The best thing to do with clay soil is to improve it by adding grit, sharp sand and compost (any type), as clay is inherently fertile but roots cannot grow well because of the poor drainage. If you add at least a 2 inch layer of compost to the surface as a top mulch, the worms will do a lot of the work for you and the soil will improve over the years. I find all brassicas do well in clay, particularly cabbages and brussels.
