Seeming conflict between the need to use N fertilizers and the need to protect groundwater quality requires better tools for distinguishing between fertilizer applications that are essential and those that are excessive. Studies were conducted to evaluate NO‐3 concentration in corn (Zea mays L.) stalks at physiological maturity as the basis for a tissue test to characterize degree of N excess during corn production. Samples of the lower portion of corn stalks were collected from plots in N‐rate experiments at 18 site‐years in Iowa. Observed relationships between grain yields and stalk NO‐3 concentrations indicated sharp breaks between NO‐3 concentrations that were not adequate and those that were adequate for obtaining maximum or near‐maximum yields. When yields were near maximum, stalk NO‐3 concentrations increased linearly with amounts of N fertilizer applied. Stalk samples collected at various times after black layering showed that NO‐3 concentrations remained constant for at least 2 wk. These observations suggest that stalk NO‐3 concentration offers great potential as the basis for a tissue test to characterize degree of N excess during corn production.

The concentration of NO−3 in the lower portion of cornstalks at physiological maturity has been proposed as the basis for evaluating the N status of corn (Zea mays L.). Here we refine estimates of the optimal range in NO−3 concentrations for this test by using relevant economic scenarios that include 900 plot‐years of new data as well as 450 plot‐years reported earlier. The results showed that rainfall had little effect on concentrations of NO−3 identified as being optimal. However, calculations showed that optimal concentrations of NO−3 in stalks were influenced by prices of corn and fertilizer. For prices that have prevailed in the Corn Belt during the past decade, an optimal range of 0.7 to 2.0 g NO−3‐N kg−1 was shown to be more appropriate than the range originally reported. When this optimal range is used, the tissue test can be used to identify deficiencies of N as well as excesses of N. Overall, the results show that the end‐of‐season cornstalk test deserves attention as a tool for providing feedback that can be used to compare and refine systems for making N fertilizer recommendations.

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Recent studies have shown good correlations between corn (Zea mays L.) yields and concentrations of NO3 in the surface 30‐cm layer of soil in late spring. Here we report additional correlations and evaluate the benefits of sampling to 60 cm instead of to 30 cm only. The study involved 45 site‐years (1346 plot‐years) of data collected in 1987, 1988, and 1989 in Iowa. Weather conditions were dryer than normal, with a severe drought occurring in 1988. Each site‐year included seven to 10 rates of N applied before planting. Samples representing the surface 0‐ to 30‐cm and the 30‐ to 60‐cm layers of soils were collected when corn plants were 15 to 30 cm tall. Nitrate concentrations in these soil layers were correlated with grain yields. The deeper sampling slightly improved the correlations between grain yields and soil NO3 concentrations, but the advantage was probably not great enough to justify the costs of the deeper sampling. The critical concentration of NO3 was 23 to 26 mg N kg−1 in the surface 30‐cm layer of soil and 16 to 19 mg N kg−1 in the surface 60‐cm layer of soil. Overall, the results support the idea that a soil test based on concentrations of NO3 in the surface 30‐cm layer of soil when corn plants are 15 to 30 cm tall has great promise for improving N management during corn production.

Soil tests for evaluating the N status of cornfields in late spring show promise as a tool for improving N management during com production. An alternative tool for evaluating N status is tissue testing, which offers the potential advantages of easier sampling and better integration of factors that influence N availability. Here we evaluate total N concentrations of whole com (Zea mays L.) plants in late spring as an indicator of N availability in cornfields. Studies were conducted at 14 site-years in Iowa during 1986Iowa during , 1987Iowa during , 1988Iowa during , and 1989. Whole plant samples were taken when corn was 15 to 30 em tall. Total N concentrations in these plants were determined. Relationships between concentrations of N in young plants and fertilizer N applied were not consistent across the 14 site-years. The concentrations of N in young plants were poor predictors of soil NO, concentrations in situations where good relationships between soil NO, concentrations and grain yields occurred. The tissue test could not detect excessive amounts of NO, in soils. Concentrations of N in young plants were greatly influenced by factors having relatively little effect on final yields. Overall, the results show that a tissue test based on the concentrations of N in young plants would not be a reliable indicator of the N availability in cornfields.