Nitrogen makes up about three quarters of all the air. It is found as free nitrogen (N2) or as nitrogen oxides (NOx). In these forms, nitrogen is useless to plants and animals.

Nitrogen must be first converted into nitrates, where the nitrogen oxide combines with another mineral such as potassium.

Nitrates are formed naturally in two ways. Lightning during thunder storms forms some nitrates.

However, the most common way to make nitrates is through "nitrogen-fixing bacteria" found in the soil. Some of these bacteria, like "Azotobacter", live freely in the soil. Others, like "Rhizobium", live on the roots of clover and other members of the pea (legume) family. That is why planting clover in a lawn, or rotating crops to include peas or beans, makes a good environmental impact. These plants replenish the nitrates in the soil naturally.

In the past hundred and fifty years, farmers have replaced this natural process by applying nitrate fertilisers directly to the soil. First they used bird dung (guano) and then created factories that "fixed" nitrogen from the air - the Haber process.

Roots of plants absorb the nitrates made in (or bought to) the soil to make amino acids, which in turn make proteins. All living organisms require this nitrogen to make proteins. Animals get the nitrogen that they need to make proteins by eating plants or other animals.

When an animal or plant dies, decomposing bacteria and fungi break down those proteins that are present in all living organisms. The amino acids in their bodies are broken down and ammonia gas (NH3) is released. Nitrite bacteria convert this poisonous ammonia back into nitrate molecules, for plants to grow again. Some plants can use ammonia directly. Ammonia is used as a fertiliser by injecting directly into the soil.

Nitrogen is removed from the nitrate in the soil by denitrifying bacteria. The nitrogen is released back to the atmosphere and the nitrogen cycle is complete.