How are some plants able to fix gaseous nitrogen from the atmosphere?

Leguminous plants have nodules on their roots that contain, symbiotic bacteria. These bacteria are capable of turning gaseous nitrogen ultimately to nitrate, which the plant can then use in order to synthesise biological molecules such as amino acids. Such bacteria are known as nitrifying bacteria. Nitrifying bacteria can also be found free-living in the soil. This is how other groups of plants absorb the nitrate they need, by simple diffusion from the soil. However, it is a far less efficient process.