 Nitrogen is sensual to life, plants and animals need nitrogen to make proteins, and proteins that are building blocks of cells and therefore life. Nitrogen makes up 78% of the air, however neither plants nor animals can take nitrogen directly from the air because nitrogen is so unreactive, however plants are able to take nitrogen compounds such as nitrates from the soil, and then animals eat these plants therefore providing animals with a source of nitrogen. In this video we are going to look at the nitrogen cycle. This is a movement of nitrogen through the environment. Nitrogen is continually cycled through the air, soil and living things. The process of nitrogen in the atmosphere being turned into nitrogen in the soils is called fixing. Here are four different ways in which nitrogen fixing occurs naturally. Firstly, nitrogen fixing bacteria in the soil turns nitrogen from the air into nitrates which the plants can then absorb. Some plants such as the legume family which includes peas, beans and clover have these nitrogen fixing bacteria in their root nodules. Secondly, decomposers in the soils break down animal excretion in dead organisms, returning nitrogen back to the soil as ammonia. The ammonia turns firstly into nitrite and then nitrifying bacteria in the soils turns the nitrite into nitrate. Thirdly, lightning can cause chemical reactions in the atmosphere resulting in nitrogen reacting with oxygen to produce nitrous oxide. The nitrous oxide then reacts with more oxygen and dissolves in rainwater to make nitrous acid. This rainwater containing nitrous acid increases the amount of nitrate in the soil. Burning fossil fuels also adds nitrous oxide to the atmosphere resulting in nitric acid. But this is of course not a natural way of nitrogen fixing. And finally, in the oceans many species of blue-green algae also called cyanobacteria can fix nitrogen. This then provides sources of nitrogen to aquatic animals. And the nitrogen goes around a similar cycle to what happens on land. These are the natural ways through which nitrogen fixing occurs. The harbour process, which is how fertilisers are made, makes up approximately 30% of the nitrogen fixing. Because nitrogen is essential for making proteins, proteins are essential for cell production. Cell production is how growth occurs. Fertilisers are therefore used to increase the nitrates in the soil to maximise plant growth. Watch our videos on the hyper process to learn more about this. And as I mentioned earlier, burning fossil fuels also adds nitrous oxide to the atmosphere, which dissolves in rainwater to make nitric acid, which then adds nitrates to the soils. This isn't as good as it sounds. It upsets the balance of the natural nitrogen cycle. Polluting ecosystems and altering the ecology of entire regions. Too much nitrogen in the soil makes it more acidic. The nitrogen also passes into rivers and lakes, where it's considered a pollutant. In some conditions, such as waterlogged soils, denitrifying bacteria in the soil break down nitrates and return nitrogen back to the air. This reduces the fertility of the soil. So from this video, you should now understand how the nitrogen cycle works. Nitrogen is essential to life, and so needs to be fixed in nitrates for plants and therefore animals to access it. Nitrogen is continually cycled for the air, soil, living things and water systems.