Welcome to part five of the column series on soybean micronutrients. The last couple of weeks I discussed boron, chloride and cobalt. I will quickly cover how copper is essential to soybeans, known deficiencies and toxicities in the region, factors of copper availability in soil, soil and plant tissue testing, and copper fertilizer recommendations. Micronutrients for Soybean Production in the North Central Region is a good regional publication that addresses copper that I utilized as a resource for this news column.
In soybeans, copper is a key micronutrient for different enzymes that are involved with photosynthesis, respiration, lignin production, and metabolism. Copper deficiency symptoms include stunted growth, yellowing of leaves, and browning of leaf tips. Copper is not very mobile within plant tissue, so symptoms show up in new growth first. There has been no documented copper deficiencies in southeast Nebraska. Deficiencies in other regions have occurred on acidic high organic matter poorly drained soils like peat and muck, and sandy alkaline soils. Soybeans are less susceptible to deficiency than other crops we grow in the region. For example, copper deficiencies are limited to small grains grown on peat soils in Minnesota.
Copper is taken up by plants and found in soil solution as a divalent cation (Cu2+) similar to other micronutrient transition metals like cobalt. Copper is held by the soil cation exchange capacity and tightly held by soil organic matter complexes even in solution. Soil test analysis (DTPA extraction method) for copper in our mineral soils is obtained along with the other metal micronutrients. Soil test DTPA values less on 0.2 ppm would suggest increased chances of a fertilizer response. Soybean copper sufficiency through tissue analysis can be determined during full bloom, typically in early July. The copper sufficiency range is 6 to 20 ppm in the tissue sample. Unfortunately, the old sufficiency range from the 1960s of 10 to 30 ppm is still in use and sometimes creates a concern not based on research that is more recent in the 1980s and 2000s.
Research in Nebraska, Kansas and Iowa has found no yield increases from copper fertilization. The Nebraska On-Farm Research Network partners with farmers to conduct randomized and replicated studies on their farm with their equipment. There has been seven studies from 2015-2018 where copper was one of the micronutrients in a mix applied as a soybean seed treatment, in-furrow, or as a foliar (resultsfinder.unl.edu). In all seven studies, there was no statistical differences between the check and the micronutrient mix, meaning the yield difference is not likely due to the copper or the other micronutrients. Numerically, the yield differences were -5, -2, 0, 0, 0, 0, 1, and 2 bushels per acre (treatment – check).
I do not recommend or see the need for copper fertilization in a soybean fertility program in southeast Nebraska. However, if you are still concerned, manure is good source of copper and manure has many other soil health benefits too. I looked forward to writing about iron in next week’s column. You can share or read this news column online through my local website for Saline, Jefferson, and Gage counties at croptechcafe.org. Know your crop, know your tech, know your bottom line.
