Wait until October! That was always the perfect time for digging my old clay soil. Or if you have a few days of good rain that will soften it up, then add plenty of compost, well rotted manure or other organic matter
Don't forget the power of hard frosts. Turn over the soil, leaving it rough in big sods, then let the frost over the winter break it down.
It's surprising how, during a dry spell late winter/early spring, you discover the sods have turned into lovely loam. At that point pile on as much compost as possible, planting on top of the compost. The worms will help mix clay and compost over the summer.
I've given up on using a fork to dig clay soil for the most part especially where its been compacted. Pick axe, full size and mini variety seems much more effective.
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It's surprising how, during a dry spell late winter/early spring, you discover the sods have turned into lovely loam. At that point pile on as much compost as possible, planting on top of the compost. The worms will help mix clay and compost over the summer.
Cambridgeshire/Norfolk border.