101 Questions answered
Nitrogen Contribution from Green Manure Vetch: Biomass Is Key
As with most things about the wonderfully complex world of soil, it depends, in this case largely on biomass production and rainfall as nitrogen fixation is directly correlated to biomass production.
Results from the Australian National Vetch Breeding Program (ANVBP) across five South Australian sites over three years showed using vetch as a green manure increased soil nitrogen by 154 kg/ha.
Green manuring fixes more nitrogen than growing vetch for hay or grain. In the trials, hay fixed 94 kg/ha while grain fixed 56 kg/ha.
For more detail, check out the Vetch GrowNotes