A soil test for N availability to corn (Zea mays L.), NO‐3‐ to 30cm when plants are 15 to 30 cm tall, was evaluated on a wide range of soils at 33 locations in Vermont and one in New York state. The NO‐3‐N present at the time of sampling is a result of an integration of all the soil and weather factors that have influenced the availability of N prior to the time for sidedress fertilizer application. There was a typical response relationship between soil test NO‐3‐N and corn silage yield on plots that only received limited amounts of starter N fertilizer at planting. The Cate‐Nelson procedure for NO‐3‐N vs relative yield (yield without sidedress N/yield with sidedress N) indicated that there was a greater probability of response to N fertilizer when NO‐3‐N was less than 36 kg ha−1 than at higher soil test values. The currently used N fertilizer recommendation procedure is based on estimates derived from cropping history, yield goal, soil type, and manure management information provided by farmers. Compared with the current method of estimating N fertilizer needs, recommendations based on the NO‐3‐N soil test call for much less N fertilizer application and are more accurate in separating sites with varying response probabilities. As a routine laboratory procedure, the proposed test has numerous advantages over other N availability procedures. However, special procedures are needed, including rapid soil treatment after sampling (to eliminate N transformations), sampling the soil at a different time than for the usual soil test, and a rapid sample turnaround time in the laboratory.

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Problems associated with using excess N for corn (Zea mays L.) production include NO3‐N pollution of groundwater as well as economic loss to the farmer. The Magdoff Pre‐Sidedress Nitrate Test (PSNT) is a soil test that provides more accurate fertilizer N recommendations for corn than previous N recommendation systems based on anticipated effects of manure and crop management. Using the test can result in significant reductions of fertilizer N use on high‐N supplying soils. This article provides the background for understanding how and why this soil test functions for corn in the humid regions of the Midwest and Northeast. The pattern of high‐crop N uptake rates relative to mineralization indicates that adequate nutrition requires the presence of a large available N pool in the soil at the start of corn's period of rapid growth. In addition, there is little NO3‐N leaching in most soils during normal growing seasons and when NO3‐N is moved deeper in the root zone, some enrichment remains in the surface 1 ft. This combination of phenomena allows NO3‐N in a 1‐ft soil sample taken just prior to the period of rapid growth to reflect fertilizer N requirements.

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Leaching loss as nitrate (NO3) is a growing concern because of its potential effect on water resources. Leaching of NO3 with drainage water from subsurface‐drained field plots seeded to maize (Zea mays L.) in 1992, 1993, and 1994 was measured on two soil types (a clay loam and a loamy sand) and for three N fertilization rates (22, 100, and 134 kg N ha−1). The 100 kg ha−1 rate was based on the results of a presidedress nitrate test (PSNT). Nitrate nitrogen (NO3‐N) leaching was similar between fertilizer N treatments at both sites in 1992, the first year after sod plowdown, but concentrations were greater than 10 mg L−1. For the subsequent two years, losses were similar for the 22 kg N ha−1 and the PSNT‐based treatments, but significantly higher for the 134 kg N ha−1 treatment on the clay loam. On the loamy sand, losses increased from the lowest to the highest N rate. Nitrate leaching losses were consistently higher on the loamy sand than on the clay loam. The N budget results showed that the 134 kg N ha−1 rate had the highest residual soil NO3‐N in the three years at both sites. On the clay loam, significant N losses occurred from denitrification following alfalfa plowdown and the subsequent fall and spring. Results indicate that N use efficiency rapidly decreases with overfertilization, even with N fertilization rates that only slightly exceed (134%) crop requirements. The PSNT‐based rate reduced N leaching losses while maintaining maize yields.