To return organic material back to the soil and stimulate the soil food web, a diverse blend of seeds can be sown to cover bare soils thus kickstarting soil repair. With different root depths and qualities, these plants are allowed to grow and are chopped and dropped just as they are ready to move out of their vegetative state into flowering and subsequent fruiting. This directly feeds the soil and replenishes fertility. Latest EDU release.
http://treeyopermacultureedu.wordpress.com/chapter-8-soils/cover-cropping/