The plants absorb mineral nutrients from the soil. The fertilizers and manures containing various nutrients like nitrogen, potassium, and phosphorus need to be added from time to time to enrich the soil. Nitrogen gas is available in large amounts but plants cannot use it directly from the air. The bacterium Rhizobium lives in the roots of the leguminous plants. Rhizobium bacteria convert the non-usable form of nitrogen. In this way, the nutrients are replenished in the soil by using fertilizers and manures, and by sowing leguminous plants.