Why farmers sometimes grow leguminous plant crops in one season and non leguminous plants in another season?

Leguminous plants help to increase the fertility of soil. Therefore farmers grow leguminous plants prior to main crop.

Leguminous plants have root nodules in which Rihizobium bacteira are present. The bacteria take nitrogen gas from the air and make it into nitrogen compounds that the plants can use. The plants do not use as much nitrogen as the bacteria fix, so the excess nitrogen compounds remain in the soil, making it more fertile for the following season's crops.