SOIL AND PLANT NITROGEN AVAILABILITY TESTS FOR CORN FOLLOWING ALFALFA

Economic and environmental incentives to accurately predict corn (Zea mays L.) N requirements emphasize the need to accurately credit N supplied to first-year corn from a previous alfalfa (Medicago sativa L.) crop. A 4-yr study (24 site-yr) was conducted to evaluate the use of preplant and pre-sidedress soil and basal stalk nitrate tests and an end-of-season stalk nitrate test to assess N availability to first-year corn following alfalfa. Pre-sidedress soil and basal stalk samples were obtained when corn was 6 to 12 in. tall and analyzed for nitrate. Grain yield was not significantly increased by applied N at any location during the study. Where no N fertilizer was applied, pre-sidedress soil nitrate concentrations in the top foot of the root zone ranged from 10 to 43 ppm N. One-half of the observations were less than the established critical value of 21 ppm nitrate-N. Low soil nitrate-N values (less than 21 ppm N) appear to have been caused by soil and climatic factors. This test provides a method for confirming adequate N availability to first-year corn following alfalfa; however, the need for N fertilization will occasionally be overestimated. Pre-sidedress basal stalk nitrate concentrations ranged from 0.15 to 1.92% N and varied considerably among years at each location. This test appears to have little potential for predicting N availability for first-year corn following alfalfa. The end-of-season stalk nitrate test was highly correlated with end-of-season soil profile nitrate; however, stalk nitrate-N concentrations indicated excessive N availability even though soil profile nitrate-N contents were low at several sites.