The difficulty in exploring patterns of alternation is the seeming lack of concrete (as opposed to abstract) examples by which the credibility of such patterns in practice may become apparent. The rotation of agricultural crops is therefore an interesting "earthy" practice to explore in the light of the mind-set which it has required of farmers for several thousand years.

Crop rotation is the alternation of different crops in the same field in some (more or less) regular sequence. It differs from the haphazard change of crops from time to time, in that a deliberately chosen set of crops is grown in succession in cycles over a period of years. Rotations may be of any length, being dependent on soil, climate, and crop. They are commonly of 3 to 7 years duration, 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.

Long before crop rotation became a science, practice demonstrated that crop yields decline if the same crop is grown continuously in the same place. There are therefore many benefits, both direct and indirect, to be obtained from good rotation (103, pp. 176-8):

1. Control of pests: with each crop grown the emergence of characteristic weeds, insects and diseases is facilitated. Changing to another crop inhibits the spread of such pests which would otherwise become uncontrollable (to the point that some crops should not be grown twice in succession). By rotating winter and summer crops, the farmer fights summer weeds in the winter crop and winter weeds in the summer crop.
2. Maintenance of organic matter: some crops deplete the organic matter in the soil, others increase it.
3. Maintenance of soil nitrogen supply: no single cropping system will ordinarily maintain the nitrogen supply unless leguminous crops are alternated with others
4. Economy of labour: several crops may be grown in succession with only one soil preparation (ploughing). For example: the land is ploughed for maize, the maize stubble is disked for wheat, then grass and clover are seeded in the wheat.
5. Protection of soil: it was once believed necessary to leave land fallow for part of the cycle. Now it is known that a proper rotation of crops, with due attention to maintaining the balance of nutrients, is more successful than leaving the land bare and exposed to leaching and erosion.
6. Complete use of soil: by alternation between deep- and shallow-rooted crops the soil may be utilized more completely.
7. Balanced use of plant nutrients: when appropriately alternated, crops reduce the different nutrient materials of the soil in more desirable proportions.
8. Orderly farming: work is more evently distributed throughout the year. The farm layout is usually simplified and costs of production are reduced. The rushed work characteristic of haphazard cropping is avoided.
9. Risk reduction: risks are distributed among several crops as a guarantee against complete failure.

The situation is somewhat different in the case of single-species forests where "rotation" is the guiding principle in the special sense of the economic age to which each crop can be grown before it is succeeded by the next one. (For example, on a 100-year rotation required for oak, one per cent of the forest would be clear cut each year, and a further 20 percent thinned out.) In total contrast to crop rotation the "monoculture" cropping system in which the same crop is grown every year. This is possible on a large scale only by the application of chemical fertilizers, herbicide and pesticides. It leads to long-term problems of soil structure and erosion, as well as to the accumulation of pollutants.

Because of the short-term advantages of fertilizers, efforts to design new approaches to crop rotation have been limited. It is only with the resurgence of interest in non-exploitive, non-polluting agriculture that such possibilities are being investigated (104).

From an agronomist's perspective, the problem is to strike a balance between harmonizing the three-fold soil-plant-climate relationship and those of the economic constraints of production. Because such threefold relationships are now fairly well understood, rotation cycles can now be considered as a whole in which the order and the plants used are of secondary importance. The problem is to ensure that the soil-plant-climate relationship is in an optimally balanced state at every moment in order to become increasingly independent of its past. The production constraints complicate this evolution and the choices possible, especially when requirements change rapidly without taking into account the recent history of a crop rotation (104)

There is a striking parallel between the rotation of crops and the succession of (governmental) policies applied in a society. The contrast is also striking because of the essentially haphazard switch between "right" and "left" policies. There is little explicit awareness of the need for any rotation to correct for negative consequence: ("pests") encouraged by each and to replenish the resources of society ("nutrients", "soil structure") which each policy so characteristically depletes.

There is no awareness, for example, of the number of distinct policies ("crops") through which it is useful to rotate. Nor is it known how many such distinct cycles are necessary for an optimally integrated world society in which the temporary failure of one, due to adverse circumstances (disaster) is compensated by the success of others. It is also interesting that during a period of increasing complaint regarding cultural homogenization ("monoculture"), voters are either confronted with single-party systems or are frustrated by the lack of real choice between the alternatives offered. There is something to be learnt from the mind-sets and social organizations associated with the stages in the history of crop rotation which evolved, beyond the slash-and-burn stage, through a 2-year crop-fallow rotation, to more complex 3 and 4-year rotations. Given the widespread sense of increasing impoverishment of the quality-of-life, consideration of crop rotation may clarify ways of thinking about what is being depleted, how to counteract this process, and the nature of the resources that are so vainly (and expensively) used as "fertilizer" and "pesticide" to keep the system going in the short-term. The "yield" to be maximized is presumably human and social development.