Do I Need to Add Micronutrients into my Soil Fertility Program

October 17, 2024 4:53 PM
Blog Post

This article is Part 5 in a series of five blogs on how best to allocate fertilizer dollars. 

One of the most frequent questions Extension professionals receive regarding soil fertility is do I need to add micronutrients to my soil fertility program? The classic answer “it depends” is really true in this case, but research done at Iowa State University and other universities across the upper Midwest shows there is no consistent yield benefit to applying micronutrients. 

What is a micronutrient? Micronutrients are essential nutrients for plant functions, but they are used in very small amounts by the plant, usually less than 1-3 pounds per acre. Micronutrients include boron (B), chlorine (Cl), cobalt (Co), copper (Cu), iron (Fe), Manganese (Mn), molybdenum (Mo), nickel (Ni) and Zinc (Zn).

Due to the parent materials on which Iowa soils formed and our rich organic matter, generally our soils supply all the micronutrients we need for crop production with a few exceptions. These exceptions include sandy soils, highly weathered soils, acid organic soils and calcareous soils. Micronutrient availability is also complicated by soil pH, soil moisture, soil temperature, and nutrient cycling. One primary example of micronutrient deficiency in Iowa is iron chlorosis deficiency (IDC) in soybean grown on calcareous soils. This is commonly seen in small field areas in north-central and western Iowa.  Research in Minnesota and Kansas has shown that in-furrow application of fluid iron chelate for soybean in calcareous soils sometimes increases yield but not to the level of noncalcareous areas in fields.

Micronutrient research for corn and soybean has been done at Iowa State University (ISU) since the 1960s and continued until about 2015.  During this time, multiple micronutrients products including stand-alone micronutrients, foliar-applied, and soil-applied products were used in dozens of studies across multiple soil types and pH ranges. In most situations there were no yield responses. Occasional yield responses were observed for zinc in corn. Yield response to molybdenum in soybean was also documented, but only in extremely acidic soils (pH about 5.4 or less). These soils should be limed, which averts the deficiency and increases yield due to other reasons. Due to a lack of consistent crop yield responses, the numerous field trials have not been successful at calibrating soil and tissue tests for micronutrients, as is done with P and K research. The exception to this is Zn, where there is sufficient data in Iowa and the northcentral region, to correlate soil test levels and crop response for corn and sorghum, see Table 1.  

Table 1.  Adapted from PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, table 15, zinc recommendations for corn and sorghum production.

Table showing Zn recommendations

The publication CROP 3135 Micronutrients for Soybean Production in the North Central Region (https://store.extension.iastate.edu/product/15259), summarizes research across several Midwestern states. Also, the Secondary and Micronutrients section (https://www.agronext.iastate.edu/soilfertility/secondarymicronutrients.html) of the ISU Extension Soil Fertility website includes presentations, articles, and reports about micronutrients.

As you evaluate the need for micronutrients in your soil fertility program, first make sure your soil pH levels are not too acidic for growing corn and soybeans, maintain optimal soil test levels for P and K, apply sulfur, and apply zinc when needed according to soil test results for corn and sorghum. Once you have achieved this part of your fertility plan, then if your budget allows it, explore if you need other micronutrients by taking these factors into consideration:

  1. Ask yourself why micronutrients might be needed since most research showed no yield responses other than zinc for corn or sorghum:
    1. Is my pH optimum for maximum crop production?
    2. Do I have very sandy soils (coarser than loamy)?
    3. Am I seeing micronutrient deficiency symptoms?
  2. If you plan to do your own field trial, be sure to:
    1. Apply different rates and measure yield to determine at what level you see a response.
    2. Include a treatment with no micronutrients so you can account for any response provided by existing levels of micronutrients in the soil.
    3. If you apply a mixture of micronutrients, use both soil and tissue testing to help you identify the deficient micronutrient.

There are many micronutrient products available. Testing if and why a specific product may work for you will ensure you are allocating your nutrient dollar resources in the best way possible. 

Special thanks to Antonio Mallarino, professor emeritus and former Extension Soil Fertility Specialist, for reviewing and contributing to this article.

A dry fertilizer applicator spreading nutrients on cornstalks.

 

 

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Angie Rieck-Hinz Field Agronomist in NC Iowa

Angie Rieck-Hinz is a field agronomist in north central Iowa for Iowa State University Extension and Outreach. She currently serves, Cerro Gordo, Franklin, Hamilton, Hardin, Humboldt, Webster, Worth and Wright counties. She has worked for ISU Extension and Outreach for over 30 years, serving in v...