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"SOUND rotations, the extensive and rational use of manure and vegetable wastes, the use of appropriate cultivation techniques, the avoidance of fertilizers in the form of soluble mineral salts, and the prohibition of agro-chemical pesticides form the basic characteristics of organic agriculture. "
Soil Association Standard for Organic Produce
SOUND rotations are essential components in any organic system. The rotation has to fulfill several very important roles, including the minimisation of weed, disease and pest problems, the maintenance of soil organic matter levels and soil structure, the provision of sufficient nitrogen' and the minimization of nutrient losses. At the same time it has to produce sufficient feed for livestock and maintain the output of livestock and cash crops so that the farmer can obtain a satisfactory income.
Without sound rotations, the farmer is unlikely to be able to farm organically successfully, at least in a manner which is sustainable.
This article explains the principles of rotation design and examines rotations which are currently in use on some organic farms in Britain, assessing their advantages and their weaknesses, and suggests areas where changes might be beneficial.
Soil fertility, health and "tiredness"
Soil fertility has been defined as the Natural strength of the soil" and it is clear that fertility and productivity depend closely on the rotation. Various aspects of this, including the formation of soil structure, nutrient cycling and organic matter, are discussed in greater detail in a Practical Handbook on Soils available from Elm Farm Research Centre.
If the soil is in good condition, with good structure, availability of organic matter and biological activity, then the natural biological control of soil-borne pests and diseases will be more effective.
Monocultures and poorly designed rotations lead to soil "tiredness" or exhaustion" - a situation where growth and development is hindered, causing yield reductions and the occurrence of plant diseases and pests as a result of reduced resistance on the part of the crop.
Soil tiredness has several components. The most obvious are nutrient deficiencies or imbalanced nutrition. Less obvious are the effects of crop residues, which if continuously of the same type may block the activities of some soil organisms, and toxic root exudates which are specific to the crop species and which create an allelopathic effect. All these effects serve to increase the susceptibility of crops to disease, a good illustration of which is the effect of takeall in continuous wheat.
Important aspects of rotation design
In general, the following ground rules should be observed:
* deep rooting crops should follow shallow rooting crops, helping to keep the soil structure open and assisting drainage;
* alternate between crops with high and low root big-mass, high root big-mass provides soil organisms, particularly earthworms, with material to live on. The grass ley is very valuable in this respect, providing an environment for earthworm activity which does not seem to be the case with pure clover stands;
* nitrogen fixing crops should alternate with high N-demand crops; ideally it should be possible to meet all the farms nitrogen requirements from within the system;
In crops which develop slowly and are therefore susceptible to weeds should follow weed suppressing crops;
* where a risk of disease or soilborne pest problems exists, potential host crops should only occur in the rotation at appropriate time intervals (e.g. brassicas);
* wherever possible, catch crops, green manures and under-sowing techniques should be used to keep the soil covered as much as possible, protecting it from erosion risks and reducing nutrient i leaching, particularly in winter.
In addition, the following factors should also be considered:
* suitability of individual crops with respect to climate and soil;
* balance between cash and forage crops; seasonal labour requirements and availability; cultivations and tillage operations.
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