Question

Your advice is sought to improve the nitrogen content of the soil to be used for cultivation of a non-leguminous terrestrial crop. [2]
(a) Recommend two microbes that can enrich the soil with nitrogen.
(b) Why do leguminous crops not require such enrichment of the soil?

Answer 1

(a) Free-living nitrogen-fixing bacteria like Azospirillum or Azotobacter are the two microbes that can enrich the soil with nitrogen. [1]

(b) Leguminous crops do not require such enrichment of the soil because they can fix atmospheric $N_2$ due to the presence of symbiotic nitrogen fixing micro-organisms like Rhizobium in their root nodules. [1]