The few perennial vegetables and a number of herbs require a permanent position but most vegetables and some herbs are grown as annuals. Those of a similar type have similar requirements of the soil and therefore need to be grown together. They also attract the same pests and diseases, however which, if the plants were continually grown in the same spot, would build up in the soil. Annually grown crops are therefore moved to a new spot and this is usually done because of lack of space on a 3-year rotation. This means that the same plants are only grown every third year in the same spot. In Victorian times a 7-year rotation was used.
Crop rotation also helps to care for the soil as one type of vegetable will remove more of certain nutrients, so the time lapse allows these to be replaced. Peas and beans add nitrogen to the soil which can then be utilized by the next crop and the rotation of shallow and deep rooting vegetables helps the soil structure.
Unfortunately, moving the position of a crop does not guarantee that you will have no pests or diseases. Airborne pests will not be deterred and pests in the soil may attack any crop. However, rotation is a valuable deterrent to those pests that are specific to one type of crop and move little — like the minute eelworms which affect potatoes and cabbages or to the build-up of diseases caused by fungus such as club root.
It is not possible to be absolutely strict about rotation, because some crops require more space than others or have a specific requirement as regards position, or preferred harvesting time — but it helps to keep to it as closely as you can.