AN ALTERNATIVE RATIONALE FOR CORN NITROGEN-FERTILIZER RECOMMENDATIONS

Nitrogen recommendations based on corn (Zea mays L.) yield goal can ov erestimate N needs if yield expectations are not realistic, and yield goal-based N recommendations may not be well correlated with observed economic optimum N rates. This study investigated the feasibility of b asing corn N recommendations on soil- and year-specific data rather th an on yield goals. Soil-specific data from multiyear corn N rate studi es, which included several soils and locations, were separated into hi gh- and low-yielding groups based on each year's maximum yield. Based on a 24-yr corn N rate study on a Rozetta silt loam soil (fine-silty, mixed, mesic Typic Hapludalfs) at Lancaster, WI, economic optimum N ra tes in high- and low-yielding years were nearly identical (160 to 170 lb N/acre). Economic optimum N rates in individual years were poorly c orrelated (R2 = 0.02) with grain yields even though maximum yields ran ged from 70 to 170 bu/acre. Similar results were obtained with other s oils and locations. Greater N uptake with high yields does not require more applied N due to higher apparent recovery of available N in year s with favorable growing conditions. A preplant soil nitrate test effe ctively adjusted optimum N rates for year-specific variations in soil nitrate content and greatly improved the accuracy of N recommendations . This test also provides a strategy for minimizing potential nitrate losses to groundwater. Our results indicate that the optimum N rate fo r corn is a soil-specific characteristic and should not be adjusted ac cording to yield goal. Corn N recommendations based on soil-specific d ata and used in conjunction with a preplant soil nitrate test offer an improved approach for identifying optimum N rates.