Legumes provide benefit in crop rotations, but data are limited on soil inorganic nitrogen (N) and soil strength responses to spring- or fall-seeded legumes as green manures for grain sorghum [Sorghum bicolor (L.) Moench] production on the prairie soils of the eastern Great Plains of the United States. With increased emphasis on conservation tillage, information is also needed on combining conservation tillage with the use of legume cover crops. This experiment was established to examine the effects of i) red clover (Trifolium pratense L.) and hairy vetch (Vicia villosa Roth.) as previous crops to grain sorghum compared with continuous grain sorghum, ii) reduced or no-tillage, and iii) fertilizer N rate on changes in soil inorganic N and soil strength. At two adjacent sites (Parsons silt loam; fine, mixed thermic Mollic Albaqualf) differing in initial pH and phosphorus (P) and potassium (K) fertility, soil nitrate-nitrogen (NO3-N) was as much as fourfold higher following kill-down of red clover or hairy vetch than following continuous grain sorghum. At the higher fertility site, soil total inorganic N [TIN: sum of ammonium-nitrogen (NH4-N) and NO3-N] shortly following kill-down of red clover exceeded TIN following hairy vetch by more than 35% and that with continuous grain sorghum by 110%; however, at the lower fertility site, the trend for soil TIN to be higher following legumes was not significant. Tillage did not affect soil NO3-N levels in the spring following legume kill-down. However, subsequent soil NO3-N levels under no-tillage tended to be higher in the spring, but lower in the fall than with reduced tillage. Increases in soil TIN by legumes and fertilizer were related to grain sorghum yield, but likely were not the only factors affecting yield. Legumes and tillage used in grain sorghum production may also provide other non-N benefits as suggested by soil penetration resistance measured at the end of the study.
