Also called a compost crop or a fallow cycle crop, green manure is a class of plant sown purely to improve the soil quality, in preparation for a later sowing of more recognisably productive crops. The crop is gently tilled into the soil when it is almost ready to harvest, usually just before it flowers. It is then left to compost in situ for about six weeks before another crop is planted into that area. Learn more about growing a green manure crop here.