Soybean is relatively nonresponsive to K fertilizer rate or placement in Manitoba soils

There has been little comprehensive potassium (K) fertility research for soybeans in Manitoba despite recent, rapid, expansion of soybean production in the province. Our main objective was to assess the efficacy of K fertilizer rate and placement combinations to increase K uptake and seed yield of soybeans grown on low K soils. Even though the seven sites had low concentrations of ammonium acetate-extractable soil test K (STK), midseason tissue K concentration increases with K fertilization, and, at several sites, visual deficiency symptoms in or near control plots, soybean seed yield did not respond to K fertilization, regardless of K fertilizer placement and rate. In a complementary field trial, barley, a crop known historically to respond well to K fertilization in Manitoba, had substantial (>20%) increases in yield with K fertilization where soybean did not respond. Ammonium acetate STK and the current 100 mg kg(-1) threshold for recommending K fertilization for soybean and barley predicted barley yield response to K fertilization in our study, but did not predict soybean yield response.