Oregon's Permaculture Networking Site in Corvallis, OR
I do these things when I have clay. In deep clay, compost will improve the top few inches of the soil, but below it will still be thick clay. Clay features millions of tiny plates which lock together, preventing air and water from passing through. I have witnessed pumice transforming deep clay into optimal, crumbling soil, a few years after planting with it. Gravel and rock in clay also improve the drainage by breaking up the clay plates and letting water drain, but to a lesser extent. You can actually see the white fungal fibers on rotten wood, which retains some moisture during our not dry summers, but drains the heavy wet rains of the winter. That is why so many trees in the forest grow out of old dead, "nurse" logs. Newly chopped wood is not porous, so it doesn't work as well as old, partially rotted wood. I do use compost. I just think that it is not enough in heavy clay.