The observed soybean K deficiency problems in parts of Kansas is likely impacting yield, and can be corrected through fertilizer applications based on accurate soil test critical values and/or plant tissue data and adequate for soil types. Preliminary work funded by the Soybean Commission more than 10 years ago showed impact on soybean yield and the need for a revised/updated this nutrient diagnostic tool.

Results will be delivered to Kansas producers and crop advisers in various ways. Information will be disseminated through field days, KSU Soybean Production Schools, extension publications, and the KSU nutrient management webpage. Results of field experiments and any revised recommendations that arise will be summarized and distributed to the public via news releases. Brief articles will be prepared for publication periodically in the Kansas State University Agronomy eUpdates. Results will be shared with county/district extension agents in the state who provide information to soybean growers on a regular basis. Finally, this work will be completed in close collaboration with the KSU soil testing laboratory, and results will be shared with the KSU soil testing lab as well as private laboratories who advise Kansas growers each year.

Update:
The 2019 field locations were harvested to measure yield response, plant samples were collected, and fall-spring soil sampling completed. Field experiments were also conducted at two locations during 2019 to evaluate the alternative soil test methods (ion exchange resin, CER). Treatments included a control (check) with no K application and one with an application of 150 lbs K2O acre-1 (high K rate).

Preliminary results showed that plant K uptake measured at reproductive stages (R2, R4, and R6) was increased by K fertilization. However, differences were not statistically significant (p < 0.05) at locations with high soil test K. In contrast, locations with low soil test K showed significantly higher plant K uptake measured at R2 (p < 0.05), R4 (p < 0.10), and R6 (p < 0.05) stages when 150 lbs K2O acre-1 was applied.

At the R6 stage, fertilized plots had 50% more K uptake and 40% more K adsorption (cumulative) by CER compared to the control. This observation suggests the potential use of CER as an indicator of K supply to soybean in field conditions, but further research is needed and ongoing in 2020. In both locations, CER was able to adsorb more K (measured as cumulative adsorption) at a high K rate. The amount of K that was adsorbed by the CER was influenced by soil moisture content. Field evaluation of crop response and soil test methods will continue during the 2020 soybean growing season.

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