[Aunt Sally]

Let me tell you the OLD way to combat club root.

In days of yore when gardeners wore a waistcoat and there was no such things as chemical treatments for club root the old gardener didn't admit defeat. 

They planted their brassica seedlings into bottomless pots which the hung up off the ground. The roots which grew out of the bottom of the pot were burnt off by the air around them.  This forced the roots in the pot to become very well established with a thick fibrous system.  When planted out into well limed ground they would survive club root as they already had an advantage. 

The only thing I could add to that would be to use a soil based compost and not a peat one.  Once peat based ones dry out they are almost impossible to wet again.  I've pulled failing plants out of moist ground to find the peaty potting soil around the roots as dry as dust.