From National Geographic
http://ngm.nationalgeographic.com/2013/05/fertilized-world/charles-text
Read this one in the old timey hard-copy magazine last week.
Mrs. P., the Master Gardner, plants fava beans and radishes in the raised beds in the fall, and turns them under in the spring. Clover, too, is a nitrogen fixer. Our small patch of blue grass is half clover, and it is greener than the neighbors. We haven't added anything from a bag in 15 years.