All living things need nitrogen in a form their bodies can use. It is important to plants and animals as well as to humans. It is a necessary part of the protein substance which is man’s body-building material. Without this substance, no one could grow or repair damaged or worn-out tissues.

While oxygen makes up only 21 per cent of the air we breathe, nitrogen makes up 78 per cent. There are about 7,200,000 tonnes of nitrogen above a square kilometer of the earth’s Surface. It is a colorless, odorless, tasteless gas that dissolves only slightly in water.

It would seem easy for living things to get the nitrogen they need since it is all around them in the air. But human bodies cannot use pure nitrogen. When breathed in, it dilutes the oxygen. So it is useful only when combined with other chemicals to form compounds. In nature, only a few plants, the legumes, such as beans, peas, and clover, are able to use pure nitrogen from the air. But all plants can use the simple nitrogen compounds of the soil in which they grow.

A nitrogen cycle is thus set up in nature which enables plants and animals to maintain life. Plants get simple nitrogen compounds from the soil and unite them with carbon to make proteins. Animals get the nitrogen they need by eating the plants. The nitrogen is returned to the soil as waste. Certain bacteria turn this waste back into simple nitrogen compounds, and the plants are able to use it again. So the cycle is complete. The supply of nitrogen is also replenished by bacteria which take nitrogen from the air and “fix” it in the ground.