Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 7, 1998
Publication Date: N/A
Interpretive Summary: Nitrogen management continues to challenge farmers and researchers. This 15-year, continuous corn study conducted in northeast Iowa compared the effects of moldboard plowing, chisel plowing, ridge-tillage, and no-tillage treatments on crop yield, uptake and removal of nitrogen, changes in soil organic matter, and nitrate loss through tile drainage lines. The amount of nitrogen removed by the corn grain was determined by yield, which was closely related to the amount of summer precipitation. Ridge-tillage and no-tillage yields were lower than chisel plow or moldboard plow treatments because of the continuous corn cropping system. Nitrogen loss through tile drainage was directly proportional to rainfall. A very important finding within all four tillage treatments was that a large amount of the nitrogen that was not removed by the crop or lost through tile drainage could be accounted for by changes in soil organic matter. This means that to understand what happens to the fertilizer nitrogen that is applied to enhance crop growth, changes in the soil must be closely monitored. Growers can use the results of this research to improve nitrogen management by switching from continuous corn to crop rotation and by using soil nitrate tests to determine the optimum rate of fertilizer nitrogen. These changes are being evaluated in a current study at this research site.
Ridge-till, moldboard plowing, chisel plowing, and no-till treatments on tile drained northeastern Iowa soil were compared using a 15-year N balance and simulations made with the Root Zone Water Quality Model (RZWQM). The N budget accounted for 98 to 104% of the fertilizer N applied between 1978 and 1992 to the four tillage treatments. Grain yields predicted by RZWQM for 13 of the 15 years averaged 9% higher than the measured values; however, the values correlated reasonably well with an average coefficient of determination of .78 for the four tillage treatments. Simulations for 1988 and 1989 failed completely because RZWQM was not able to accurately predict the hydrology associated with the low rainfall seasons. Predicted total N accumulation by the corn plant was much higher than what was measured in 1990, 1991, or 1992. Estimates of profile nitrate, N mineralization, seepage loss of N, and denitrification were not satisfactory, presumably because the model failed to accurately simulate hydrology for the different tillage practices. Although simulation results were used to compare the four tillage practices, they were not suitable for predicting the fate of fertilizer N. The simple N budget and simulation results suggest that adopting ridge-tillage, without concurrent adjustments in N applications rates, methods of application, and associated soil and crop management practices, will not reduce the potential for off-site water quality degradation.