Brassicas prefer a rich soil that contains some lime; use a pH tester and if your soil is acidic add lime before sowing or planting. The leafy, non-rooting brassicas also prefer a firm soil - otherwise they won't produce solid, compact hearts - so lightly compact the soil by walking over it with your weight on your heels before planting out. Or leave several months between digging over and planting out.
With the leafy brassicas you'll get better results if you can set aside an area as a seed bed and then transplant the young plants to their final growing positions.

Broccoli
Brussels sprouts
Cabbages
Cauliflowers
Kale/borecole
Kohl rabi
Swede
Turnip.