First look at plants that are high in nitrogen and see if they can grow in you area. Comfrey, alfalfa,clover,certain grasses and even weeds can be grown and the harvested. I dont see the need to store for later use though. If your using your garden every year, it needs to be fed. I went to my local feed store and got alfalfa meal,blood meal and oyster shell mixed well, and put in each hole for squash and tomatoes that were being planted in sand and gravel. Results were amazing. Be sure to mulch and do not use wood chips or sawdust. They take nitrogen out of soil to breakdown.Sheeps wool is good also.