Why four beds beat one big plot

Most of us start out on an allotment by planting wherever there happens to be a gap. It works for a season or two, and then the trouble begins: clubroot in the brassicas, white rot in the onions, eelworm in the potatoes, and carrots that fork and sulk no matter what you do. None of that is bad luck. It is the natural result of growing the same family of plants in the same ground year after year.

A four-bed rotation is simply a promise to yourself that next year's crops will not go where this year's relatives grew. Group your plants by family, move each group on by one bed every year, and you get a plot that fights pests and diseases without any chemicals, feeds itself rather than relying on you to remember the fertiliser, and stays manageable because you always know roughly what is going where. Four beds is the classic allotment arrangement because it fits the four groups neatly and gives each group a full year's break from its own kind.

The four groups your crops fall into

Group by plant family, not by what you do with them in the kitchen. Onions and carrots share a bed in most rotation plans because they share the same needs, not because they are related.

• Potatoes and relatives — earlies and maincrop potatoes, plus tomatoes, peppers, aubergines and outdoor cucumbers. Keep these strictly apart from the potato bed in following years; they share blight and eelworm.
• Legumes — broad beans, runner beans, French beans, peas and mangetout. Their roots carry nodules that pull nitrogen out of the air and leave it in the soil, so they are the generous group in the rotation.
• Brassicas — cabbages, sprouts, kale, calabrese, cauliflower, swede, turnip, radish, rocket and mustard. Hungry, lime-loving, and all vulnerable to clubroot and cabbage root fly.
• Roots and alliums — carrots, parsnips, beetroot, onions, garlic, leeks, shallots and spring onions. Beetroot and chard are botanically cousins of the roots here, so treat them the same way.

Anything that does not fit neatly — sweetcorn, courgettes, salad leaves, spinach — can be slotted into whichever bed has a gap. Rotation is a guide, not a religion.

The rotation order, year by year

Run the loop the same way every year: potatoes follow roots, legumes follow potatoes, brassicas follow legumes, and roots follow brassicas. Then start again.

• Bed one — potatoes. Manured the previous autumn, planted in spring. Earthing up and the deep cultivation clear the ground and leave it in good heart.
• Bed two — legumes. Following the muck, they need no feeding at all. Sow broad beans in autumn or peas and beans in spring, and dig in the roots when the crop finishes rather than pulling them out.
• Bed three — brassicas. They want nitrogen and firm, limed ground, and they get both after the legumes. Lime in winter, tread the soil firm before planting, and net against pigeons from day one.
• Bed four — roots. No fresh manure, no lime. The brassicas will have left the ground settled, which suits carrots and parsnips perfectly. Thin them early and resist the urge to feed.

Next autumn, the potatoes move into what was the legume bed this year, and everything shifts round by one. Simple to remember, hard to get wrong.

What to add to each bed, and when

• Potato bed: two bucketfuls of well-rotted manure per square metre spread in autumn and left for the worms, or a handful of general-purpose organic fertiliser at planting if you are short of muck.
• Legume bed: nothing. Overfeeding beans gives you leafy plants and few pods. A sprinkle of wood ash as the flowers set is plenty.
• Brassica bed: a handful of lime per square metre in winter, and a comfrey or nettle liquid feed in midsummer if the leaves look pale.
• Root bed: leafmould or spent compost forked in the previous autumn if your soil is heavy, and nothing else. Fresh manure is what makes carrots fork.

Making it work on a real allotment

Real plots are not tidy rectangles of equal size. Permanent crops — rhubarb, asparagus, fruit bushes, herbs, a comfrey patch — sit outside the rotation altogether and should be given a corner of their own so they do not disrupt the four beds. If your beds are different sizes, rotate by area rather than by bed: the group that occupied the largest area this year should move to the largest area next year.

Winter cropping needs a little thought. Sprouts and kale will hold their bed until March, which clashes with planting potatoes there. Either accept that the rotation slips by a few weeks, or lift the last brassicas into a nursery corner and replant them to free the bed. Courgettes, sweetcorn and squash can travel round as a fifth, makeshift group whenever you are short of space.

Keeping the thread from one year to the next

In October, before the plot goes quiet, sit down with a notebook and draw four rough rectangles. Write in what grew where, what you limed, and what you manured. Note the failures too — the bed where the onions bolted, the corner where the carrots came up forked — because those notes are what turn a rotation plan into a rotation habit. Next spring you will not have to remember anything at all. You will simply look at the page and move everything on by one.

The first year of rotating feels fussy. By the third, it feels obvious, and by the fifth you will wonder why the old boys on the site, who have grown the same potatoes in the same spot for twenty years, still ask you why your brassicas look so healthy.