The answer is organic matter. This means amendments such as compost, aged manure, seaweed, sawdust and wood chips. Do not expect quick results, however. The organic matter will decompose slowly to improve the clay. There is a shortcut. Use the rocks to build raised beds at least 24 inches high. Then fill the beds with great topsoil that you can purchase and layer this on top of your poor soil.  Clay soil drains slowly so having a raised bed will help the soil to drain and the new layer of soil that is two feet deep can be planted into immediately. The good news about clay soil is that it holds more nutrients than sandy soil so is more fertile - but only if your plants can survive the challenge of growing in clay. The dirt-cheap way to improve the soil would be to team up with a dairy or horse farm and get loads of composted and aged manure. Add grass clippings, brown leaves and wood chips. Next spring you'll be ready to plant. Building great soil is worth the work. Keep growing, Marianne Binetti