Development and Evaluation of an Optimum Nitrogen Based Manure Application Rate for Corn Production and Groundwater Protection

Abstract

The objective of this two year project was to determine an optimum manure and or fertilizer application rate to obtain optimum com yields and to protect groundwater from excess nitrate contamination. Demonstration plots were planted to com following alfalfa. Plots were treated with dairy manure at application rates between 7.5 and 23 ton per acre, commercial fertilizer at rates of 45 and 68 pounds per acre, or left untreated as a control. Groundwater monitoring was conducted with multi-port well nests, sampling at one-foot intervals three to five feet into the aquifer. A total of thirty-seven well nests were used to monitor the groundwater upgradient and downgradient of each of the twenty-four plots. The amount of nitrate-nitrogen leached from each plots was calculated from collected groundwater samples. In addition to collecting groundwater samples, residual soil nitrate samples were collected from each plot and analyzed for nitrate and ammonium nitrogen. Each year representative com samples were taken from each plot and treatment to test for yields. All these data were used in mass balance calculations to determine an optimum nitrogen based manure application rate. A total of 2030 groundwater samples were collected and analyzed form the project site wells. Nitrate concentrations in the upper three-feet of groundwater downgradient of the test plots ranged from 1 to 116 mg/I NO3-N. Weighted average calculations of nitrate-nitrogen concentrations of groundwater flowing from under the test plots ranged from 16 to 86 Kg/Ha during the two years. The total average amount of nitrate-nitrogen in the outflow from the field ranged from 89 to 563 Kg as NO3-N during the two years. Comparison of N inputs to N leaching losses showed that 12 to 50% of the applied N over the two year period leached to groundwater. The shallowest wells were used to determine leaching losses to groundwater, which ranged from an average of 13.2 to 100 lb/A lost. Com yields varied greatly each year, mainly due to the timing of precipitation during critical periods of the growing season. Yields ranged from a treatment average of 84 to 96 bushels per acre the first year to between 94 to 141 bushels per acre for the second year treatments. Several mass balance calculations were performed using all the sample data. During each series of calculations, additional nitrogen inputs and outputs were included to observe the resulting balance. Soil and groundwater sample data indicated that by waiting until spring to till a stand of alfalfa, rather than tilling the previous fall, reduced the residual soil nitrate in the Spring by about 50%. This in tum reduces the amount of nitrate available for leaching during spring recharge. It was also determined that soil ammonium-N inputs, in addition to residual soil nitrate, should be analyzed and given credit as part of a N management. Comparing groundwater nitrate concentrations between plots revealed losses of nitrate from denitrification in addition to leaching. Results suggest that supplementing first-year alfalfa credits with 25 to 45 pounds of nitrogen from manure or fertilizer can ensure optimum com yields the first year of crop rotation and possibly reduce leaching nitrates to groundwater, except in less than ideal growing conditions which reduce yields and result in excess soil nitrate that is likely to leach to groundwater.