All leguminous plants are nitrogen fixers. The legume family is peas and beans. These plants have the ability to take nitrogen out of the atmosphere and out of the food and soil and make these little white nodules which are choc-full of nitrogen. Be sure when pulling up legumes to look for these nodules and scrape them off the roots and back on to the soil. Better still, chop off the stalk at ground level and leave the roots to rot. This is a great, natural way to improve your soil.