Why rotation earns its keep on a small plot

On a half-acre you can get away with being relaxed about where things go. On a five-rod allotment or a modest back garden, you cannot. The pests and diseases that cause the most heartbreak — clubroot, onion white rot, potato cyst eelworm, the larvae that leave carrots riddled with tunnels — live in the soil and build up steadily whenever the same family of crops returns to the same spot. Rotation interrupts that cycle simply by moving the host plant somewhere else.

There is more to it than pest avoidance. Different crops make different demands on the ground. Brassicas are hungry and like firm, limed soil. Legumes fix their own nitrogen and leave the ground richer than they found it. Roots and alliums ask comparatively little and resent freshly manured ground, which forks carrots and makes onions bolt. Potatoes want plenty of organic matter and their dense canopy smothers weeds. Put them in a sensible order and the soil improves as the years pass rather than wearing out.

The four-group system, and why it still works

The traditional four-course rotation divides everything you grow into four blocks. It has survived because it is genuinely practical, not because it is traditional.

• Legumes — peas, broad beans, runner beans, French beans. Leave the roots in the ground when you clear the crop; those nodules are free nitrogen.
• Brassicas — cabbages, kale, sprouts, calabrese, plus swedes, turnips, radish and rocket. Follow the legumes, which have enriched the ground they need.
• Roots and alliums — carrots, parsnips, beetroot, onions, garlic, leeks, shallots. These follow the brassicas, using what is left without needing more feeding.
• Potatoes — with the catch-all group alongside them: courgettes, sweetcorn, lettuce, spinach, chard. The ground gets a heavy muck and a good clean the following winter.

Each group moves on one bed every year, so a full cycle takes four seasons. Clubroot spores can survive a decade, but four years is long enough to break most cycles and to stop problems becoming entrenched.

Fitting four groups into three beds

Very few small plots divide neatly into four equal beds, and trying to force it leads to abandoned plans. A three-bed rotation is the realistic compromise. Put legumes in bed one, brassicas in bed two, and roots with alliums in bed three, then grow potatoes in bags, a spare strip of ground, or as a one-off elsewhere. Beans and peas can also be worked along a fence or up a wigwam, which frees a whole bed.

If you have two long beds, split each in half widthways and you have your four blocks. Where a shed, a fruit bush or a permanent rhubarb crown sits in the middle of a bed, treat that area as fixed and rotate around it. Perennial crops never move, so draw them onto your plan first.

Bed sizes and the practical layout

Beds of 1.2 metres wide and 3 to 4 metres long are ideal. You can reach the middle from either side without standing on the soil, which keeps the structure open and makes weeding a pleasure rather than a chore. Allow 45 to 60 centimetres for paths — wide enough for a wheelbarrow, and worth the lost growing space.

Sow and plant in blocks rather than long single rows. A block of carrots is harder for carrot fly to find, holds moisture better and shades out weeds between plants. On a small plot, blocks also let you interplant: a quick row of radishes between brassicas, or lettuce tucked under sweetcorn before the canopy closes.

Keep a permanent marker at the end of each bed — a painted stone, a slate, a stout label — showing which group was there this year. It saves an awful lot of head-scratching in February.

Green manures, lime and the gaps in between

The rotation plan is also a feeding schedule. Manure goes on the ground that will hold potatoes and brassicas the following season, ideally in autumn so it has time to settle. Lime goes on ahead of brassicas, but never in the same year as manure, and never before potatoes, which dislike alkaline soil and will scab.

Where a bed is empty from October to March, sow a green manure. Field beans and winter tares suit the ground ahead of hungry crops and fix nitrogen as they grow. Grazing rye is unbeatable for soaking up nutrients and smothering weeds. Cut and dig them in a fortnight or so before you want to plant, and you have added organic matter for the price of a packet of seed.

Keeping records that actually help

The plan only works if you can remember it. A simple notebook is plenty: bed number, crop, variety, sowing date, and a line on how it did. Add the date you cleared each bed and what went in afterwards. A photograph from the same spot each month is worth a page of notes.

Come January, spread the notebook out and shuffle next year's groups onto the beds before you order any seed. Half an hour with a pencil in the warm will save you a season of avoidable trouble, and you will find the whole plot gradually getting easier to manage — which is exactly what a good rotation is for.