Nitrogen is essential for life. It is the most abundant element in the air we breath and is the building block of nucleic and amino acids. Despite its abundance in the air, it shows up in roughly 5 parts per million in the soil and is one of the biggest limiting factors of plant growth. Nitrogen is an important plant macronutrient and the first number listed on various fertilizers.
Nitrogen is not a highly reactive element. N2 in the air must first be converted to either ammonia (NH3) or nitrogen oxide (NO2) before it can be taken up by an organism. This conversion is called nitrogen fixation and is primarily fixed in the soil by bacteria such as Clostridium butryicum. To speed up growth of plants, farmers often apply nitrogen to the garden as a fertilizer.
Another way to get nitrogen into the soil and available for plants is to grow cover crops of the legume family. Legumes partner with symbiotic bacteria to facilitate this process. These bacteria often grow in nodes in symbiosis with the host plant directly on their root system. Nitrogen is then made available to the host plant or to other plants when host plant dies and nitrogen from the root nodules are released.
Examples of nitrogen fixing cover crops include clover, alfalfa, and soybeans. Nitrogen fixation can also occur naturally in the air from lightning.
Cover Image by Melanie J Watts, used under its Creative Commons license.