Environmental Science question on Farming?

Question

I've been asked to explain how "legumes" contribite to increased soil fertility. Can someone please help me out?

Answer 1

legumes develop a symbiotic relationship with certain bacteria which are capable of fixing atmospheric nitrogen. Since legumes have a relatively short life cycle, after they die they release the fixed nitrogen.

Answer 2

They fix nitrogen from the atmosphere.  Some plants have a symbiotic relationship with nitrogen fixing bacteria. They provide a safe home for bacteria, whilst the bacteria convert nitrogen gas into nitrates.

Dead matter from the legumes also adds to the fertility of the soil.

http://www.dawn.com/2008/01/14/ebr4.htm

http://en.wikipedia.org/wiki/Nitrogen_cy...