Long-Term Effects of Manure, Fertilizer, and Tillage On Soil Phosphorus Availability and Crop Yield.

This study evaluated the long-term (12-years) effects of residue management and phosphorus (P) source, rate, and method of application on soil test P (STP) and crop yield. Manure and triple-superphosphate (TSP) were applied at pre-plant and incorporated before corn (Zea mays L.) in a corn-soybean (Glycine Max) rotation in spring 1999 and autumn 2001 and 2003.  Target P rates were 0, 80, and 160 kg P₂O₅ kg^-1. Manure placement was broadcast and sweep injected, and TSP was broadcast and banded; tillage practices were no-tillage and chisel-plow. Soil samples were collected for P analysis in autumn following soybean harvest. In all years, corn grain yield increased with increasing P rate and was significantly greater in manured plots than in TSP plots. Corn yield on plots under chisel-plow were in most cases significantly higher than those under no-tillage plots, and this effects was more evident for the TSP treatment than for the manure treatment. Soil test P in the 0 to 15 cm depth, was greater for the higher rates of manure and fertilizer than in the lower rates for all years; in addition manure tended to increase STP more than TSP. From 2000 to 2010, STP in TSP treatment increased linearly for both placements in no-tillage; no changes were observed for the chisel-plow broadcast plots; and STP increased linearly in the years after manure application (1999-2003) and decreased linearly in the residual years (2004-2011) for the knifed plots. For the manure broadcast plots, STP increased linearly in the no-tillage and decreased linearly in the chisel-plow; and in the sweep injected STP increased linearly in the years after manure application and decreased linearly in the residual years. The results of this long-term study show that P source and placement can have a significant effect on corn grain yield and on STP levels.