Question

how does a soil become rich in nitrates when leguminous plants are grown in it

Answer 1

Leguminous plants have grow nodules on their roots which may harborRhizobia,a class of bacteria which can fix atmospheric nitrogen. Fixed nitrogen may be taken up by the same plant or may be transferred to other plants by contact exchange. On decomposition of plant material nitrogen so fixed is released in to the soil for later use by plants or just may get lost by leaching away or deep into the soil or merely get lost into air by denitrification.

Answer 2

Roots in leguminous plants have nodules. A bacterium called Rhizobium lives in these nodules and converts atmospheric nitrogen in nitrates. These nitrates are used by leguminous plants.