For years, we have been fertilizing for two crops ahead of corn and leaving our soybean crop in year two, utilizing the leftovers from the previous year’s corn. As we have begun to push soybean yields to new heights with management practices such as seed treatments, early planting and fungicides, we should not be overlooking fertility.
One key piece of the puzzle when we look at managing soybean fertility is the amount of plant-available potassium (K). K is especially important because it is essential for the plant to properly regulate the stomata (leaf openings), and insufficient K levels can induce stomatal closure, lowering the plant's ability to perform photosynthesis.1 Not only can insufficient K interfere with photosynthesis but because of its effect on the stomata, insufficient K can especially affect the plant’s ability to perform in heat and water-stressed situations.
To go along with K being a key component for photosynthesis, it is also in very high demand for the soybean plant and seed development, removing 1.4 pounds of K2O per bushel. Meaning the maintenance application rate for a 60-bushel-per-acre soybean crop is 84 pounds of actual K2O or 140 pounds of muriate of potash (0-0-60) per acre.2 In contrast, it would take a 300-plus-bushel-per-acre corn crop to remove a similar amount of K.
Understanding the Soil Test
Laboratory results may be reported in parts per million (ppm) or lb/acre.
To convert ppm to lb/acre, multiply ppm by 2 (lb/acre = ppm X 2).
To convert lb/acre to ppm, divide lb/acre by 2 (ppm = lb/acre ÷ 2).
Three forms of K are in equilibrium in the soil: unavailable slowly available, and readily available. About 90% to 98% of total soil K contained in feldspars and micas is unavailable. Slowly available K is “fixed” between soil clay layers and may be released when soils become wet; however, soil tests do not provide a value for fixed K. Readily available K is measured by soil tests because of its solubility in water. Be sure not to soil sample during a drought as sampling then may result in an underestimated value for available K because the dried soil “fixes” K. The underestimation can result in a soil report indicating a need for additional K to be applied. It is recommended to maintain soil K levels between 0 and 30 ppm above critical K levels to help maximize soybean yields. To calculate the critical level, multiply the cation exchange capacity (CEC) by 2.5 and add 75.2,3 For example, the critical K level for a soil having a CEC of 12 meq per 100 grams is 105 ppm [(12 x 2.5) + 75]. The maintenance range for soybeans is 30 ppm, so the K soil test level for this soil should be maintained between 105 ppm and 135 ppm.2,3
Getting Potassium to the Plant
When deciding on how to provide K to a soybean crop, we need to keep in mind the large amount of K that is required to grow a soybean crop each year. That large requirement generally calls for a preplant application as foliar feeding will not be capable of delivering a large enough supply for the growing crop. For soybean production, broadcasting and incorporating K fertilizer sources prior to planting have proven to be the most effective for deficient soils. In reduced tillage situations, banding K can be effective, but an awareness of the salt concentration and the effect it can have on the crop is needed.
If we are going to keep pushing the envelope on growing higher-yielding soybeans each year, it is important that we understand the needs of the crop. Regular soil sampling and accurate yields will let us accurately know how much potassium we need for the soybean crop.
1Benlloch-Gonzálz, M., Romera, J., Cristescu, S., Harren, F., Fournier, J.M., and Benlloch, M. 2010. K+ starvation inhibits water-stress-induced stomatal closure via ethylene synthesis in sunflower plants. National Library of Medicine. National Center for Biotechnology Information. J Exp Bot. 2010 Feb; 61(4):1139-45. https://pubmed.ncbi.nlm.nih.gov/20054030/.
2Staton, M. 2014. Phosphorus and potassium fertilizer recommendations for high yielding, profitable soybeans. MSU Extension. Michigan State University. https://www.canr.msu.edu/.
3Staton, M. 2013. Nutrient management recommendations for high-yield soybean production. MSU Extension. Michigan State University. https://www.canr.msu.edu/.
Web sources verified 12/23/2020.