The nitrogen cycle is the movement of nitrogen from the Earth's soil to the atmosphere and back again.

Most of the Earth's atmosphere - about 78% - is made up of nitrogen (N2). Most living things, including human beings, cannot use it in this form.

When we breathe, we inhale nitrogen and then exhale it. We don't use the nitrogen that we take in from the air.

However, nitrogen is necessary for life. All organisms contain nitrogen compounds. Nitrogen is an essential component of proteins and nucleic acids (DNA and RNA).

Many microorganisms that live in soil decompose proteins, breaking it down into simpler materials.

During this process, ammonia (NH3) is produced as a waste product.

When ammonia, which is a gas, is dissolved in soil water, it reacts with hydrogen ions to form ammonium ions (NH4+).

Sometimes, the roots of plants will absorb these ammonium ions and convert them back into proteins.

However, plants often do not absorb ammonium ions directly.

Instead, bacteria in soil convert ammonium ions to nitrates, which are the main source of nitrogen for most plants.

Soil contains two groups of bacteria known as nitrifying bacteria.

One group converts ammonium ions to nitrite ions (N02-). The second group then changes these nitrite ions into nitrate ions (NO3-)

Nitrifying bacteria only break down ammonium ions when there is oxygen in the soil water.

Oxygen dissolves into soil water from the air spaces that normally occur in soil. However, if all the spaces become filled with water, leaving no room for air, then the soil water has no source of oxygen, and nitrifying bacteria cannot do this.

In anaerobic conditions (when there is no oxygen available) a group of bacteria known as denitrifying bacteria change remaining nitrates to nitrogen gas, which gradually escapes into the atmosphere.

Lightning changes smalls amount of gaseous nitrogen to nitrogen compounds.

When these are dissolved in rainwater, they form weak nitrous and nitric acids. In soil, these combine with other elements to form nitrates and nitrites.

Some bacteria, known as nitrogen-fixing bacteria, change nitrogen from the atmosphere into nitrogen compounds. They play an important role in returning gaseous nitrogen to the soil.