Utilizing soil phosphorus and potassium reserves for soybean production on a claypan soil

Under temporary economic stress, producers may elect to grow crops without fertilization. However, information is lacking regarding crop and soil response to growing multiple crops without phosphorus (P) and potassium (K) fertilization on claypan soils in the eastern Great Plains. The objective of this study was to determine the effect of utilizing soil‐test P (STP) and soil‐test K (STK) reserves on yield of 5‐yr continuous soybean [Glycine max (L.) Merr.] grown on a claypan soil and on soil‐test values. In the first two low‐yielding years, soybean yield and yield components were unaffected by STP and STK concentrations. In the subsequent three average‐yielding years, soybean yields were up to 0.35 Mg ha−1 less when STP was initially 5 mg kg−1 or STK was initially 57 mg kg−1 than at greater STP or STK. Greater STP increased pods plant−1, whereas greater STK increased pods plant−1, seed pod−1, and seed weight. Even though P and K uptake at R2 (full bloom) did not correlate directly with yields, greater P uptake at R2 in average‐yielding years increased pods plant−1 and greater K uptake at R2 increased seed pod−1. Both STP and STK declined at more than 3 mg kg−1 yr−1 for larger initial STP and 15 mg kg−1 yr−1 for larger initial STK. While using residual soil P and K of varying concentrations in this claypan soil to grow soybeans marginally affected yields, the 5‐yr decline in STP and STK values may greatly affect subsequent sensitive crops and require high rates of fertilization.