Farmers Often Grow Legumes Such as Soybeans Every Few Years

Farmers often grow legumes such as soybeans every few years and plow much of the plant parts into the soil. What does this practice mainly serve?

A) to replenish the soil with useful forms of carbon
B) to replenish the soil with useful forms of nitrogen
C) to replenish the soil with useful forms of phosphorus
D) to replenish the soil with useful forms of oxygen