I grow Clapton and Candid Charm caulies and don't use any chemicals whatsoever. I'm no expert, but always have good sized, tight, clean heads. They are greedy feeders, and I usually use fish blood and bone to give them a long, steady feed, together with plenty of water. I grow them under scaffold netting and I stitch up the little slots that you sometimes get in it because the butterflies will squeeze through these. Clapton doesn't suffer from it, but Candid Charm is possibly prone to club root if you have it on your plot, so I always lime the bed well a few weeks before planting. I would imagine that they need a decent soil and probably wouldn't grow very well on thin, sandy soils, so I'm lucky to have quite a heavy soil.