Nitrogen fixation is the process of taking nitrogen from its atmospheric form (which is unusable by plants) and converting it into nitrogen compounds (like ammonia, which is usable by plants). Nitrogen fixation is performed naturally by different bacteria. Microorganisms that fix nitrogen are called diazotrophs. The most common plant that can fix nitrogen are plants of the legume family, but some non-legumes can fix nitrogen also.
- Main Article: Legume
Legumes make up the vast majority of nitrogen fixing plants. Many legumes are important crop species, such as peas, beans, peanuts, and lentils.  Legumes are the most common nitrogen fixing plants because nitrogen-fixing nodules are able to grow on their roots. These nodules are Rhizobium bacteria molecules, and they are some of the only organisms that fix nitrogen.(Purves) Legumes are an important part of farming, because they replenish the nitrogen in the soil.
Although most nitrogen fixing plants are comprised of legumes, there are some organisms that can fix nitrogen that are not legumes. These organisms include 22 different genera of woody plants called actinorhizal plants. Along with these plants, a lot of these non-legume nitrogen-fixing organisms are cyanobacteria, which are found in a variety of land and sea habitats. Another plant that can fix nitrogen is the Azolla fern, which is the only fern that is capable of this. It can fix nitrogen because of a symbiotic relationship with cyanobacteria.  Although nitrogen-fixing plants that are non-legumes are uncommon, they still are important for the survival of other plants that cannot fix nitrogen.
Nitrogen fixation is only a small part of the nitrogen cycle. There are three other processes that help cycle nitrogen through the atmosphere: decay, nitrification, and denitrification. Decay is when proteins made by plants produce organic nitrogen compounds through excretions or dead organisms. Nitrification is the process of converting organic nitrogen into nitrites and nitrates. Finally, denitrification is the process of replenishing the atmosphere by changing the nitrates into nitrogen gas. All of these processes is an extremely important part of the nitrogen cycle, and without them plants could not survive. 
Nitrogen fixing plants are an extremely important part of agriculture. These plants supply the ground with the nitrogen that it loses from other crops. Farmers will usually alternate crops year by year with nitrogen fixing plants and non-nitrogen fixing plants in what is called a crop cycle. Nitrogen fixation is extremely important because of how much nitrogen it provides. Nitrogen fixation fixes twice as much nitrogen per year (175 x 106 metric tons) as other sources like lightning, industrial, and fires (80 x 106 metric tons). This shows that nitrogen fixation in plants is an invaluable source of fixed nitrogen, and without it plants would not be able to grow healthily and provide us with the things that we need. 
- Nitrogen Fixation
- The Microbial World
- Nitrogen Cycle
- Purves, William. Life the Science of Biology. 2005. Couriers Company Inc.