PO 14. Describe how the following soil factors affect symbiotic nitrogen fixation.

Soil conditions affect the degree to which all nitrogen cycle processes occur.  Fixation in particular is an important process for legume management, and is influenced by many soil properties.  As pH drops, fixation decreases; very little will occur below a pH of 5.0.  Excessive wetness or dryness will cause fixation to slow, and even to stop under severe drought conditions.  The population of correct Rhizobia species is essential, as different bacterial species require different plant hosts. Good fixation is dependent on correct inoculation of the legume seed with the appropriate Rhizobium.  The soil nitrogen level can influence fixation; as readily-available N from other sources increases (manure, fertilizers, biosolids, OM), the amount of N fixed will decrease.  Rhizobia are aerobic and require oxygen to function, so under wet conditions, N fixation will decrease.  High soil organic matter content will also cause a drop in N fixation, especially when coupled with high available N.