Editing Pea (section)

=== Nitrogen fixation ===
[[File:Snow pea flowers.jpg|thumb|upright|Pea flowers]]
Peas, like many legumes, contain [[symbiosis|symbiotic]] bacteria called ''[[Rhizobia]]'' within [[root nodule]]s of their [[root|root systems]]. These bacteria have the special ability to [[nitrogen fixation|fix nitrogen]] from atmospheric, molecular nitrogen ({{chem2|N2}}) into [[ammonia]] ({{chem2|NH3}}).<ref>{{cite web|url=http://www.biology.ed.ac.uk/archive/jdeacon/microbes/nitrogen.htm|title=Biological Sciences|website=Biology.ed.ac.uk|access-date=21 August 2017|archive-date=16 January 2013|archive-url=https://web.archive.org/web/20130116214211/http://www.biology.ed.ac.uk/archive/jdeacon/microbes/nitrogen.htm|url-status=dead}}</ref> The chemical reaction is:
:{{chem2|N2 + 8H+ + 8e- -> 2NH3 + H2}}
Ammonia is then converted to another form, [[ammonium]] ({{chem2|NH4+}}), usable by (some) plants by the following reaction:
:{{chem2|NH3 + H+ -> NH4+}}
The root nodules of peas and other legumes are sources of nitrogen that they can use to make [[amino acid]]s, constituents of proteins. Hence, legumes are good sources of [[plant protein]].<ref>{{Cite journal|last1=Lawrence|first1=John M.|last2=Grant|first2=D. R.|date=September 1963|title=Nitrogen Mobilization in Pea Seedlings. II. Free Amino Acids|journal=Plant Physiology|volume=38|issue=5|pages=561–566|issn=0032-0889|pmid=16655833|pmc=549973|doi=10.1104/pp.38.5.561}}</ref>

When a pea plant dies in the field, for example following the [[harvest]], all of its remaining nitrogen, incorporated into [[amino acid]]s inside the remaining plant parts, is released back into the [[soil]]. In the soil, the amino acids are converted to nitrate ({{chem2|NO3-}}), that is available to other plants, thereby serving as fertilizer for future crops.<ref>Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Press, Cambridge UK</ref><ref>{{Cite book |author=Smil, V |year=2000 |title=Cycles of Life |publisher=Scientific American Library}}</ref>