Rhizobia are able to survive because they form a symbiotic relationship with legumes,  versus free living bacteria which do not need host.  A symbiotic relationship means that both the bacteria and the plant get what they want.  Rhizobia receive energy and carbon-based molecules while the plant gets nitrogen (Tallack).

Process:

1. Rhizobia go into the root hairs of a plant.

2. The bacteria multiply and start forming nodules on the roots.
3. In the nodules, rhizobia convert free nitrogen, which is found easily in the air, to nitrates.
4. Now the plant uses the nitrates to grow and provides the rhizobium with energy.  (Nitrogen fixation)

[1]

"Inoculation of Forage and Grain Legumes." Department of Crop and Soil Sciences - Cooperative Extension. Penn State. 16 Dec. 2007 <cropsoil.psu.edu/extension/facts/agfacts11.cfm>.

[2]

Footnotes:

[1]"Inoculation of Forage and Grain Legumes." Department of Crop and Soil Sciences - Cooperative Extension. Penn State. 16 Dec. 2007 <cropsoil.psu.edu/extension/facts/agfacts11.cfm>.

[2]"nitrogen fixation: roots and nitrogen-fixing bacteria." Online Art. Encyclopædia Britannica Online School Edition. 16  Dec.  2007  <http://school.eb.com/eb/art-7>.