Question

why is nitrogenous fertilizer not added in soil in which leguminous plants are grown

Answer 1

because already it has rhizobium bacteria for converting atmospheric nitrogen to nitrate

Answer 2

Answer:

The rhizobium bacteria present in leguminous plants atmospheric nitrogen into  soluble nitrogenous compounds. Hence soil in which legumes are grown becomes rich  in nitrogen and does not require nitrogenous fertilizers.