Micronutrients: Do They Pack a Big Punch?

By Dan Kaiser, Department of Soil, Water and Climate, University of Minnesota

There have been many questions focusing on micronutrient application. Micronutrients by definition are essential elements needed for plant growth and development, but quantities needed are small when compared to macronutrients such as nitrogen. Most soils contain high enough concentrations of these micronutrients that we do not need to supply them as fertilizers. However, most of them have optimum pH ranges in which they are available. Since many of them are metals they are most available at low soil pH values. Even though parts of the tri-state area contain high pH soils, in general there are only a small number of micronutrients that are important for crop production and in most circumstances soil tests can be used to evaluate the need for their application.

In general the most noticeable and widespread problem is iron deficiency chlorosis (IDC) in soybean. However, this problem is not related to low iron levels in the soil but rather a poor utilization of iron within the plant. Most producers deal with this problem on a yearly basis in the Red River Valley and other areas where calcareous soils persist. Because it is a utilization problem there is no reliable soil test method to determine where areas of IDC will be an issue for soybean. Right now, choosing a tolerant variety is the best management strategy to deal with this issue.

Zinc and copper can also be an issue in areas of northwest Minnesota. Zinc is mainly an issue for corn or dry edible beans while copper can be a problem on small grains planted on organic soils. A six inch soil test can be analyzed for both zinc and copper to determine if either needs to be applied. Current recommendations for zinc call for fertilizer application if the soil test comes back lower than 1.0 ppm for corn or 0.75 ppm for edible beans. For spring wheat grown on organic soils the same test can be analyzed for copper which is marginal at 5.0 ppm and deficient at 2.5 ppm or below. The final micronutrient that may be recommended is chloride on small grains. However, fields that have potash applied to them will likely not be deficient since a significant amount of chloride is applied with potash. Check with your local extension if chloride is of importance in your area and what soil tests may be available.

Manganese has been another micronutrient that has been discussed for soybean. This stems from work in Indiana that noted problems with glyphosate resistant soybean utilizing manganese. A yellowing of leaves a few days after application of glyphosate has been noted in fields, especially in areas where a high rate of glyphosate has been applied. This effect has been termed “glyphosate flash” and has been debated as to possible negative yield effects. In Minnesota research on applications of manganese to glphosate resistant soybeans has not provided evidence of a need for application.  This has been the case for most research conducted in states west of Michigan and Indiana. High pH areas in Minnesota were targeted for research sine the availability of manganese significantly decreases when pH level are above 7.4. Results of this work however have questioned whether other yield limiting factors such as IDC or dry weather patterns limit yield more than what could be gained by an application of manganese.  At this time there are no soil tests or recommendations available for manganese application to soybean.

Many micronutrients can be applied as a foliar application and there are many fertilizer sources out there that contain multiple nutrients as a single package. Remember though that there only are a few specific ones that have been proven to be important for crops grown in the tri-state area.  While it may be tempting to tank mix micronutrients with glphosate to try to get more bushels out of the field, it is unlikely that there will be a large yield increase and some nutrient sources can have negative impacts on the effectiveness of glyphosate and ultimately weed control. This has been well documented in research with metals elements tank mixed with glyphosate. If you do choose to take mix make sure your source is fully compatible with glyphosate.  Since many chelated micronutrient sources can be expensive and represent a large cost that may not be needed, remember to make sure to make informed decisions before anything is used to get the best results for your particular cropping system.