Fixation of nitrogen in the soil
The nitrogen fixation ability of legumes is enhanced by the availability of calcium in the soil and reduced by the presence of ample nitrogen.
This useful book is published in full on the FAO site
HERE (also click on book cover)
if link stops working, search for "Forest Tree Legumes in Tropical Agriculture")
Legumes have been used in agriculture since ancient times. Legume seeds or pulses were among the first sources of human food and their domestication and cultivation in many areas occurred at the same time as that of the major cereals. Nutritionally they are 2-3 times richer in protein than cereal grains and many also contain oil. Leguminous mulches have always been used as a source of nutrient-rich organic matter and nitrogen for crops. In more recent times, legumes have become important as high quality forages for livestock both in cultivated pastures and in naturally occurring associations.
Of all plants used by man, only the grasses are more important than the legumes but it is the legumes that show the most promise for future exploitation and development.
The legumes are the third largest group of flowering plants comprising over 18,000 species in 650 genera which are well distributed in most environments throughout the world.