The nitrogen cycle is the continuous movement of nitrogen through Earth's air, soil, water, and living things. Nitrogen makes up 78% of the air we breathe, but most organisms can't use it in that form. They need it transformed first.

Here's how the cycle works: Special bacteria in soil convert atmospheric nitrogen into forms plants can absorb through their roots. Plants use this nitrogen to build proteins and grow. When animals eat plants, they get the nitrogen they need. When plants and animals die and decay, or when animals produce waste, other bacteria break down these materials and return nitrogen to the soil. Some bacteria eventually convert nitrogen back into gas, releasing it into the atmosphere to complete the cycle.

Without the nitrogen cycle, life as we know it couldn't exist. Proteins, which every living cell needs, are built from nitrogen. Farmers understand this cycle when they plant crops like soybeans and peas, which host nitrogen-fixing bacteria in their roots, naturally enriching the soil. They also understand it when they add fertilizers, which are essentially concentrated nitrogen compounds that speed up the natural process.

The cycle can take years or decades to complete, with nitrogen atoms moving from air to soil to plant to animal and back again in an endless loop that has sustained life on Earth for billions of years.