According to the Royal Horticultural Society the principle of crop rotation is to grow specific groups of vegetables on a different piece of land each year. The aim of a 4 year plan (one of the most common) is to move groups of plants around in sequence thereby avoiding the same groups of plants returning to a plot within a 3 year period.

Benefits of a successful crop rotation system.

Pest and Disease Control
Soil pests and diseases tend to attack specific plant families, incorporation of a crop rotation system interupts the pests life cycle and build up is reduced.

Weed Control
Crops such as potatoes and squashes can, by their nature, suppress weeds helping to minimise problems for following crops.

Soil Fertility
Differing crops have different soil requirements and as a result benefits. The changing of crops year on year minimises any deficiencies allowing the soil to replenish.

Soil Structure
Alternating between deep rooted and fibrous rooted crops helps to improve the overall soil structure.

Plant Families and Groups

When planning any crop rotation system it helps to know which group (or family) the various plants belong to. This is because plants within the same group tend to suffer at the hands of the same pests or diseases as well as having somewhat similar requirements.

Cruciferae/Brassicaceae
One of the cornerstones of a successful crop rotation system. Tend to have fairly high pH requirement and may benefit from the addition of lime prior to the seasons planting.
Includes Cabbage, Kale, Cauliflower, Sprouts, Spinach, Turnips, Swede

Solanaceae
This group has a slightly acidic preference combined with a high nitrogen requirement and as such will benefit from manure applied in the preceeding autumn.
Includes Potatoes, Tomatoes, Aubergine

Leguminosae
With the aid of root nodules this group fixes nitrogen from the air, helping to provide a good proportion of their own nutrient requirements.
Includes Beans (French, Runner, Broad), Peas

Allium
Includes Onion, Garlic, Shallot, Leek

Curcubitaceae
Includes Cucumber, Marrow, Courgette, Squash, Pumpkin

Umbelliferae
Includes Carrot, Parsnip, Celery, Celeriac, Fennel, Beetroot

Where to start

The four year system starts by dividing the allotment into a number of working plots. Four (for rotation purposes), one permanent bed (for the likes of asparagus, rhubarb and herbs) plus remaining beds to be worked for those plants that do not fall into the typical groups used in the four year system (alternatively crops such as corn and curcubits could be used to fill gaps around plants in the rotating beds or fitted anywhere into the rotation system).

Ideally the plot needs to start being worked in the autumn of the first year.This is to enable manuring of the what is to be termed Plot 1 prior to the planting of potatoes the following spring (typically this is done on Good Friday, but with a flexible holiday and global warming I'm sure this age old practice will change).
When the potatoes go in the ground this is an ideal time to lime Plot 2, if felt it is needed, and to apply a good preferably home made compost to the remaining plots.
The heavy, clay soil so typical of Redhill will greatly benefit from a good digging over at the same time as Plot 1 is manured. This is to allow the frosts to break up the soil structure over the winter. Once the soil structure improves a winter application of of green manure will help to lock in nutrients and increase humus levels in the soil.

Four Year Crop Rotation Plan