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Nitrogen is essential for the formation of amino acids which form proteins. The nitrogen cycle is a model that explains how nitrogen is recycled.

There's lot of nitrogen in the air – about 78% of the air is nitrogen. Because nitrogen is so unreactive, it cannot be used directly by plants to make protein. Only nitrates are useful to plants, so we are dependent on other processes to convert nitrogen to nitrates in the soil.

1. Nitrogen gas from the air is converted to nitrate compounds by nitrogen-fixing bacteria in soil or root nodules. Lightning also converts nitrogen gas to nitrate compounds. The Haber process is a man-made process where nitrogen gas is converted into ammonia which is used to make fertilisers. Farmers use fertilisers like ammonium nitrate to help crops to grow and increase yields.
2. Ammonia is converted to nitrates by nitrifying bacteria in the soil.
3. Plants absorb nitrates from the soil and use these to build up proteins. The plant may be eaten by an animal, and its biomass used to produce animal protein.
4. Decomposers break down the bodies of dead organisms, urine and faeces resulting in nitrogen being returned to the soil as ammonia. This ammonia is converted to nitrates by nitrifying bacteria.
5. In some conditions denitrifying bacteria in the soil break down nitrates and return nitrogen back to the air. This is usually in waterlogged soil. Improving drainage reduces this effect, making the soil more fertile by retaining more nitrates.

Farmers can increase the nitrate content of soil using two methods:

• crop rotation
• using fertilisers

Crop rotation and improved soil fertility

Farmers often grow crops such as peas, beans or clover as these crops can form nitrate, as they have nitrogen-fixing bacteria in their roots. This will increase the nitrate content and fertility of the soil. Crop plants will take in the nitrate and use it to make proteins for growth. One year the farmer will grow one of these crops and then the following years, the farmer will plant another crop in the nitrate rich soil. Growing different crops each year in a cycle is called crop rotation.

Using fertilisers

Natural fertilisers such as manure or compost are used by farmers to provide a source of nitrate to increase crop yield. Expensive artificial fertilisers such as ammonium nitrate can be applied to the fields.