Manure placement depth impacts on crop yields and N retained in soil

The objective of this study was to determine the impact of manure placement depth on crop yield and N retention in soil. Experimental treatments were deep manure injection (45 cm), shallow manure injection (15 cm), and conventional fertilizer-based management with at least three replications per site. Water infiltration, and changes in soil N and P amounts were measured for up to 30 months and crop yield monitored for three seasons following initial treatment. Deep and shallow manure injections differed in soil inorganic N distributions. For example, in the manure slot the spring following application, NO(3)-N in the surface 60 cm was higher (p<.01) when injected 15 cm (21.4 mu g/g) into the soil than 45 cm (11.7 mu g/g), whereas NH(4)-N had opposite results with shallow injection having less (p = 0.045) NH(4)-N (102 mu g/g) than deep (133 mu g/g) injection. In the fall one year after the manure was applied, NO(3) -N and NH(4)-N were lower (p = 0.001) in the shallow injection than the deep injection. The net impact of manure placement on total N was that deep injection had 31, 59, and 44 more kg N ha(-1) than the shallow injection treatment 12, 18, and 30 months after application, respectively. Deep manure injection did not impact soybean (Glycine max L.) yield, however corn (Zea mays L.) yield increased if N was limiting. The higher corn yield in the deep injected treatment was attributed to increased N use efficiency. Higher inorganic N amounts in the deep injection treatment were attributed to reduced N losses through ammonia volatilization, leaching, or denitrification. Results suggest that deep manure placement in glacial till soil may be considered a technique to increase energy, N use efficiency, and maintain surface and ground water quality. However, this technique may not work in glacial outwash soils due to the inability to inject into a rocky subsurface.