Cornish Cabbage Plants

Interestingly, the Cabbage people say that:-

"Raising pH levels above 6.5 is essential. Placing sea sand or volcanic rock dust in the bottom of the hole when planting will lessen the likelihood of club root disease. "
https://www.cornishcabbageplants.com/plant-care

Tried 'Rockdust' once, can't say I was impressed and it was very expensive. I would have thought sea sand would be salt laden? (unless there is such a thing as washed sea sand?) , salt, as we know is toxic to plants?
It is well known that a mildly alkaline soil is beneficial to brassica growth.

Having said all this, if they have been going for some time without negative results, good luck to them!