Organic farmers know that planting legumes, like the bell beans seen above, will increase the amount of nitrogen for the next crop.
Nitrogen is so vital because it is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide (i.e., photosynthesis). It is also a major component of amino acids, the building blocks of proteins. Without proteins, plants wither and die.
So how does this work? The simple answer is that symbiotic bacteria, Rhizobia, which are found within the nodules on the roots of legumes. These bacteria convert nitrogen from the air into usable nitrogen for the plants to live. When the plant is tilled into the soil, the nitrogen is release into the soil and increases soil fertility.
But in simple terms, we simply call it a cover crop.