The process by which bacteria convert nitrogen in the atmosphere into a usable form for plants is called nitrogen fixation. This process is vital for agriculture, as most plants cannot use atmospheric nitrogen directly. Specific bacteria, such as Rhizobium, associate with the roots of leguminous plants and convert atmospheric nitrogen (N₂) into ammonia (NH₃), which plants can assimilate. This symbiotic relationship between bacteria and plants enhances soil fertility and supports crop growth, underscoring the importance of microorganisms in nutrient cycling within ecosystems.