The advice not to plant brassicas on the same plot more than once every three years is not unusual; crop rotation is recommended for all vegetables, as nutritional needs and intake vary slightly from one vegetable group to another; therefore, growing the same group on the same plot year after year may deplete the soil of a particular nutrient and lead to unhealthy crops or, at the very least, a lower yield. In addition, it is likely to result in a build-up of the particularly wide range of pests and diseases that are specific to brassicas.

The reason why we are often advised not to compost their roots is, again, because they are susceptible to so many pests and diseases, and there may be a risk of contamination. My own plot, for instance, is contaminated with Club Root; I have overcome the problem by potting on my cabbage seedlings several times, so that they are very robust when I plant them out and can withstand the disease; I also line the planting holes with potting compost, and thanks to this approach, I've been able to keep the disease at bay, although it is still present in the soil. However, unless there are signs that your crop is diseased or has been attacked by pests, I see no reason why you shouldn't add discarded leaves and roots to your compost.