The manure should be aged or else it will hold up the nitrogen and you should till it in . Google "liquid gypsum". It works better than dry and will really help break up your clay soil and help with drainage. Also, till in some compost to help with organic matter. I grow giant pumpkins and add a few yeards each fall before I do soil testing in the spring. I had a new plot last year and had terrible soil. After some other amendments (like kelp, humic acid, etc) , compost, aged manure and liquid gypsum, I was able to grow a 525 pounder, 450 pounder and ~350 pounder with only 6 hours of sun.