To keep closely related vegetables (botanical families) together and grow them on a different piece of land each year, ideally not repeating for three to four years. (This is not always achievable in a small garden)
Pest and disease control
The build-up of soil born pests and diseases can be controlled when plants from different families are planted successively.
Moving weed suppressing plants such as potatoes from plot to plot can minimise weed problems for crops that follow. It also helps to keep a particular weed from getting out of hand.
Different crops require different nutrients and trace elements from the soil. Growing one crop continually will deplete the soil of certain nutrients. By changing the crops from year to year and hence the soil management, nutrients will be replenished. Legumes (peas and beans) in particular build up the nitrogen in the soil for following crops.
Soil structure is improved when alternating between crops with deep tap roots and those with shallow fibrous roots.
It is not always easy to stick to a strict crop rotation in a small garden. Sowing and harvesting times may not always coincide and there may be places in the garden that are well suited to one crop and not well to another.
I follow a simple 4 crop rotation but I also mix my plants so that some of the crop rotation benefits are taking place simultaneously eg growing some leguminous plants in all beds so that nitrogen is being fixed in the soil for all other crops. The exception to this is planting legumes with the onion family. Not only do onions not require a lot of nitrogen they are classic ‘non companion’ plants.
The four crop rotation is:
Potatoes – Legumes – Brassicas – Roots
Potatoes include all members of the Solanaceae family including potato, tomato, eggplant and peppers
Legumes include peas and beans (broad, runner, French etc)
Brassicas include cabbage, broccoli, sprouts, kale,
Roots include vegetables from the Umbelliferae / Apiaceae (Carrot, parsley, celery, and parsnip) and Chenopodiaceae (beetroot, spinach, chard) families
Complete the rotation with a green manure even if it is direct seeded amongst plants that are completing their life but not ready to pull out. Best sown in autumn.