An allotment usually has relatively large beds of crops, each hosting plants of one family. Growing the same crop in the same bed year after year causes 2 problems:
An ideal environment in which the pests and diseases for that vegetable will thrive; and the crop makes the same demands on the same specific nutrients it needs. Put the two together and you have a recipe for epidemics striking down even more sickly plants.
Rotation is essential – the KISS principle, just keep the crops moving! Leave at least a year, or better a 2/3 years gap, before a crop returns to the same bed.
Set aside an area for the “permanents” – the fixed beds for asparagus, comfrey, fruit bushes, herbs, rhubarb, and strawberries.
Know your crops strengths and weaknesses.
Rotation applies to vegetable/plant families, NOT to individual crops.
Think about how susceptible your crops are to soil-borne disease, bugs and pests.
Some crops “must have” rotation, as they are VERY susceptible to disease and pest damage – potatoes and brassicas are obvious examples of crops which should not return to the same bed for 3 to 4 years.
Some crops “are keen on” a rotation – the onion family (unless you have white rot); and the carrot family. Leeks are probably the least susceptible of all the onions, so they are quite flexible – and can fit in a number of places (mine go in after early potatoes).
Some crops “are generally not that choosy – but it’s worth humouring them” – peas and beans.
And some, bless their cotton socks, are just “eager to please”, and can go anywhere – beetroot, the leaf-beets, spinach, lettuce, the squashes and cucumbers, sweetcorn, salsify and scorzonera.
Some crops benefit the following crop.
Beans and peas fix nitrogen – so let nitrogen-hungry brassica follow them (possibly in the same season);
Potatoes don’t like lime, so follow them with liming in the winter; next season, plant the crops that like lime most (brassica, or beans/peas);
Roots dig deep and break up the soil – so follow them with potatoes;
Peas and beans like the rich deep dug soil left behind after the potatoes;
Brassica like a firm soil, so don’t follow potatoes very happily;
The onion family are generally happy in the firm soil left by the brassica.
Working out all the permutations gives an “ideal” 5 or 6 year sequence like this –
Year 1 – Beans/peas,
Year 2 – followed by Brassica,
Year 3 – followed by Onion family,
Year 4 – followed by carrots/parsnips,
Year 5 – followed by potatoes, (add lime in winter),
and back to the beginning
Think of winter treatments –
Digging in manure in the winter before the potatoes;
Adding lime before the peas and beans;
Adding compost before the brassicas;
Growing a winter cover of green manure as preparation for the onion, and roots crops.