Corn Response to Nitrogen Fertilization in a Soil with High Residual Nitrogen

County, a major fruit- and vegetable-producing area. High N fertilizer rates (100 to 300 kg N ha 1 ) are applied High levels of residual NO3–N are present in the soils of the Arkanby onion growers to optimize yields without regard for sas River Valley in Colorado where alfalfa (Medicago sativa L.), grains, fruits, and vegetable crops are produced. This study evaluated soil test NO3–N levels (Bartolo et al., 1995, 1997). the use of continuous corn (Zea mays L.) to reduce residual N levels Soil test results from the Colorado Arkansas Valley in a furrow-irrigated, silty clay soil. Fertilizer N needed to maintain area and Otero County indicate high levels of residual optimum corn yields following watermelon [Citrullus lanatus (Thunb.) soil NO3–N (Dr. Lorenz Sutherland, USDA-NRCS, LaMatsum. and Nakai], and its impacts on NO3–N leaching potential Junta, CO, personal communication, 1998). Producers were also evaluated. Treatments evaluated from 2000 through 2003 in the area often think that the soils are just inherently included two N sources (urea and Polyon) and six fertilizer N rates. high in available N and do not associate the high level Corn grain yields were not significantly increased by N fertilization of residual soil N with past and current N management the first year following watermelon but increased with increasing practices. Halvorson et al. (2002) found 355 kg NO3–N residual soil NO3–N levels the second year without additional N fertilha 1 in the 0- to 60-cm soil profile and 785 kg NO3–N ization and increased by N fertilization in the third and fourth years. Nitrogen source did not significantly affect corn grain yields, residual ha 1 in the 0- to 180-cm soil depth before planting onion soil NO3–N, or N fertilizer use efficiency (NFUE). Nitrogen use effi- near Rocky Ford, CO in a furrow-irrigated, moldboard ciency generally decreased with increasing level of available N. Aver- plow tillage production system. Most soils in this area age NFUE based on grain N removal over 4 yr was 55% at the lowest are generally well drained (no need for tile drainage fertilizer N rate and 30% at the highest N rate. Excluding the first system) but often have water tables within 4 m of the corn crop, grain yields and gross economic returns less N costs were soil surface. Because of high residual soil NO3–N levels, maximized with about 265 and 258 kg ha 1 of available N (soil plus high N fertilization amounts applied to shallow-rooted fertilizer N), respectively. Soil residual NO3–N levels declined follow- crops like onion, the short distance to water tables, and ing the second, third, and fourth corn crops. Reducing N application excess water application to control soil salinity, there is rates (based on credits for residual soil NO3–N and previous crop) a high NO3–N leaching potential in this area (Ceplecha