Making Good Use of Organic Crop Rotations

Brenda Frick, Ph.D.

Organic producers repeatedly rate crop rotation as the most effective tool they use to maintain soil fertility and manage weeds, diseases and insects. A crop rotation is a planned sequence of different crops. Effective crop rotation is essential to any sustainable cropping system, but it is especially important in organic systems, where producers can't hide the problems of monoculture with quick fixes.

Organic producers frequently include legumes associated with nitrogen fixing bacteria as part of their fertility management. Perennial legumes such as alfalfa, and biennials such as sweet clover, offer the greatest nitrogen benefits, especially if any mowed material is returned to the soil. Red and white clovers are short-lived perennials that can be used as biennials, either with or instead of sweet clover. Annuals such as pea, black lentil, black medic or various vetches can also add nitrogen. Most of the nitrogen benefit of a pulse is lost if grain is removed.

Nitrogen is not the only soil factor to be considered. Buckwheat is a deep-rooted crop that has an acidifying effect on the soil, and thus helps mobilize phosphorus and make it available to subsequent crops. Mustards may also help with phosphorus uptake.

Cereals such as oat, barley, and fall rye, and forage grasses can also be used as cover crops, and plowed in to increase soil organic matter. The use of perennial plants, and of cover crops can improve soil tilth and structure.

Crops such as fall rye and mustard can have an allelopathic effect; their residues may inhibit the germination of weeds in following crops.

Soil moisture should also be considered in crop rotation. Broadleaf crops such as pea, lentil and flax are generally shallow rooted. Cereals tend to be intermediate. Perennial crops can access moisture at deeper levels. If moisture is limiting, perennials may need to be followed by a partial fallow to allow water recharge. Shallow rooted crops following a perennial crop may be able to access recharge water at the surface, while leaving deeper recharge reserves to medium rooted crops that follow them.
Crop rotation is also an excellent tool for weed management. By altering warm season crops such as beans, cool season crops such as wheat, winter crops such as fall rye, biennials such as sweet clover, and perennials such as alfalfa, a producer keeps altering the weed environment in a way that prevents the weeds from adapting to it. Specific crops can be used to target weed problems as they arise. For instance, early emerging weeds such as wild oats can be managed with mechanical controls in late seeded crops. Canada thistle can be effectively managed in alfalfa fields by a combination of mowing during the crop and tillage after plowdown.

Crop rotation also keeps some kinds of insect pests from building up. Rotations are most effective for insects that feed on only one crop, that over-winter in that crop, and are not very mobile. Unfortunately that does not describe grasshoppers. Still, there may be some insect advantages to greater crop diversity, as it fosters a greater diversity of the predators and pathogens that naturally control crop pests.

Rotating susceptible and resistant crops can reduce the incidence of crop diseases as well. Generally, crops that are most similar, share diseases. Rotation is especially effective against diseases that are soil or stubble borne. Again, greater diversity increases the likelihood that other natural controls will be present to mitigate the effects of the disease.

The ideal crop rotation is site specific, and depends on the soil and agronomy factors mentioned above as well as the markets available for different crops, the equipment and facilities available, and the comfort level of the producer. In general, diversity leads to stability, and reduced risk. The greatest diversity that the producer can manage usually gives the greatest benefits.

Brenda Frick, Ph.D., P.Ag,. is the Prairie Coordinator for the Organic Agriculture Centre of Canada and is located at the University of Saskatchewan. She welcomes your comments at 306-966-4975 or via email brenda.frick@usask.ca .

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This article first appeared in The Western Producer, and is published here on the OACC website with permission.