From 1984-1993, continuous corn was grown with spring chisel tillage, while in 1994 a no-till soybean-corn rotation was begun with a winter wheat cover crop after corn to “trap” nitrogen remaining in the soil. Fertilizer N rates were decreased over the 15 years, as N fertilizer “philosophy” and recommendaions changed. Preplant anhydrous ammonia rates were 250 lb/A in 1984-88, 200 lb/A in 1989-93, 175 lb/A in 1995, and 155 lb/A in 1997 and 1999, with small additional amounts of N applied as starter.
Nitrate-N concentrations in drainflow decreased greatly over the 15-yr period, as a result of the changes in management practices (Fig. 1). Concentrations were consistently in the 20 to 30 ppm range in the 1985 to 1988 period and in the 7 to 10 ppm range in the 1996 to 1999 period. Fertilizer N rates during the 1985-88 period were high by today’s standards but were the recommended rates at the time. The conversion from continuous corn with high fertilizer N rates, to a corn-soybean rotation with lower fertilizer N rates and a winter cover crop following corn, resulted in concentrations of nitrate-N in tile drainage below 10 ppm, on this low organic matter, silt loam soil.
Higher total amounts of nitrogen were lost per acre from the narrower drain spacings compared with wider drain spacings, especially in years with higher overall losses (Fig.2). Since more water is drained from the narrower spacings, more pounds of N are also lost in the water (since concentrations are similar among all spacings). This underscores the importance of using cover crops or other practices to trap or treat nitrate leaving the soil in drainage, especially as drainage intensity is increasing.