I've used a garden plot here for 3 seasons, and after many disappointing yields, I'm realizing the soil is to blame. It's clay. When it rains, the garden becomes a pool and the plants rot. I've put a bunch of compost and composted horse manure in it over the last 3 years, but it seems it all goes away and only clay and rocks are left behind. What to do to fix the soil? What should I add? I can till the garden with our tractor, but it doesn't go too deep. Should I just pile good dirt on top of the clay? Thanks.