Alfalfa is a leguminous deep rooting cover crop.  It fixes nitrogen, which means it has a symbiotic relationship with nitrogen producing bacteria that live in nodules on its roots-these bacteria take nitrogen from the air and convert it into a form plants can use.  When the plant dies and it's roots begin to decompose, residual nitrogen in the nodules becomes available to other plants.  It also has a deep taproot which can pull up minerals that are deep in the ground as well as helps to penetrate compacted ground.