Potatoes
Dig in plenty of manure or compost. Potatoes are hungry, and their dense leaves smother weeds. Tomatoes belong here too.
Four beds, four years
Crop rotation
Grow each family of vegetables in a different bed each year. Pests and diseases that live in the soil lose their host, and each crop makes use of what the last one left behind.
Year 1
The four families
Who follows whom
Peas and beans
They follow the potatoes into ground that is still rich. Their roots take nitrogen from the air and leave it in the soil.
Cabbage family
Leafy and greedy for the nitrogen the beans left. Lime the bed first if your soil is acid, and firm it well.
Roots and onions
Carrots and parsnips fork in fresh manure, so they come last, when the richness has been used up. Then the cycle starts again.
What does not rotate
Salads, spinach, chard, courgettes, squash and sweetcorn have few soil diseases of their own. Use them to fill gaps in any bed. Fruit bushes, rhubarb, asparagus and most herbs stay put for years, so give them a permanent bed at one end of the plot.
If you have one bed
In a small garden you cannot rotate properly, and you need not worry much. Just avoid growing the same family in the same spot two years running, and take extra care with potatoes, onions and the cabbage family, whose diseases last longest in the soil.