I'm on heavy clay too and agree with others. Adding plenty of organic matter - home-made compost, leaf mould, manure - will give you a fertile clay loam over time. Planting carrot seeds into a row of sharp sand works well for me but I wouldn't add it in large quantities. Small amounts of "clay breaker" (incorporating horticultural gypsum) also works well in particularly difficult places, but this is expensive too. Organic matter, organic matter and more organic matter is the answer.
Good luck!