So is it preferable to maximize legumes on the transition period? Are three years under green manure better than two or one?
Kristy Borelli and other researchers compared different transition phase rotations: out of 9 different systems, two included 3 years of grain, two included two years of grain followed by one year of green manure, two included one year of grain followed by two years of green manure and one included three straight years of green manure.
At the end of the transition period, the researchers compared the total gain in nitrogen: it was higher for any rotation that had at least one year of green manure. Three years of green manure resulted in slightly higher levels of N than rotations with only one or two years of green manure. More importantly, spring wheat in the first certified organic year was higher after three years of green manure than after one or two; but winter wheat yield on the second year of the certified organic phase was no different whether the transition had had one, two or three years of green manure. Protein levels in spring or winter wheat were the highest in any rotation than had had one, two or three years of green manure.
Bottom line: successive years of green manure in the rotation to transition to organic are likely not necessary. A transition phase rotation should plan on a green manure legume for at least its last year: rotations that only had small grain translated in lower soil N levels, lower yields and lower protein content in the certified organic phase.
To go further: read more about this study here and here.