Soil and tissue tests for determining nitrogen status of corn
Soil and tissue tests are important tools for adjusting N fertilization rates to maximize the profitability of corn production and(or) minimize the adverse effects of N fertilizers on the environment. When used early in the season, these tools give site-specific information that can be used to fine-tune N fertilizer recommendations. When used late in the season, they give feedback that is essential for adjusting future fertilization practices toward optimal. The need for better soil and tissue tests is shown by mounting evidence that many producers are applying N fertilizers at rates higher than those associated with maximum profit;Studies involving 1346 plot-years were conducted to refine the late-spring soil NO[subscript]3[superscript]- test, which previously has been identified as a promising tool for improving N management in Iowa. These studies provided the first correlations for this soil test to depths of 60 cm, and they demonstrated the great stability of critical concentrations (NO[subscript]3[superscript]- or NO[subscript]3[superscript]- plus exchangeable NH[subscript]4[superscript]+ to depths of 30 or 60 cm) across a wide range of conditions. Studies utilizing [superscript]15N tracers in 414 plots documented the ability of the late-spring soil test to reduce uncertainty associated with carryover of fertilizer N, mineralization of N from soil organic matter or plant residues, and highly variable interactions of fertilizer-N losses during the first few weeks after application. Results from 420 plots showed that the late-spring soil test was superior to a tissue test based on the N content of plants collected at the same time;A visual rating system was developed and calibrated for midseason evaluations of N status. The system is based on firing, the premature death of lower leaves. The system was shown to be at least as reliable as a tissue test based on the N concentrations of the ear leaf at silking, and it offers the advantages of requiring less time, equipment, and expense;Nitrate concentrations in lower cornstalks at the end of the season were shown to be extremely reliable as an indicator of N status across a wide range of conditions. This new tissue test is unique because it is the only tool currently available for evaluating N status in the optimal-to-excess range and because its critical concentrations are directly linked to economic criteria. This test, especially when used with the late-spring soil test, should prompt significant voluntary improvements in N management by revealing when it is profitable to reduce N fertilization rates.