If it's available where you are, mix in some expanded shale with soil, manure and composted leaves or whatever. Expanded shale looks like pea-gravel, but it's shale that has been heated to 1700 degrees F, so that it pops like popcorn. It holds moisture or air and loosens clay soil or makes sandy soil less subject to drying out (less porous?). Tomatoes, peppers, roses, etc. love the stuff. It's not avalable everywhere, I'm afraid.