In an era of climbing costs, farmers need to scrutinize every part of their operation to control expenses and maintain profitability. One expense many farmers can skip is foliar fertilizers.

Foliar fertilizers are applied to the leaves of plants, and the leaves take up the nutrients and use them for plant nourishment. Generally, foliar fertilizer is used to add micronutrients – nutrients that the plant requires in very small amounts for proper development.

However, foliar fertilizer in research tests conducted on crops not exhibiting signs of nutrient deficiency have not been shown to improve yields, said Greg Schwab, soil management specialist with the University of Kentucky Cooperative Extension Service. In addition, it is not uncommon to see a reduction in yields where the plant was burned as a result of the application.

Fertilizers containing many different nutrients are expensive and are not appropriate to apply in a random fashion, Schwab said. Often these products are promoted to cure “hidden hunger,” but no such hidden nutrient deficiencies have been identified in Kentucky.

“In Kentucky we are fortunate that the soil adequately supplies micronutrients so long as the soil pH is in the appropriate range for the crop,” he said.

The only nutrients with a valid soil test in Kentucky are phosphorus, potassium, magnesium, boron and zinc, Schwab said. To identify deficiencies for any other plant nutrient, you have to look at the plant itself. A new UK publication explains plant tissue testing which is the only way to test for these deficiencies.

Schwab said he wants farmers to understand that before applying a foliar fertilizer, they need to know what the nutrient deficiency is that they are trying to correct.

“I’m not going to go out and spend a lot of money on a fertilizer that contains several elements and apply it regardless if those elements have been identified as deficient,” he said.

It is also common to see foliar fertilizer products that contain macronutrients like nitrogen, phosphorus and potassium. Because the plants need large amounts of these nutrients, the small amounts supplied by these fertilizers are inconsequential to the plant.

“Neither method is a very good use of your fertilizer investment dollars,” Schwab said. “You want to supply macronutrients as soil applied fertilizers and then use tissue analysis and supply only the limiting micronutrient.”

There are some specific instances where micronutrient deficiencies in Kentucky can be corrected with a foliar fertilizer containing just that missing element, he said.

Manganese deficiency in soybeans is one where a targeted micronutrient application may be worthwhile. The symptom is a yellowing of leaves often in a wet area. If tissue analyses in these areas show manganese deficiency, then Schwab said UK recommends a foliar application of manganese sulfate. Because the plant takes up these nutrients through the leaves, plants need to have plenty of leaf area to absorb the fertilizer to maximize fertilizer uptake efficiency.

A micronutrient application might also be in order for zinc deficiency in corn. This deficiency is characterized by wide white spots in the leaf followed by whole leaves turning white and dying.

“If you see this you may be want to apply a rescue application of zinc fertilizer,” he said.

But, the best time to apply zinc is when applying other nutrients to the soil. To determine the amount of zinc to apply, a producer will need to review his soil tests for phosphorus and soil pH levels. The higher these two levels, the more zinc they will need to apply. A chart for this can be found in UK publication AGR 1.

“If you don’t apply zinc as a soil applied fertilizer when applying other fertilizers, a foliar fertilizer may be needed,” he said.

To learn more about tissue sampling and soil nutrient needs, contact a county office of the UK Cooperative Extension Service. Offices are located in every county in the state.