N supply in stockless organic cereal production under northern temperate conditions. Undersown legumes, or whole-season green manure?

Summary

Two systems for nitrogen (N) supply to organic spring cereals were compared under Norwegian conditions. Repeated undersowing of clover in the cereals in four growing seasons was compared to a whole-season green manure in the second year. Cereal yields were higher in the treatments with clover than in the controls. The yield increasing effect of undersown clover was residual. One year of whole-season green manure increased subsequent cereal yields significantly, but not enough to compensate for the loss of yield over the total four-year period. If phytopathological problems can be avoided, repeated undersowing of legumes seems to be more profitable than green manure each fourth year in stockless organic cereal production systems. The soil mineral N decreased during the study, demonstrating a negative N balance. Hence, additional N sources should be found for stockless organic cereal systems under Norwegian conditions.