English: Nitrogen is the most commonly limiting nutrient in plants. Legumes use nitrogen fixing bacteria, specifically symbiotic rhizobia bacteria, within their root nodules to counter the limitation. Rhizobia bacteria fix nitrogen which is then converted to ammonia. Ammonia is then assimilated into nucleotides, Amino Acids, vitamins and flavones which are essential to the growth of the plant. The plant root cells convert sugar into organic acids which then supply to the rhizobia in exchange, hence a symbiotic relationship between rhizobia and the legumes .