While it may be tempting when planning a garden to designate each crop to a specific area to keep things simple, i.e. a bed where only lettuce is grown, this practice is very counter-productive. You should instead rotate the crops in each garden bed.
When a single crop is sown repeatedly in the same area it's known as a monoculture. This system of growing crops creates many negative side-effects including soil erosion, increased pest and pathogen populations, and nutrient depletion. And sometimes, to answer the problems people turn to synthetic, often toxic, pesticides and fertilizers. Chemical inputs further damage the system until the system cannot support itself without successively heavier and heavier inputs. Monoculture is inefficient, damaging, unsustainable, and can be very expensive.
One of the main answers to the problems presented by monoculture is to rotate your crops. Rotating crops is a method that has been practiced for many thousands of years to keep land fertile long before technology allowed us to sustain monocultures with our synthetic chemicals.
A myriad of benefits comes with rotating crops when compared with monoculture. Yields can increase by up to 25%. Soil erosion, sediment transport, and surface run off are greatly reduced. Different plants affect the chemical environment of the soil in different ways, because of this the soil will remain fertile instead of slowly depleting. Pests and pathogens are usually linked to a specific crop family. When different plants are introduced, the pest and pathogen reproduction cycle associated with the previous crop is either broken or severely hindered. Soil structure is improved. Water retention is increased.
To rotate crops, it's as straight forward as following two guidelines. -
Don't grow the same crop, or a member of the same family, in the same bed 2 seasons in a row.
Follow the crop rotation of heavy givers, then heavy feeders, then light feeders.
There are a few ways these guidelines can be bent and manipulated. Light feeders are ultimatley optional in the rotation. A rotation of heavy givers can be replaced by a fallow season or a large input of green manure/organic matter.
• Legume Family: beans, peas, clover, vetch, lupin
• Brassica Family: broccoli, brussels sprouts, cabbage, cauliflower, chinese cabbage, collards, cress, kale, kohlrabi, radishes, turnips.
• Aster/Daisy Family: artichokes, chicory, endive, lettuce, sunflowers.
• Beetroot Family: beets, spinach, Swiss chard.
• Grass Family: grains, oats, rye, wheat.
• Solanaceae/Nightshade Family: eggplant, peppers, tomatoes, potatoes.
• Cucurbit Family: cucumbers, melons, summer and winter squash, pumpkins, watermelon.
• Allium Family: Garlic, onions, leeks, shallots.
• Umbellifer Family: carrots, celery, anise, coriander, dill, fennel, parsley.
If being planted, light feeders should be planted after heavy feeders and followed with heavy givers. Most root crops will actually suffer in a high nutrient environment, focusing more on leaf growth rather than root growth.