The intricacies of crop rotation

The purpose of crop rotation is to prevent the build-up of a pest or disease to which a crop is susceptible. But there is more to rotation than merely switching crops.

- 

For example, many farmers follow the principle that a leaf crop should be planted after a root crop. Generally, this is a beneficial practice, but it can end up achieving nothing.

In the case of carrots (a root crop) and Swiss chard (a leaf crop), for instance, both vegetables are vulnerable to root knot eelworm. So rotating between them will not get rid of the pest.

Similarly, if beet cyst eelworm is present, you should avoid following beetroot with a Brassica such as cabbage, as the build-up of the pest will continue. (If the pest is not on the farm, of course, this rotation presents no problem at all.)

For the same reason, it is a mistake to follow tomatoes with an eelworm-susceptible crop, unless you use one of the new tomato hybrids that are resistant to eelworm.