The rate and temporal pattern of ? mineralization from decomposing residues of hairy vetch (Vicia villosaRoth) and rye (Secale cerealeL.) were compared with the pattern of ? uptake by corn (Zea maysL.) planted under no‐tillage into these cover crops in the field. In a second field study where no corn crop was grown, the pattern of ? mineralization from killed fescue sod was compared with the pattern from killed hairy vetch plus sod. Hairy vetch was far more effective than rye in supplying corn ? requirements. In both experiments, the majority of ? mineralized during the corn growing season became available only after a period corresponding to corn silking. The results suggest poor synchronization between ? release via cover crop mineralization and corn ? uptake in the no‐till system.