Monoculture crop production has been associated with an increase in pests and soil-borne disease. Using crop rotation and companion planting can help to break disease cycles and improve soil health.

Crop rotation is one of the most basic principles of vegetable production and should always be practised. It’s best described as a system of crop production in which various crops are grown in such a way that no crop is planted on the same piece of land more than once in three planting cycles (but preferably four).

While crop rotation is recommended for improving soils and for conservation purposes, its greatest benefit lies in the reduction of disease levels in the soil. Many pathogens can persist in the soil after the crop has been removed (such as black-rot in cabbage).

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Failure to practise crop rotation will result in an increased rate of infestation, in turn leading to higher pest management costs. Crop rotation might also reduce unwanted insect populations and perennial weed infestations.

An example of a crop rotation system using five plots, over five growing cycles, is given in the table. The crops planted include:

• Legumes:      Beans, peas, cowpeas, pigeon peas and bambara
• Brassicas:      Cabbage, Chinese cabbage, kale, radishes and cauliflower
• Root crops:   Carrots, onions, beet, sweet potato, amadumbi and onions
• Solanaceae:   Tomatoes, peppers and potatoes
• Leafy crops:   Swiss chard, amaranthus and cleome
• Cucurbits:      Pumpkins, squash, melons and cucumbers
• Green mealies can be included in the rotation at any point.
• Another general rule is not to plant an underground bearing crop in consecutive seasons in the same soil.

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STAGGERED PLANTING

A continuous supply of vegetables can be achieved by staggered planting – making smaller, but regular, plantings at regular intervals (for example every four weeks) during the planting season to ensure a continuous supply of the crop.

Maturity can be predicted in part by using days from planting to harvest for each crop.

Continuity can also be achieved to some extent from single plantings of crops which don’t require a certain stage of maturity to be ready for harvesting, such as sweet potatoes which can be harvested when the size is adequate for marketing.

COMPANION PLANTING

Companion planting also has value to protect plants from pests. It’s based on the theory that the companion plants – for example, flowers growing next to a food crop – disrupt the search pattern of pests looking for host plants. They detect the host plants but become confused because of the more diverse planting style.

• Separating rows of cabbages, broccoli or other brassicas with rows of onions is a popular combination, and possibly works because the onion’s strong scent disrupts cabbage pests.
• Tomato plants grow well next to cabbages, which seem to deter caterpillars.
• Growing leeks near carrots seem to repel carrot flies.
• Planting marigold or calendula between vegetables may reduce unwanted nematodes in the soil.

Nematodes occur naturally in the soil, but monoculture can result in a build-up of species that could be harmful to specific crops. These nematodes might feed on the root system of host plants and can cause considerable damage.

A variety of herbal plants planted amongst the vegetables or around the vegetable patch may have beneficial effects. Basil planted with tomatoes and lettuce may deter insects. Oregano planted with broccoli may repel cabbage flies.

Table: An example of a crop rotation system

• This article was originally written by Sunette Laurie and Lulama Mkula from the Agricultural Research Council in South Africa and first appeared in Farming.