There's lot of nitrogen in the air - about 78 per cent 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.
Farmers can increase the nitrate content of soil using two methods:
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.
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.