Managing Potassium and Phosphorus Soil Test Levels in a Rice–Soybean Rotation

A long‐term rice–soybean (Oryza sativa L.–[Glycine max (L.) Merr.]) rotation was conducted at Glennonville, MO, to determine how quickly farmers need to increase soil test levels in low‐phosphorus (P) and ‐potassium (K) fertility fields. Plots received P and K fertilizer to increase soil Bray‐1 P and ammonium acetate–extractable K in 1‐, 4‐, or 8‐year buildup programs. In most years, soybean and rice yields from plots receiving P and K fertilizer treatments were significantly greater than yields from untreated controls. Two soil test P and K target levels were tested. The soil test target for rice using University of Missouri recommendations was 22% lower for P and 36% lower for K than the soybean target. No justification was found for recommending lower soil test P and K target levels for rice than soybean. Averaged across buildup programs, rice produced significantly greater yields using the soybean P and K soil targets than the rice P and K targets. Averaged over years, soybean and rice yields were not affected by the length of time in P and K buildup programs. Farmers can prolong buildup of soil test P and K over 4 to 8 years without yield losses.