Adding cowpeas to the buckwheat cover crop reduces your need to add a layer of compost before seeding your fall garden since you greatly increase the amount of available nitrogen produced. Like many legumes, cowpeas
fix nitrogen out of the air, and they do that even better when there's very little nitrogen in the soil for them to suck up. Buckwheat steals soil nitrogen from its neighbors, tricking the cowpeas into fixing more nitrogen than they would if grown by themselves in the same soil. You cut the duo at the same time you would have cut the buckwheat (which is several weeks earlier than you would normally cut cowpeas) and end up with somewhere between 50% to 100% of your nitrogen needs for the fall garden taken care of.