Green manures are slightly more specific than cover crops. Green manures are usually legumes, mustard and grasses that are grown specifically to increase soil organic matter and nutrients. Green manures help add nitrogen and other nutrients back into the soil.
The Black Crowder cowpea is a high yielding variety of long pods. The cowpeas themselves have a deep purple color when first shelled which then turn black when dried. The Black Crowder's green pods are easy to shell. Try with cowpea inoculant for maximum Nitrogen fixation.