If somebody tells you your corn and soybean crops can benefit from micro- or secondary nutrients, ask for hard evidence before you buy. University soil scientists and crop consultants in the Corn Belt generally don't see a strong need for these elements.
Micronutrients are boron, chloride, copper, iron, manganese, molybdenum and zinc. The secondary nutrients are calcium, magnesium and sulfur.
"Deficiencies of any of these nutrients are not extensive in the Midwest," says University of Illinois soil scientist Bob Hoeft. "However, if there is a deficiency, it will result in a significant yield reduction unless corrected."
Sulfur is a secondary element that will need to be watched, Hoeft points out. Deficiencies could develop because air-quality standards have markedly reduced the amount of sulfur received from the atmosphere. Also, higher soybean yields are removing more sulfur.
In Minnesota, the most-often-deficient micronutrient is zinc for corn and edible beans, says University of Minnesota soil scientist George Rehm. "We also sometimes get iron chlorosis in soybeans and need supplemental iron."
Ric Fritz, a crop management specialist with Mowers Soil-Testing Plus, Toulon, IL, says company personnel don't see many micro- or secondary nutrient deficiencies in the multi-state Midwestern area they serve.
"Those nutrients sometimes are needed in horticultural crops, but seldom in corn and soybeans," says Fritz. "The exception is on sandy soils with low nutrient levels or on very high-pH soils (7.5+)."
Mowers personnel occasionally find iron chlorosis and manganese deficiency in soybeans on high-pH soils. "In those situations we generally recommend bean varieties that can tolerate the soils," he says.
Fritz points out that the best way to detect micro- or secondary nutrient deficiencies is with tissue tests. "Soil tests for those elements are on the low end of the accuracy scale."
For example, the University of Illinois Agronomy Handbook rates the reliability for a phosphorus soil test at 85 on a scale of 0-100. In contrast, a soil test for boron (in corn and soybeans) is rated at 10, manganese at 10 and iron at 10 (for soils with less than 7.5 pH). Sulfur, calcium, magnesium and zinc have soil test reliability ratings in the 40s.
"We are on the lookout for micronutrient deficiencies as we scout fields," says Fritz. "If we see visual symptoms, we can do a tissue test. A plant tissue test that includes nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, manganese, iron, copper, aluminum and sodium is available." ?