One of the major functions of roots is to obtain nutrients from the soil and each plant species has its very own particular nutrient requirements. Growing the same plants in the same spot year after year can severely diminish the available nutrients particularly when growing vegetables. It can also cause a build up of pests and soil diseases specific to that species.
Crop rotation is a fairly old idea that is still very useful for gardeners today. It was originally practised as a way of maintaining soil fertility by alternating crops that deplete the soil of it’s minerals with crops that actually improve the soil and add specific nutrients.
A fine example of why crop rotation is important is Cabbage. Cabbage can be prone to club root, a fungal disease that devastates brassicas (i.e. plants from the cabbage family), but is harmless to other species of vegetables. If you have four beds in your vegetable plot and grow cabbage in bed one the first year, bed two the second year and so on, by the time you plant cabbage in bed one again (year five), the fungal spores will have disappeared and your cabbage will be safe.
Brassicas also require a lot of nitrogen for good leaf growth and so have a tendency to deplete the soil of nutrients. In a crop rotation system, you might choose to follow cabbages with legumes such as beans or peas as these species fix the nitrogen in the soil by adding their own nitrogen back in as they grow. Alternatively, you might choose to follow Brassicas with carrots which prefer very low nitrogen and other nutrients.
Crop rotation generally relies on growing plants from the same family and ABC Gardening Australia has created a handy template as a suggestion for a four-year crop rotation plan see below: