Welcome to part four of the column series on soybean micronutrients. The last couple of weeks I introduced the series and discussed boron and chloride. I will quickly cover how cobalt is essential to soybeans, known deficiencies and toxicities in the region, factors of cobalt availability in soil, soil and plant tissue testing, and cobalt fertilizer recommendations. Micronutrients for Soybean Production in the North Central Region is a good regional publication that addresses cobalt that I utilized as a resource for this news column.
You have not likely learned a lot about cobalt as a micronutrient. In the past, cobalt was considered a beneficial nutrient, not truly an essential micronutrient. In soybeans, cobalt is needed by Bradyrhizobium bacteria for nitrogen fixation, so cobalt is needed to raise a good soybean crop. Researches are still learning more about cobalt and its role in enzymes that affect plant growth and metabolic function. In soybeans, deficiency symptoms would be similar to nitrogen with plant yellowing and reduced growth. However, there has been no documented cobalt deficiencies in soybeans in the region.
Cobalt is taken up by plants and found in soil solution as a divalent cation (Co2+) similar to other micronutrient transition metals like iron and copper. Cobalt is held by the soils cation exchange capacity, which is different from the two previous micronutrients of boron and chloride. There has been not been much research done on cobalt fertilization, soil and plant tissue testing in the region. Work done in other parts of the world show that deficiencies in cobalt more likely occur on highly leached acidic sandy soils, highly calcareous soils, or peat soils that do not exist in southeast Nebraska. Even in these extreme conditions elsewhere in the world, only one ounce per acre of cobalt superphosphate would be recommended (Havlin et al., 2005).
The Nebraska On-Farm Research Network partners with farmers to conduct randomized and replicated studies on their farm with their equipment. There has been six studies, including one in Saline County, from 2015-2018 where cobalt was one of the micronutrients in a mix applied as a soybean seed treatment or in-furrow (resultsfinder.unl.edu). In all six studies, there was no statistical differences between the check and the micronutrient mix, meaning the yield difference is not likely due to the cobalt or the other micronutrients. Numerically, the yield differences were 0, 0, 0, 0, 0.6, and 2.0 bushels per acre (treatment – check).
In summary, we have more to learn about cobalt as a micronutrient, but current soil and agronomic information in the region points to it not being a concern for soybean production in southeast Nebraska. You can email () or call me (821-1722) with your cobalt questions. I looked forward to writing about copper 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.
