Long term effects of phosphorus and conservation tillage management on crop yield and soil phosphorus in corn-soybean rotations

As awareness of the importance of conservation tillage for controlling soil erosion and P loss from fields increases there is a need for more knowledge of its effects on crop yield, soil P stratification, and efficiency of fertilizer placement methods. A study established in 1994 at five Iowa research farms was conducted to gain a better understanding of these relationships in corn (Zea mays L.) soybean [Glycine max (L.) Merr.] rotations. The research farms were in Northeast (NERF), Northern (NIRF), Northwest (NWRF), Southeast (SERF), and southwest (SWRF) regions of the state. This study used two tillage systems (no-till and chisel-disk), broadcast and planter-band placement methods, several P rates, and three replications to determine long-term effects and relationships between grain yield, soil-test P (STP) and P stratification, and the efficacy of different sampling depths for predicting yield responses. Over the 12 years of this study tillage often increased corn yield compared with no-till, although the long-term average effect was statistically significant only at NERF, NWRF, and SERF (0.6, 0.5, and 0.5 Mg ha). Tillage had infrequent, small, and inconsistent effects on soybean yields, and long-term average effects favored tillage at SERF (0.12 Mg ha) and no-till at SWRF (0.14 Mg ha). Tillage by placement interactions were