Little is known about legume green manure productivity on organic farms. Soil and plant tissue were sampled in annual, biennial, and perennial green manures on 41 fields in the eastern prairies. Green manure biomass averaged 4572 kg ha^-1; 53% was legume plant material and 18% was weeds. Soil test P and plant tissue P concentrations were below critical levels in about half of all green manures. Mean N fixation was estimated at 71 kg ha^-1. This observational study provides a baseline for future research to optimize green manure and nutrient management in organic grain production systems.