The nitrogen cycle, similar to the carbon cycle, is defined as the series of processes by which nitrogen and its compounds are interconverted in the environment and in living organisms. The natural nitrogen cycle consists of fixation of nitrogen from the atmosphere, changes among nitrogen forms in the soil, and loss of certain nitrogen molecules back to the atmosphere. During these natural processes, these nitrogen-containing substances are made useful to living things.
Most plants get the nitrogen they need from soil. Many farmers use fertilizers or manure to add nitrogen to the soil to help plants grow larger and faster. This is because we remove nitrogen from the ecosystem when we eat the food, and without using fertilizer to replenish it, the soil would not be fertile.
All living things, including us, require nitrogen to build proteins. However, because of the chemical nature of nitrogen gas, we cannot obtain nitrogen directly from the air. Instead, we must depend on the nitrogen cycle to obtain nitrogen indirectly. For example, we eat plants or eat plant-eating animals.