Gardening

Rotation principles in organic farming

The most important principle of rotations is that the crops that follow each other in the rotation must have different characteristics and, therefore, different requirements. For example, a main crop will normally consume a lot of nitrogen and humus, therefore, in the rotation it will be followed by a crop that accumulates more humus, more nitrogen, improves the structural conditions of the soil, etc.
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This can be designed taking into account the family to which it belongs – since crops from the same family cannot be repeated -, the depth of the roots or even according to the usable part of the plant. Sometimes there is even incompatibility between different families. For example, nightshades and cucurbits should not succeed each other. Sometimes, it is necessary to suppress certain crops that present serious health problems for a few years, especially during the conversion period.
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But it is also necessary to take into account the conditions of the soil and it is, therefore, necessary to know the physical, chemical and biological characteristics of our soils (antecedents, chemical analyzes, etc.). This will determine the quality of the soils and if it is necessary to prepare them previously with some treatment. It can also guide or determine the types of crops to be established in the rotation and the alternatives.
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Table 5. Crops that should not be repeated by family.
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When organizing the rotation and in view of its success, it must be taken into account that the soil must always be covered, normally by cultivated or natural plant covers (spontaneous flora), since they will avoid the loss of nutrients, energize the biological processes of the soil, provide organic matter, protect the soil against erosion, increase the effectiveness of irrigation by improving the water retention capacity, etc. The inclusion of green manures in the rotation is, in addition, a complementary strategy both in terms of fertilization and in the control of pests and diseases.
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Table 6. Distribution of crops that should not be repeated by depth of their roots and usable part.
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Therefore, the design of the rotation requires the following orientations among others:
– Determine the physical, chemical and biological characteristics of the soil.
– Keep the floors covered.
– Establish first, the main crops and later, secondary crops (short cycle, few demanding, etc).
– Cultivate species of different families, different usable parts, with roots of different depths and shapes – and associate with plants with complementary needs.
– Alternate weeding crops with soil improver crops.
– Once an adequate rotation has been established, variations must be minimized
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Table 7. Some criteria for the design of rotations.
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Green manures in rotation. A complementary strategy in the control of diseases and plants .

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Green or sidereal manures are cultivated plants that are incorporated into the soil, generally during the flowering period, in order to make an agronomic improvement. They are located between lanes in fruit orchards or between two main crops in the rotation, as an intercropping.
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The fundamental function of green manures is to supplement the nutrition of the crops in the rotation, either through the fixation of free nitrogen, or by their effectiveness in making nutrients available to crops that would otherwise be inaccessible. However, a secondary objective of green manures may be the control of pests and diseases.
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Green manures are used in organic farming on a regular basis to control adventitious plants. Thus, for example, the mixture of broad beans and rye, sown in autumn as green manure, and buried in late March or early April, controls the growth of adventitious plants in the following crop. This is due, on the one hand, to the rapid and abundant growth of green manure “drowns” the other plants; and, on the other hand, because rye releases substances into the soil that poison the adventitia (allelopathic compounds), not allowing them to grow properly. On the other hand, beans accumulate nitrogen in the soil, after cutting and burying the green manure, which is used by the next crop, which produces more than without the use of green manure.
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The incorporation of green manures is also capable of reducing problems caused by soil fungi and reducing the populations of nematodes and soil insects. Table 8 lists some examples. This is mainly due to three mechanisms that we explain below.
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1. Increase of biological activity in the soil.
The incorporation of green manure leads to an increase in the population and activity of numerous organisms present in the soil (spiders, insects, fungi, bacteria, protozoa, etc.). Most of these organisms are positive or neutral for the culture, and they limit the populations of those that are harmful. This control is carried out indirectly, through competition for resources, and, directly, because they are predators, parasites, or produce toxic substances that harm pathogens.
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2. The formation of toxic substances during the decomposition of green manure.
The vegetal remains incorporated into the soil are transformed into simpler ones by the organisms present in the soil. As a result of this degradation, some intermediate organic substances are formed that are toxic to other organisms that are harmful to plants.
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3. The use of green manure as a trap plant.
Thus, the control of nematodes by green manures can be carried out using host plants that stimulate their development, but that are eliminated before the life cycle of the nematode is completed. An example is the control of fodder beet nematodes that can be partially achieved by using cruciferous green manures as a preceding crop, which stimulates the emergence of cyst larvae, after which the plants must be buried in the soil. Fava beans have a similar effect before sugar beets.
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Table 8. Examples of the use of green fertilizer for crop protection.
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Before undertaking the design of the rotation to start the conversion period, we must therefore know the complex of diseases and pests associated with each species.
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Afterwards, plant species that are not sensitive or are not attacked by the same diseases or pests should be alternated. In general, it is recommended that successive crops do not belong to the same family, and that a crop not be repeated until at least four years have passed, although it obviously depends on the most frequent pest or disease in the growing area.
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