lucerne is good for nitrogen fixing in the soil. There are better plants, but it is cheap and easy to get.
Perhaps you may have an interest reading this rivrhut?:
Estimated symbiotic N2 fixation by annual legume and lucerne pastures on 2 Vertosols with and without applied gypsum.
http://www.regional.org.au/au/asa/2010/pastures-forage/legumes-broadleaf/7200_dearb.htm
