When corn plants lack nitrogen, their embarrassing pale yellow color is pretty obvious to neighbors who drive by. Phosphorus or potassium deficiencies have their own telltale signs.

But if there's a lack of micronutrients, nobody — including the affected farmer — may be aware of it. And that's bad, because deficiencies in micronutrients like manganese, copper, zinc, iron or boron can put a hit on yield.

“You may have plants that are a little stunted and yields that are not great, and not know why,” says Don Huber, a Purdue University plant pathologist. “Micronutrient deficiency may be the reason.”

Huber points out that a micronutrient deficiency also can have a striking impact on a plant's susceptibility to disease, and on the severity of the disease.

“Micronutrients regulate a plant's physiology,” he explains. “It doesn't take a lot of micronutrient to mobilize a plant's disease resistance, but it is critical.”

Cary's Pioneer Farm, Alma, MI, has experienced some micronutrient deficiencies. It's also seen an excellent payoff from correcting those deficiencies. The Cary operation grows corn, soybeans and edible beans.

In 1994 the operation began using the services of independent crop consultant John Fedewa, Dennings & Associates, Elsie, MI. The Dennings group is affiliated with Brookside Laboratories, New Knoxville, OH. Brookside-affiliated consultants routinely soil test for all micronutrients as well as the major and secondary elements. In his initial testing, Fedewa uncovered deficiencies in boron, copper and zinc in the Cary's fields.

“The copper test was so low in 1994 that we couldn't even get an accurate reading,” Fedewa reports.

“The Carys started applying 4 lbs/acre of zinc, ½-1 lb/acre of boron and 1 lb/acre of copper to dry starter fertilizer,” says Fedewa. “They are in a corn-bean rotation and apply those micros plus manganese in the corn and edible bean years. Levels of all three elements have improved dramatically.”

In his area of central Michigan, says Fedewa, the ideal test readings are 1.6-2 lbs/acre (0.8-1 ppm) for boron, 6 lbs/acre (3 ppm) for copper and 10 lbs/acre (5 ppm) for zinc.

Although the Carys have not run yield checks that compared the previous micronutrient levels to present levels, they have enjoyed excellent production in recent years.

“We weigh all our crops at harvest,” says Scott Cary. “Our corn has been about 15 bu/acre higher than eight or 10 years ago. Soybeans are up by at least 5 bu/acre.

“We used to think a 125-bu/acre corn average was good for this area, and lately we have averaged 140 bu/acre. At times we have even gotten 200-bu corn and 60-bu soybeans,” he says.

Cary claims part of their yield improvement has been due to good weather. But he's convinced that correcting the micronutrient levels also has been a factor.

He points out, however, that micronutrients — although a profitable investment for his operation — are expensive compared to major and secondary elements.

Fedewa says micronutrient deficiencies are most often seen on muck ground, black sands or where pH is high (more than 7.0), phosphorus is excessive (150 ppm or 300 lbs/acre), or sulfur is elevated (100 ppm or 200 lbs/acre).

“Very high phosphorus levels can tie up zinc, and excessive sulfur can bind up molybdenum,” Fedewa explains. “High pH, high-organic-matter soils (above 5%) and muck soils can lead to a manganese deficiency.”

Fedewa points out that a micronutrient deficiency often is crop-specific. For example, a boron deficiency in alfalfa or sugar beets may not be a deficiency in corn or soybeans.

Don Brucker, an independent crop consultant from Melvin, IL, in the heart of corn and soybean country, sees occasional micronutrient problems.

“Usually, it involves the pH being out of whack and tying up the micros,” he reports. “However, there are times when the pH is right on and we still have micro deficiencies.”

Brucker says most micronutrient problems can be corrected with one or two years of soil application of the needed element. That normally takes care of things for a long time. This is particularly true for copper, zinc and boron.

“I've found some soils that don't respond to a soil application of manganese,” says Brucker. “In those cases we do a foliar feeding.”

The upshot on preventing micronutrient deficiency, says Michigan consultant Fedewa, is regular and thorough soil testing. It also pays to do occasional tissue testing, especially under those conditions where a deficiency is more likely to occur.