This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
In the following pages the various vegetable crops are described chiefly in alphabetical order for the sake of convenient reference. It may be as well, however, to set them out in their natural groups as far as possible, so that one may see at a glance how the principles of rotation may be applied.
Broccoli, Borecole or Kale, Brussels Sprouts, Cabbage (including Savoy), Cauliflower, Kohl-rabi, Sea-kale, Radish, Turnip. These are all subject to attack from the "Club-root" fungus (Plasmodiopliora brassicce).
Crops belonging to the same family have somewhat similar natures, and are as a rule subject to the same diseases and pests. Their roots have a similar effect on the soil, and it is possible that so-called "soil sickness" is due to the superabundance of certain secretions from the roots. Consequently when plants of the same nature are grown in the same soil continually, without a reasonable lapse of time, their roots find themselves surrounded by the injurious organic secretions of previous crops. And yet, if plants of a totally different nature were put in the same soil, the secretions (no doubt bacteria of some kind), instead of being injurious, would in all probability be of great benefit to them. It is thus easy to conceive a state of affairs in the soil by means of which the refuse of one crop may be of use to another of a dissimilar nature. By changing the various crops from one piece of land to another, therefore, not only does the ground get a "change", but the new crops are likely to be in a position to make use of what their predecessors threw away; in other words, one plant's meat may be another plant's poison. From this it follows that it would not be wise cultivation to crop soil in succession with plants belonging to the same natural groups outlined above. The crops in any one group may follow those in any other, thus securing a beneficial change of soil and food.