 6.4 up to 7 and McFarland recommends growers “stay well away from low levels. As pH goes below 5.5, aluminum toxicity can begin to be a problem.”