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. Research Question In the upper Midwest, corn is frequently grown in rotation with alfalfa. The previous alfalfa crop often provides most, if not all, of the N needed by the corn crop. Many producers, however, are reluctant to fully credit this legume N contribution due to a perceived risk of N deficiency. The objective of this study was to determine if soil or plant nitrate tests can be used to predict legume N credits to first‐year corn following alfalfa and thereby promote more complete crediting of the legume N contribution. Literature Summary Currently, N credits for a previous alfalfa crop are estimated from stand density, age of stand, and cutting management. Use of a soil or plant diagnostic test to predict legume N contributions could remove some of the producer uncertainty related to these credits. The pre‐sidedress soil nitrate test, a pre‐sidedress basal stalk nitrate test, and an end‐of‐season stalk nitrate test have been proposed as methods of assessing N availability to corn. The pre‐sidedress nitrate tests have potential for detecting N released from organic N sources during the early part of the growing season. Study Description Soil and stalk nitrate tests were used to predict N response in first‐year corn following alfalfa at 24 site‐yr from 1988 through 1991. Sites were located on major soils used for corn production in Wisconsin. Corn yield response to applied N was measured at all sites. Sidedress N rates were 0, 50, 100, 150, and 200 lb N/acre from 1988 through 1990 and 0, 30, 60, and 90 lb N/acre in 1991. Soil samples for nitrate analysis were, obtained in 1‐ft increments from 0 to 3 ft before planting and at the end of the growing season. Pre‐sidedress soil and basal stalk samples were obtained from unfertilized plots when corn plants were 6 to 12 in. tall. End‐of‐season stalk samples from unfertilized plots were obtained after physiological maturity. Applied Questions Can pre‐sidedress soil or basal stalk nitrate tests predict N needs of first‐year corn following alfalfa? The pre‐sidedress soil nitrate test successfully identifies sites that will not respond to added N using a 21 ppm nitrate‐N critical value. It also identifies some nonresponsive sites as responsive based on this critical value resulting in unnecessary N applications. These instances usually occurred on sandy soils where some of the nitrate leached from the top foot and on a fine‐textured soil where N mineralization was probably delayed by cool, wet soil conditions. The pre‐sidedress basal stalk nitrate test appears to have little potential for predicting N needs for first‐year corn following alfalfa. Does the end‐of‐season stalk nitrate test provide a reliable evaluation of the N supply to first‐year corn following alfalfa? End‐of‐season stalk nitrate concentrations were highly correlated with end‐of‐season soil profile nitrate contents in the top 3 ft. However, the end‐of‐season stalk nitrate test, indicated excessive N availability at several sites even though end‐of‐season soil profile nitrate contents were low. Is corn grown in rotation with alfalfa an environmentally and economically sound N management practice? Since optimum corn grain yields were obtained where no N fertilizer was applied and little nitrate remained in the soil profile at the end of the growing season, first‐year corn following alfalfa with little or no N fertilizer maximizes yield, efficiently recovers mineralized N, and minimizes potential N losses to the environment. Recommendation The pre‐sidedress soil nitrate test can be used to identify sites where response to applied N will not occur based on a 21 ppm nitrate‐N critical level. Although N response is unlikely in first‐year corn following alfalfa and our results show no response to N even at low pre‐sidedress soil nitrate‐N values, occasional response to 30 to 50 lb/acre of applied N has occurred in other studies. Where soil nitrate‐N values are less than 21 ppm, application of 30 to 50 lb N/acre will insure optimum yields while using most of the alfalfa N contribution. Pre‐sidedress soil and basal stalk nitrate‐N concentration, end‐of‐season stalk nitrate‐N concentration, and grain yields with and without applied N for 24 site‐yr. Grain yield Year County Soil nitrate‐N Stalk nitrate‐N End‐of‐season stalk nitrate‐N Without N With N ppm % % ‐‐‐‐‐bu/acre‐‐‐‐‐ 1988 Clark 26 0.26 ‐ 21 23 Wood 21 0.91 ‐ 106 109 Grant 12 0.15 ‐ 78 77 Columbia 18 0.70 ‐ 136 132 1989 Clark 13 0.90 0.01 134 133 Wood 27 1.26 0.13 131 125 Grant 20 1.51 0.16 131 137 Columbia 43 1.45 0.63 188 186 Monroe 12 1.92 0.24 138 137 Dane 21 1.28 0.20 176 173 Marathon 26 0.87 0.04 126 122 Manitowoc 34 1.22 0.53 115 114 1990 Clark 10 0.70 0.05 139 135 Wood 27 0.73 0.21 129 132 Grant 28 0.92 0.19 174 173 Columbia 32 1.46 0.50