Currently, phosphorus (P) fertilizer recommendations for double-cropped wheat (Triticum aestivum L.) and soybeans (Glycine max L. Merr.) are based on data from monocropping conventional tillage systems. The purpose of this 3-year study on an eroded Ultisol was to compare times (fall, spring, and fall plus spring) and rates (0, 32, 64, and 128 kg ha−1 year−1) of P applications for double-cropped wheat and soybeans grown with disk tillage and no-tillage production practices. Disk tillage was used to incorporate P; otherwise, tillage implements were not used. Although most of the applied P was concentrated in the surface 7.5 cm of soil, it did not affect extractable soil levels of K, Ca, Mg, Cu, Zn, and Mn. Applied P did, however, reduce Cu and Zn concentrations in both soybean and wheat tissue, but the reduction probably was not great enough to affect yields. Yield response to applied P was obtained, but this response was primarily with the lowest P rate; average yields for 0 and 32 kg ha−1 year−1 were 2,330 and 2,750 kg/ha, respectively, for wheat and 2,210 and 2,690 kg/ha, respectively, for soybeans.
Time of application did not affect yields or nutrient concentrations of either crop. Nutrient concentrations in soybean leaves did not differ among tillage systems, but soybean yields averaged 200 kg/ha higher with no-tillage than disk tillage. With wheat, nutrient concentrations were lower and forage and grain yields were higher with disk- than no-tillage. Yield differences between tillage systems were essentially the same for no P as for any rate of applied P, which suggests that these differences between tillage systems were due to tillage rather than method of application. The data suggest that current P recommendations for monocropping systems are suitable for double-cropping systems but P recommendations for disk- and no-tillage systems should be based on soil samples taken from the upper 7.5 cm of soil rather than the more common 0- to 20-cm depth.