Soybean Response To Potassium Fertility And Fertilizer In Manitoba

Production of soybean [Glycine max (L.) Merr], a high potassium (K) consuming crop (18 - 23 g K2O kg-1 or 1.1 - 1.4 lb K2O bu-1), has rapidly expanded in Manitoba in recent years. However, there has been little comprehensive historical K fertility research for soybean in the province. A two-year study was developed to assess the frequency of soybean yield response to K fertilization and determine the effectiveness of K fertilizer rate and placement combinations for increasing seed yield. The study included small plot trials, field scale on-farm trials and paired microplots within each on-farm trial. Small plot trials were primarily used to determine the efficacy of different K fertilizer rate and placement combinations (33 or 66 kg K2O ha-1 sidebanded, and 33, 66 or 132 kg K2O ha-1 broadcast and incorporated) for improving seed yield. Although the potential for K fertilizer response was present at midseason in some site-years, as indicated by an increase in tissue K concentration of the whole plant (WP), uppermost mature trifoliate (UMT) leaves and the stem, there was no significant seed yield response to K fertilization at any small plot site-year. On-farm trials were used to investigate the frequency of soybean response to K fertilization across a range of NH4OAc soil test K (STK) concentrations (52 - 451 mg kg-1), and microplots within on-farm trials were used to investigate the response relationships at a more detailed scale. Three out of nineteen site-years had significant seed yield responses to K fertilization at a field-scale; one response was negative and two were positive. In the microplots, the potential for fertilizer K response was also present in some site-years at midseason with increases in UMT K concentration. However, the site-years with midseason tissue K responses did not have significant seed yield differences at maturity. There were two site-years that did have a significant yield increase with K fertilization in microplots, but these were not site-years where K fertilization increased midseason tissue K concentrations. Consistent with the small plot and field scale results, there was no significant relationship between STK and relative seed yield in the microplots.