This is done by planting such crops as hemp, clover or cowpeas.
"The Library of Work and Play: Gardening and Farming." by Ellen Eddy Shaw
These are the leguminous plants; such as clover, beans, cowpeas, alfalfa, soy bean, etc.
"The First Book of Farming" by Charles L. Goodrich
If you sow wheat in the autumn it is ready to be harvested in time for planting cowpeas.
"Agriculture for Beginners" by Charles William Burkett
Rye may be sown in the fall and plowed down in May, and cowpeas planted to be disked into the soil.
"Crops and Methods for Soil Improvement" by Alva Agee
Many leguminous plants, such as alfalfa, cowpeas, peanut vines, etc., may be cut fine and used as an absorbent for molasses.
"The Philippine Agricultural Review" by Various