By Mark Bernard

When it comes to micronutrients for Minnesota crops, the watchwords are “buyer beware.”

Micronutrients are essential for healthy plant growth. Zinc, manganese, boron, iron and other micronutrients are necessary for critical plant physiological processes. Unlike macronutrients such as N, P and K, micros are taken up by plants in tiny quantities — usually less than a pound per acre.

Micronutrient fertilization pays only in crops that are sensitive to deficiencies — and only when a recent soil test identifies a need for the micronutrient. Tissue tests are not a reliable predictor of micronutrient needs. (Source: Dan Kaiser, University of Minnesota)

Still, a micronutrient deficiency can have an outsize effect on crop yield. That’s one reason farmers hear a lot of marketing pitches for micros. Where do these nutrients fit in a profitable fertilizer program?

The good news is, micronutrient deficiencies are rare in Minnesota. Our soils usually contain an ample supply for crop production. And we almost never see micronutrient deficiencies in soils that receive regular manure applications.

What’s more, micronutrients are no more deficient now than in decades past, despite increases in crop yields and nutrient removal rates, says Dan Kaiser, a University of Minnesota Extension soil scientist who is leading micronutrient research.

In Minnesota, typically the only micronutrients that might be needed in a fertilizer program are:

• zinc for corn
• iron for soybeans
• boron for alfalfa

Corn is the primary Minnesota crop that responds to zinc fertilizer application. (Source: Dan Kaiser, University of Minnesota)

Zinc for corn
Corn is the most widely grown Minnesota crop that may respond to zinc (Zn) fertilization. However, zinc is not needed on all cornfields.

Zinc response is most frequent in corn grown on soils with a high pH. Deficiency symptoms usually appear in the first two or three weeks of the growing season and are characterized by broad bands of striped tissue on each side of the leaf.

The University of Minnesota recommends a soil test to determine if zinc is needed as part of your corn fertilizer program. The soil test for zinc is reliable and will accurately predict the need for zinc.

The greatest likelihood of a response to zinc will be in fields that test less than 0.50 parts per million Zn (based on the DTPA test). Zinc will not increase corn yield potential in soils testing 0.75 ppm or greater, Minnesota research shows.

Zinc can be applied in a band or broadcast. Because zinc deficiency is generally limited to distinct portions of fields, variable-rate application works well, if you have that capability.

Other micronutrients, including iron, copper, manganese and boron, will not increase corn yields, Kaiser says.

Micronutrients such as copper, zinc and boron play a vital role in plant development. (Source: Dan Kaiser, University of Minnesota)

Iron for soybeans
Soybeans grown on calcareous soils with a pH of 7.4 or greater may suffer from an iron deficiency called iron deficiency chlorosis (IDC). The soil generally has plenty of iron (Fe), but because of the soil’s chemistry, soybean plants are not able to absorb the nutrient. Leaves turn yellow, and in severe cases, plants may die.

Choosing tolerant soybean varieties is the best way to manage IDC. Recent Minnesota research also found the application of an ortho-ortho-EDDHA chelated iron fertilizer such as Soygreen in the seed furrow at planting can improve iron uptake on problem soils. Also, seeding a companion crop of oats before soybean planting has been shown to reduce IDC in severely affected fields.

Once symptoms of IDC appear, foliar applications of iron fertilizer may improve leaf greenness, but seldom produce a profitable yield increase, Kaiser says. Minnesota research shows that soybeans do not respond to applications of magnesium, zinc, manganese or copper, he adds. And boron may actually cause yield decreases in soybeans.

Boron for alfalfa
Alfalfa grown on sandy, nonirrigated soils in east-central and north-central Minnesota is the only Minnesota crop that has been shown to respond to boron (B) fertilizer.

Boron deficiency shows up as yellow, stunted leaves. Plants have a bushy, umbrella-like appearance and winter-kill easily.

Boron should be applied based on a recent soil test. The most likely response to boron is on eroded, low-organic-matter soils testing less than 1.0 ppm B. U-M recommendations suggest applying 2 to 4 pounds B per acre. Boron fertilizer can be blended with other fertilizers.

Caution about tissue testing
Tissue testing is often promoted as a way to assess the need for foliar-applied micronutrients during the growing season.

But tissue tests are not a reliable predictor of micronutrient needs, Kaiser says.

“Tissue testing was never meant for in-season nutrient corrections,” he says.

For one thing, micronutrient sufficiency ranges have not been established for tissue tests, he says. That means there is no demonstrated link between yield and the concentration of micronutrients in plant tissue. In addition, nutrient values can vary widely, depending on when and how plant tissue samples are collected.

Instead, the best way to identify micronutrient deficiencies is with a proper soil test.

Bernard is a certified professional agronomist (CPAg) and certified crop adviser (CCA) from Richland, Minn. Find information and links to Minnesota CCAs at mcpr-cca.org.