The fertility of soil can be improved by practising crop rotation. The process in which different types of crops are grown alternately in the same field or soil is called crop rotation. In the rotation of crops, the leguminous crops like pulses, peas, beans, groundnut and Bengal gram (Chana) etc., are sown in-between the seasons of cereal crops like wheat, maize and pearl millet (bajra), etc. That is, in crop rotation, the leguminous crops are grown alternately with non-leguminous cereal crops to restore the fertility of the soil. We know that nitrogen is one of the elements which is essential for plant growth and it is required in greater amounts by the plants. Although nitrogen is abundant in the air as nitrogen gas but the cereal plants cannot utilize it directly. The cereal crops obtain their nitrogen supply from the nitrogen compounds present in the soil. So, repeated growing of the cereal crops like wheat and maize removes a lot of nitrogen from the soil. The soil becomes deficient in nitrogen and this reduces its fertility. And ultimately this results in decreased food-grain production. On the other hand, there are certain bacteria in the roots of leguminous plants like pulses, peas, beans, groundnuts etc. which can directly fix the nitrogen gas present in the air to form nitrogen compounds in the soil. Thus, leguminous crop plants have the ability to fix atmospheric nitrogen to form nitrogen compounds. These nitrogen compounds go into the soil and improve its fertility. Some of these nitrogen compounds are used by the leguminous crop itself, and the rest of these nitrogen compounds are left in the soil to enrich it. Thus, planting a legume crop like pulses, peas, beans or groundnut in a field will result in nitrogen-rich soil. So, the deficiency of nitrogen in the soil by growing a cereal crop like wheat has been made up by growing a leguminous crop in the next season. And the soil becomes ready to support the next cereal crop. Thus, in crop rotation, the cereal crops like wheat, maize and pearl millet are grown alternately with leguminous crops like pulses, peas, beans and groundnut, etc. When the maize crop is grown first, it takes away a lot of nitrogen from the soil for its growth and makes the soil nitrogen deficient. And next, when the leguminous crop like pulses or groundnut is grown in the same field, then the leguminous crop with its nitrogen-fixing bacteria, enriches the soil with nitrogen compounds and increases its fertility. And when another cereal crop like wheat is grown after that, then wheat can utilize this extra nitrogen from the soil for its growth and produce a bumper crop. In this way, rotating different crops (leguminous and non-leguminous crops) in the same field replenishes the soil naturally and leads to an increase in crop production.
Advantages of Crop Rotation:
Rotation of crops improves the fertility of the soil and hence brings about an increase in the production of food grains.
Rotation of crops saves a lot of nitrogenous fertilizer. This is because the leguminous crops grown during the rotation of crops can fix atmospheric nitrogen with the help of their nitrogen-fixing bacteria, and there is no need to add nitrogenous fertilizer to the soil.
Crop rotation helps in weed control and pest control. This is because weeds and pests are very choosy about the host crop plant which they attack. When the crop is changed, the weeds and pests associated with that crop usually disappear.