Azotobacter: A genus of nitrogen-fixing bacteria. Unlike legume bacteria, azotobacter live in the soil independent of association with higher plants. They use soil organic matter as a source of energy, rather than carbohydrates from a host plant. When these microorganisms die, the nitrogen in their bodies is left in the soil in the form of proteins and related compounds. This nitrogen fixation is encouraged by organic matter with a high carbon content. These bacteria need available calcium, and their activity decreases in acidic soil. A ph higher than 6.5 gives the best results. Azotobacter are responsible for a large amount of natural nitrogen in the soil. Nitrates from high-nitrogen, artificial fertilizers decreases their efficiency. Sugar and humic acid products stimulate the growth of these organisms.