Winter Legumes as a Nitrogen Source for No‐Till Grain Sorghum1

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Abstract
Because winter legume cover crops may provide significant quantities of N while conserving soil and water resources, the role of legumes in conservation tillage production systems warrants renewed attention. The objectives of this study were (i) to determine the N contribution of several legumes to a subsequent crop of grain sorghum [Sorghum bicolor(L.) Moench] under no‐tillage management and (ii) to determine the influence of legume cover crops on soil fertility status. A field experiment was conducted in which four winter legumes, one nonlegume, and a no cover crop treatment were studied over a 3‐yr period. In addition, four fertilizer N rates (0, 28, 56, and 112 kg N ha−1) were applied to no‐till grain sorghum. Grain sorghum did not respond to fertilizer N when following a legume cover crop but responded to as much as 99 kg N ha−1when following a nonlegume cover crop or no cover crop. A mean estimate of 72 kg N ha−1was obtained for the fertilizer N replaced by the legume. The effects of the legume cover crops on soil fertility status included: (i) a lower pH, (ii) a redistribution of K+to the soil surface from deeper in the soil profile, and (iii) a lower C/N ratio in soil organic matter. Since fertilizer N represents a sizeable portion of the fossil fuel energy required for nonleguminous row crop production, the estimated N contribution of legume cover crops represents a significant energy savings, enhancing the conservation value of a no‐tillage production system.