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In order to maintain the fertility of a field, it has traditionally been the practice to alternate between different crops in some (more or less) regular sequence. It differs from the haphazard change of crops from time to time. Rotations may be of any length, being dependent on soil, climate, and crop. They are commonly 3 to 7 years in duration, but usually with 4 crops (some of which may be grown twice in succession). The different crop rotations on each of the fields of the set making up the farm as a whole, constitute a 'crop rotation system' when integrated optimally.
Metaphor
It is possible to perceive the alternation in parties in power in a multi-party system as a form of crop rotation.
[Features]The manner in which each crop is deliberately chosen to correct for the weaknesses of the previous crop (e.g. control of pests, soil degradation, soil fertility, etc) and benefit from its enhancement of the soil. Understanding of the merits of particular crops under particular conditions.
[Contrast]The chaotic switch between policies of the 'right' and the 'left' without any understanding of the merits of each, the need periodically to compensate for the negative consequences ('pests') of the other and to replenish the particular resources which each policy so characteristically depletes.
[Keys]Balancing the rotation cycle. Fallow period. Short-term advantages of the use of fertilizers (with dangerous long-term implications). Use of each crop to inhibit the pests encouraged by the previous crop.