N fixation diagram by Nefronus

Some symbiotic or free-living microorganisms can fix inert di-nitrogen (N) from the air into reactive organic nitrogenous compounds.  Most biological N fixation (BNF) in farming systems occurs in the root nodules of legumes where rhizobium bacteria take photosynthates from the plant in exchange for fixing atmospheric N and returning ammonium or amides which the plant uses to form amino acids, proteins, etc.   Plants need more N than any other nutrient and N commonly limits plant growth in many ecosystems. 

Crop plants that fix most or (often) all of their N requirements by BNF include peas, beans (both Vicia & Phaseolus), clovers, lucerne (alfalfa), and lupins.  Amounts of N fixed often exceed the legume's needs so rotations, companion crops and crop mixtures with non-leguminous crops are commonly employed within cropping systems. 

Rhizobium species are specific to legume species, and may not be native in some soils, so BNF by some legumes may require their inoculation with suitable Rhizobium strains. 

Research over many years has sought to introduce BNF into non-leguminous crop species, thereby overcoming or reducing their need for fertiliser N.  Some BNF supports commercial sugar cane cropping, but the benefits of BNF to other crops remain minimal.    

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