Question

How do soil become rich in nitrates when leguminous plants are grown in it?

Answer 1

Leguminous plants have grow nodules on their roots which may harbor Rhizobia, a class of bacteria which can fix atmospheric nitrogen. ... The nitrogen fixed by the bacterial cells are leached to soils from the roots of leguminous plants in the form of ammonium, which is converted to nitrate by the nitrifying bacteria.

Answer 2

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Bcoz rhizobium bacteria present in the roots of well as many as plants converts the atmospheric nitrogen into a soluble nitrogen compound and thus restores the nitrogen content of the soil.