the same crop repeatedly in the same place eventually depletes the soil of various nutrients. One way that farmers can avoid a decrease in soil fertility is to practice crop rotation, by which different crops are planted in a regular sequence so that a crop that leaches the soil of one kind of nutrient is followed during the next growing season by a crop that returns that nutrient to the soil. If crop rotation is done properly, farmers can keep their fields under continuous production, without a need to let them lie fallow or to apply artificial fertilizers, both of which can be expensive.
Legumes, plants of the family Fabaceae, for instance, have nodules on their roots which contain nitrogen-fixing bacteria. It therefore makes good sense agriculturally to alternate them with cereals (family Poaceae) and other plants that need nitrogen. A common modern crop rotation is alternating soybeans and maize (corn). In subsistence farming, it also makes good nutritional sense to grow beans and grain at the same time in different fields.
Crop rotation is also used to control pests and diseases that establish in the soil. Families of plants tend to have similar pests. By regularly changing the planting location, the pest cycles can be broken or limited. This principle is of particular use in organic farming, where pest control is achieved without synthetic pesticides, herbicides and fungicides.
Some plants follow one another favorably and others do not which brings risks of yield losses, development of diseases and weeds.
The choice of the rotation depends on the nature of the soil, which determines the species of plants that one can cultivate, but also on animal breeding, which plays a role in the choice of the rotation (fodder crops, straw). Finally a good rotation takes into account the proportion of natural meadows.
The choice of the rotation is also determined by the economical aspects (local needs, market accessibility).
Crop rotation was already mentioned in the Roman literature, and referred to by great civilizations in Africa and Asia. From the end of the Middle Ages until the 20th century, the three-year rotation was practised by farmers in Europe with a rotation: rye or winter wheat, followed by spring oats or barley, then letting the soil rest (fallow) during the third stage. The fact that suitable rotations made it possible to restore or to maintain a productive soil has long been recognised.
A Four field rotation was pioneered by the Dutch and popularised by the British agriculturist Charles Townshend in the 18th century. The system (wheat, barley, turnips and clover), opened up a fodder crop and grazing crop allowing livestock to be bred year-round. The four field rotation was a key development in the British Agricultural Revolution.
In the Green revolution, the practice of crop rotation gave way in some parts of the world to that of simply adding the necessary chemical inputs to the depleted soil, e.g. replacing nitrogen with ammonium nitrate and restoring soil pH with lime.