Certainly Clay is very fertile but it takes an awful lot of energy to work. In my case 16 years. It was a real battle but I feel that I won in the end. Double digging was the begining of the battle and sharp sand was the weapon used plus masses of FYM and own compost. If you only use FYM the Clay simply binds up again somehow absorbs the matter but liberal helpings of sharp sand prevents this. To begin with it was impossible to grow Potatoes because if you managed to plant them you couldn't earth them up and if you managed to grow any you would need the pickaxe as mentioned by Richard to harvest them.
I perfected methods of growing Carrots and Parsnips, which I have described on this forum, and they became easier to grow as the years went on.
When I put up my greenhouse I double dug the entire area and put 12" of manure at the base and more at the 1 spit level and masses of sharp sand as the digging progressed and the Tomatoes that it grew grown direct into the soil were magnificent.
So if you have clay there is a way to tame it but it's bloody hard work and today it would be beyond my capabilities it's youngmans work!
Richard,
With soil like you have it is only really fit for grass and grazing. This is where raised beds would be ideal and in the years that you and I have know each other you should by now have a least 12" of soil on top of the awful soil you have. I do not envy you. Living not to far from "The Levels" I would think that some of that wonderful black Peat would help you no end. You did say that you have the luxury of being organic without having to subscribe to any organization so their restriction on the use Peat should be no problem.
To have soil like you have is a real bummer and again I say I do not envy you. Fortunately Clay is not like that normally it is generally quite deep layered. Of course you could move and sell it to somebody else! Apart from that raised beds would serve your needs perfectly.
JB.