Micronutrients are essential plant nutrients that are found in trace amounts in tissue but play a vital role in plant growth and development. Of the 17 elements essential for plant growth, eight are micronutrients: boron (B), chlorine (CI), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), and nickel (Ni). Other nutrients such as cobalt (Co), sodium (Na), silicon (Si), selenium (Se), and vanadium (V) can also benefit crop function, although are generally not considered essential to all plants. Most soils in the tri-state region contain adequate quantities of micronutrients. This is particularly true for fields that regularly receive livestock manure. Field crop deficiencies of Cl, Mo, and Fe have rarely been observed in this region of the United States. Some soils, however, may be deficient in B, Cu, Mn, and Zn, which can cause plant abnormalities, reduced growth, and yield loss. When called for, micronutrient fertilizers should be used judiciously and with care. Some micronutrient fertilizers can be toxic if added to sensitive crops or applied in excessive amounts.
Diagnosing Micronutrient Deficiencies
Both soil testing and plant analysis are useful in diagnosing micronutrient deficiencies. Relative to soil pH, micronutrient soil tests are not as reliable and so plant analysis can play an important role to complement a soil test. Combining plant analysis with soil tests provides more accurate assessment of the micronutrient status of crops and soils. The original tri-state recommendations called for using different extractants depending on the micronutrient of interest: 0.1 N HCl for Mn and Zn and 1.0 N HCl for Cu. In the tri-state region, however, soil testing labs most often use the Mehlich-3 extractant to estimate micronutrient availability. But how effective the “universal” Mehlich-3 extraction is at characterizing soil micronutrient availability is not well understood, as recommended micronutrient concentration ranges based on the Mehlich-3 extraction have not been developed for the tri-state region. The lack of yield responses in micronutrient fertilization trials is a major limitation to developing soil test extractants that can predict micronutrient deficiencies. Despite the uncertainties that exist, Mehlich-3 extractable micronutrients likely relate meaningful information about availability of each nutrient and can be useful information to have.
Summary of Micronutrient Trials
A recent effort to summarize micronutrient trials in Ohio found a total of 194 trials (17 alfalfa, 33 corn, and 144 soybean trials) that tested a micronutrient fertilized treatment relative to an unfertilized control treatment. Overall, yield responses to micronutrient fertilization were rare, with the only responses observed when Mn was applied to soybean: go.osu.edu/micronutrients.
The original tri-state recommendations provided a sufficiency table for micronutrient concentrations in field crop components. These values remain unchanged in this update.
The micronutrient soil tests recommended for use in Michigan, Ohio, and Indiana are 0.1 N HCl for Mn and Zn, and 1.0 N HCl for Cu using a 1 to 10 soil-to-extractant ratio. Note most micronutrient soil test recommendations are based on Mehlich-3 extractable levels which have not been systematically calibrated to crop response in the tri-state region.
Micronutrient availability in both mineral and organic soils is highly regulated by soil pH. The higher the soil pH and the lower the soil test, the more micronutrient fertilizer is needed to correct a deficiency. Micronutrient fertilizers can be soil or foliar applied. There are many micronutrient fertilizer formulations currently sold in the tri-state region. These exist as straight grade nutrients, combinations or blends, and granular or foliar formulations. All reputable fertilizers should be labeled for rate, spray volume, placement, and important precautions. Growers are encouraged to follow the manufacturers' labels closely to minimize any risks of crop damage.
Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa
Executive Summary | Soil Sampling, Handling, and Testing | Soil pH and Lime Recommendations | Nitrogen | Phosphorus and Potassium | Calcium, Magnesium, and Sulfur | Micronutrients | Additional Resources | Authors and Acknowledgements
This website provides a summarized version of the Tri-State Fertilizer Recommendations, reporting only the main points of the document, but lacking comprehensive detail. For complete information, please see the full version which will be available soon from The Ohio State University Extension Publications Store.