What is the nitrogen cycle?

In the nitrogen cycle , in what form will plant roots absorb the nitrogen from the soil?

• I understand that for root nodules of leguminous plants the N2 can be absorbed and be converted directly into ammonia by Rhizobium bacteria. But what about non-leguminous plants ? Do they have to absorb nitrates from the soil and convert it step-by-step to nitrites then ammonia or do they have the ability to directly absorb ammonia from the soil? Any answer will be extremely appreciated.

• Answer:

  http://hortipm.tamu.edu/ipmguide/nutrient/nitrogen.html http://en.wikipedia.org/wiki/Nitrogen_cycle http://www.rainbowplantfood.com/agronomics/efu/nitrogen.pdf Plants can take up N in 4 forms: NH4 Ammonium NO3 Nitrate Organic Nitrogen Molecular Nitrogen