Your advice is sought to improve the nitrogen content of the soil to be used for cultivation of a non-leguminous terrestrial crop.

(a) Recommend two microbes that can enrich the soil with nitrogen.

(b) Why do leguminous crops not require such enrichment of the soil?

Our atmosphere contains enough nitrogen, but the plants cannot take up this atmospheric nitrogen directly. It needs to be converted into absorbable forms (nitrates and nitrites) by certain microbes such as bacteria and cyanobacteria.

(a) Nitrogen-fixing bacteria like Azospirillum and Azotobacter fix atmospheric nitrogen while living freely in the soil and helps in enriching the nitrogen content of the soil.

(b) Leguminous crops such as pea, beans and pulses do not require such enrichment of the soil. These crops contain Rhizobium bacteria in a symbiotic association in their root nodules. Rhizobium is capable of fixing atmospheric N₂ only in the symbiotic form. They cannot fix N₂ independently. This bacterium makes the soil fertile.