lime, lime , and do not grow greens for a couple of seasons, but grow every thing else and welcom Richard,we can do with a few more from Gods country.

Well Dick, this is a capital problem. To help this it's calcium that is required in the soil, crop rotation of at least 4 years. You may kill it in your plot, but next door it is still available on the next crop of brassicas you grow, so it is a collective effort to reduce it.

Apart from liming and growing your brassicas in large pots to ensure a sizeable root ball before you plant out, some people claim real breakthroughs by using seaweed based products as a fertiliser and in the planting hole.
I can't verify these, but you can take a look and make up your mind at:
http://www.seaweedproducts.co.uk/node/616