County, a major fruit-and vegetable-producing area. High N fertilizer rates (100 to 300 kg N ha Ϫ1 ) are applied High levels of residual NO 3 -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 NO 3 -N levels (Bartolo et al., 1995(Bartolo et al., , 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 NO 3 -N (Dr. Lorenz Sutherland, USDA-NRCS, La-Matsum. and Nakai], and its impacts on NO 3 -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 NO 3 -N residual soil NO 3 -N levels the second year without additional N fertilha Ϫ1 in the 0-to 60-cm soil profile and 785 kg NO 3 -N ization and increased by N fertilization in the third and fourth years. Nitrogen source did not significantly affect corn grain yields, residual ha Ϫ1 in the 0-to 180-cm soil depth before planting onion soil NO 3 -N, or N fertilizer use efficiency (NFUE). Nitrogen use effi-

Furrow‐irrigated onion (Allium cepa L.) production, with high N fertilization rates, may be contributing NO3–N to ground water in southeastern Colorado. This study determined the growth and N uptake patterns of onion grown on a silty clay soil, N fertilizer use efficiency (NFUE) of onion, and recovery of residual N fertilizer by corn (Zea mays L.) following onion in rotation. Onion was sampled biweekly from 18 May to 15 Sept. 1998 from plots receiving 0 and 224 kg N ha−1. Nonlabeled N and labeled 15N fertilizer were band‐applied near the onion row in split applications of 112 kg N ha−1 each on 18 May and 25 June. Onion dry matter accumulation was slow from planting to about late May, followed by a rapid increase in biomass production and N uptake. Because residual soil NO3–N was high, N fertilization resulted in only a small increase in bulb yield. Greatest demand for N by onion occurred during bulb development. Fertilizer N recovery by onion was 11 and 19% for May and June N applications (average 15%), respectively. Much of the fertilizer N remained in the upper 60‐cm soil profile at harvest and had moved toward the onion bed center. Fertilizer 15N detected at 180‐cm soil depth indicated leaching losses from the root zone. The unfertilized 1999 corn crop recovered 24% of fertilizer N applied to onion for a total fertilizer N uptake by the two crops of 39%. Delaying N fertilizer application until onion bulbing begins may improve NFUE. Planting corn directly on the previous onion bed may result in greater N fertilizer recovery by corn.

Water and nitrogen (N) are major inputs in the production of onions in the Arkansas Valley of Colorado. Because nitrates move with irrigation water, the effect of different rates of application of both N fertilizer and water on nitrate leaching were studied simultaneously. After a 2-year field study (1990-1991), it was concluded that >50 t·ha-1 of onions could be obtained without any N fertilizer when >42 ppm of nitrate nitrogen (NO3-N) were initially present in the top 33 cm of soil and up to 112 cm of irrigation water was applied. Total onion yield was not improved by applying more than the calculated irrigation requirement. The 2-m profile of soil under these experiments was found to contain >1400 kg·ha-1 of residual NO3-N prior to fertilizer treatments. When twice the estimated irrigation requirement was applied, >1000 kg·ha-1 of NO3-N was unaccounted for and presumed to have been mostly leached below the 2-m profile and partly denitrified. In both years, the onions were planted on land that had been fallowed the previous season, which does not help explain the presence of the high levels of nitrates found in the soil profile. It was concluded that sound water and N management practices in onion fields are crucial for preservation of water quality.

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