SWINE MANURE RATE, TIMING, AND APPLICATION METHOD EFFECTS ON POST-HARVEST SOIL NUTRIENTS, CROP YIELD, AND POTENTIAL WATER QUALITY IMPLICATIONS IN A CORN-SOYBEAN ROTATION

Management practices for applying liquid swine manure to minimize surface runoff water pollution and improve crop production can be evaluated by conducting long-term field studies. This article documents results from a six-year (1996-2001) central Iowa field study that evaluated the effects of swine manure application management practices on soil nutrients, organic matter, pH, crop yield, and discusses potential water quality implications. Swine manure management practices included single-rate (SR) and double-rate (DR) nitrogen (N) based application rates (168 and 336 kg N ha(-1), respectively), three timings (fall injection [FI], winter broadcast [WB], and spring injection [SI]), and two methods (broadcast and injection) of liquid swine manure. Analysis of these practices involved comparing levels of residual soil total phosphorus (P) as Bray-1 available P (RSP), residual soil nitrate-nitrogen (RSN), percent organic matter (OM%), pH, carbon: nitrogen (C: N) ratio, and crop yields (kg ha(-1)) in a corn-soybean rotation cropping practice. Manure application rates were based on standard crop-available N levels as applied to corn plots and were adjusted for environmental losses. Soil samples were collected immediately prior to commencing the study and after harvest each year and analyzed for RSP, RSN, OM%, pH, and C:N ratio at selected depths in the top 0.30 m of the soil profile. Deep soil core (0-1.22 m) samples also were collected after harvest in 1996 and 2000 and analyzed for RSN as a function of depth in the soil profile. When averaged over the six project years and manure application times for corn plots, these findings showed that DR application plots had significantly higher accumulation of RSP and RSN (32.6% and 36.5%, respectively) versus SR application plots in the top 0.30 m of the soil profile. The RSP for the SI (38.2%) and WB (32.8%) treatments was significantly higher than for the FI treatment on corn plots. The RSN accumulation also was found to be significantly higher for the SI (33.4%) and WB (17.4%) treatments than for the FI treatment on corn plots. The 0-1.22 m deep soil core analysis indicated a higher RSN accumulation in up to 88% of the 1.22 m soil profile depth range for the DR and SI treatments. When averaged across the six years and application rates, corn yield was significantly higher for SI plots (9,596 kg ha(-1)) versus FI plots (9,236 kg ha(-1)). The reduction in FI plot corn yield results may have been due largely to excessive leaching of nutrients through the soil profile from post-harvest rainfall during the fall-spring duration. Swine manure SI plots with the higher DR application rate produced the highest average corn yield (10,093 kg ha(-1)). When averaged over project years and application times, the SI treatment showed an increase of 4.0% in corn yield over the FI treatment. While there were short-term significant increases in OM% for the SI1 treatment during 1996 and 1997, there were no significant cumulative differences in OM% as well as pH and C:N ratio in the soil profile after six years of N management practices at the study site. Although no manure was applied to soybean plots, residual effects of N management practices using the WB and SI application methods in the DR treatment plots during corn years significantly increased average values of RSP, RSN, and soybean yield. Results of this study indicate that long-term application of higher liquid swine manure rates during winter and spring application times resulted in significantly higher post-harvest accumulation of residual P and N in the soil profile. These results also show that incorporation of swine manure during the spring application time produced significantly higher corn yields compared with fall and winter application times. Overall results suggest that while residual soil P and N content may be significantly higher from spring versus fall manure application times, these nutrient runoff losses and the potential threat to surface water quality may be substantially lower during spring and summer compared with fall and winter due to effects from crop nutrient uptake, microbial activity, leaching, and evapotranspiration during the growing season.