Nitrogen (abbreviated as N) is an element. It is an essential plant nutrient. While the atmosphere contains more than 78 percent of nitrogen, most plants are unable to extract it from the air. Some nitrogen-fixing plants, which form symbiotic relationships with some strains of bacteria, are able to extract the nitrogen and use it. Once the plant drops leaves or other material, and microbes compost it, the nitrogen becomes available in the soil.
A trio planting method surrounds plants that need nitrogen with plants that are able to fix nitrogen.