Would that be bentonite clay? Is it very alkaline? If you desire – compost right where you plan to plant. Nutrients leach out of compost piles that would be better to leach into where you want it. Using thick mulch while you garden is tantamout to on-location compost too…
Several years ago, we had a new home built. It was built on not red clay, but yellow clay. The first year, I put about 4 bales of hay on the ground and tilled it in. We planted and got a meager return. That fall, we collected all the leaves in the neighborhood and put them on the garden and tilled them in. We did that for the next 5 or 6 years, hay in the spring and leaves in the fall. When we sold that house, the garden plot was the nicest black soil I ever saw, and it would grow anything.
I’ve found a lot of usefull information about biulding up good soil from our local University Extension office. One thing in particular I reacall reading recently advised against adding sand to heavy clay soil. Unless you add a »large’ amount, (seems like it was 30-40% by volume), it will lead to greater compaction problems. Humas as others have suggested is you best bet. I called my local county seat to ask what they had been doing with the grass clipopings and leaves they collect and found that they are just stock piling them and letting them rot! I was told I could take all I wanted for free! Check for something similar in your area. Some larger cities collect such yard waste and compost them to sell at reasonable rates. Also, just to stir the pot a bit,…..What is organic? I’m desperatly trying to define what the current or popular use of this term when referring to gardening \ farming. From a stricly scientific point of view oil is organic as it is a product of that which was once living and is combustable, but I don’t think any of us would advocate spraying oil on our gardens.
If you can fix up clay, you’ll get the best possible soil. I’m still working with sandy soil, and while it’s not impossible to work with, it’s never going to be as fertile as well-maintained, clay-based soil. All in all, though, I’m glad for my „beach” garden. At least it drains well, and here on the rainy side of the Cascades, that can be a blessing!