As soybean yields edge upward, there's a growing risk that nutrient deficiencies will put a lid on further yield gains.

The problem that may have snuck up on you, especially if you're pushing hard for high yields, is shortages in the secondary and micronutrient complex. Those elements are seldom considered, unless an obvious deficiency raises a red flag.

"They're not common, but deficiencies of calcium, magnesium, sulfur, boron, iron, manganese, molybdenum and zinc do occur," says Bob Hoeft, University of Illinois extension soil specialist. "And deficiencies can result in significant yield reductions if not properly corrected."

Calcium is seldom deficient in soils from Iowa to Ohio and Pennsylvania. If soil pH falls, or is naturally below 5.0, however, calcium could be deficient. Hoeft says that raising the pH by adding calcitic limestone usually solves the problem.

Magnesium deficiencies are rare, but have occurred on acid soils. They're usually limited to soils with low cation-exchange capacity, which usually occurs in fields with a history of liming with calcitic rather than dolomitic limestone.

Hoeft notes that highly acid soils (pH under 4.5) are extremely rare, but are most likely to respond to supplemental magnesium, generally in the form of magnesium potassium sulfate or dolomitic limestone.

A 60-bu soybean crop requires approximately 25 lbs of sulfur and only about 40% of that amount is left in crop residues after harvest.

"With this high a removal rate, one would expect to see responses to applied sulfur, particularly on soils where positive responses to sulfur have been reported in other crops," Hoeft says. "In field studies, though, response to sulfur has not been consistent.

"While there have been few documented cases of sulfur deficiency in soybean- producing areas, this is one we need to watch for several reasons," Hoeft adds.

The first reason is federal air-quality standards, which are lowering the amount of sulfur plants get from the air. Less sulfur is being used in pesticides and fertilizers, too. And as crop yields increase, more sulfur is being removed from the soil.

Hoeft says current soil tests for sulfur are not very reliable.

"If test levels are high, we can be fairly certain there will not be a deficiency. However, if test levels are medium to low, there may or may not be a deficiency. Plant analysis is a more reliable predictor of sulfur deficiency."

Small quantities of boron exist in most soils, with most being held in the soil's organic fraction. Hoeft points out that, while soybeans need boron, deficiencies are rare in the Midwest. And because a thin line separates the right amount from too much, when it becomes toxic, he recommends against applying boron unless there is a conclusive need for it.

Sandy soils may be an exception. They tend to be lower in organic matter, and boron doesn't readily attach itself to other soil components. Once released from organic matter, it's easily leached below the root zone.

Iron deficiency has been identified in several states, almost exclusively in soils with 7.3 pH or higher. In these alkaline soils, iron is present but most is in a form that can'tbe used by the plants. Plant leaves become chlorotic (yellow to white color between the veins), and food production in the leaf is inhibited.

Soil tests do a poor job of detecting iron deficiency, since current tests can't differentiate between forms of iron that can and can't be used by plants. But if iron deficiency symptoms develop on a high-pH soil, you'll probably have the same problem every time you plant soybeans on that field, says Hoeft.

Plant iron chlorosis-tolerant varieties as a first defense against it. Second, 0.15 to 0.2 lb/acre of chelated iron fertilizer applied foliarly can remedy the situation. Fertilizer should be applied within three to seven days after symptoms appear, but before the third trifoliate leaf emerges.

Soybeans require just the right amount of manganese. Too little causes a yield- robbing deficiency; too much can be toxic.

Like iron deficiency, manganese deficiency occurs in soybeans almost exclusively on high-pH soils. And it usually happens because the manganese is unavailable to the plants. Initial symptoms are similar to those of iron deficiency. As the deficiency worsens, leaves develop rust-colored spots.

Hoeft says manganese chelate fertilizer, applied foliarly at 0.15 to 0.2 lb/acre, can correct the deficiency.

"Apply as soon as symptoms are detected and only to areas where symptoms occur," he says. "Plants not showing symptoms are not likely to be deficient, so would not benefit from additional manganese."

Applying manganese as a starter fertilizer is also effective in preventing deficiency, but it should be limited to deficient areas.

Manganese toxicity generally occurs on low-pH soils (5.2 or lower). Symptoms are cupped or crinkled leaves and stunted plants.

The least expensive and most effective prevention method is liming soils to increase pH to a level of 6.0 or greater.

Molybdenum becomes more available as soil pH increases. So very acid (below 5.0 pH) soils are most prone to deficiencies. While molybdenum fertilizers are available, the most economical way to correct a deficiency is through liming.

"Molybdenum is essential to the nitrogen fixation process," Hoeft says. "Because of this, symptoms of deficiency are similar to nitrogen deficiency symptoms."

Too little zinc results in stunted, yellow-to-light green plants, which drop lower leaves prematurely. Flowering is reduced and any pods that set are abnormal and mature slowly.

Zinc deficiency generally shows up in corn before levels drop low enough to be a problem in soybeans. While zinc deficiency has been diagnosed across the Soybean Belt, it's most likely to occur on sandy soils, soils high in pH and phosphorus, and/or soils low in organic matter.

On those soils, Hoeft recommends monitoring zinc through soil tests and plant tissue analysis.

"If both of these indicate a deficiency of zinc, then apply some on a trial basis. Research has not always shown a yield response, even when deficiency was suspected.

"So make sure you have a problem by verifying that you get a yield response before going to the expense of applying zinc fertilizer."