Nitrogen (N) fertilizer is used extensively in corn production in Iowa and the north-central region. In 1997, the 12.2 million acres of corn grown in Iowa received an average of 121lb/acre of N fertilizer; corresponding values for the region were 62.2 million acres and 130 lb/acre (USDA-NASS, 1998). Excess water leaving the root zone from this cropland is needed to replenish surface and groundwater supplies; however, the nitrate-nitrogen (N03-N) this water carries can contaminate both of these resources. Baker et al. ( 197 5), Baker and Johnson ( 1981 ), and others in the region (e.g., Gast et al., 1978; Kladivko et al., 1991) have shown that N03-N in subsurface drainage water from row-crop land usually exceeds 10 mg/L, the drinking water standard, with annual leaching losses averaging over 15 lb/acre. This water, if not intercepted by tile drains, can percolate to groundwater and cause contamination there. Drainage water intercepted by tile drains and short-circuited back to surface waters can cause contamination there. Studies (e.g., Johnson and Baker, 1982 and 1984; Hatfield et al., 1995) have shown that streams and rivers in Iowa, whose flow is often dominated by shallow subsurface drainage, can have sustained high levels ofN03-N in the range of 10 mg/L. In addition to concerns for drinking water quality, N03-N contamination of water in drainage to the Mississippi River from Iowa and the north-central region has been implicated as a possible cause of a hypoxic or "dead zone" in the Gulf of Mexico (Rabalais et al., 1996). Therefore, improved management of N fertilizer is needed to decrease contamination from this source.