Answer: Clay soil can be a real challenge and you are right to figure it out before laying the sod or it will remain a problem. You can begin, on a small scale, to improve the soil by working in lots of compost or other organic matter. Work it in to a depth of at least six inches, deeper if you can. If you were going to improve soil for garden beds, you'd continue to do this year after year to develop loamy soil, however, you won't want to dig up your lawn every year!
Adding Gypsum may help break up the clay, but it doesn't work for all clay soils. I recommend you contact your Michigan State Macomb County Cooperative Extension office. They will have detailed info on regional soil types and can tell you the best mix of amendments before sodding. Good luck!
Macomb County MSU Extension
21885 Dunham Road, Suite 12
Clinton Township, MI 48036
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