According to the UN Food and Agriculture Organization, world supply of fertiliser exceeded 200 million tons in 2007-08, and world fertiliser use is projected to increase in the years ahead. Fertiliser has become essential to modern agriculture.
Plants produce the sugars they need to synthesise organic compounds through photosynthesis. They also require, however, a number of soil nutrients like nitrogen, phosphorus and sulphur. In nature, when plants die and decay, the nutrients they extracted from the soil are returned to the soil through decomposition. When crop plants are harvested, however, the nutrients are not returned to the soil, so soil nutrient levels can decrease over time.
Adding fertilisers compensates for the nutrients extracted by the plants to sustain their growth. If fertilisers are not added, soil nutrient levels may eventually become insufficient to sustain crop production.
In the past, farmers used to rotate crops and allow fields to lie "fallow" or barren instead of using fertiliser. When a field was left fallow for a year, the activity of soil bacteria and other organisms could gradually increase soil nutrient levels so that the farmer could plant crops there in the next year. Rotating crops also helped to maintain soil nutrient levels, since some plants like legumes harbour symbiotic nitrogen-fixing bacteria that increase soil nitrogen levels. Modern agriculture, however, relies on fertilisers to achieve a higher rate of production.
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