Happy Memorial Day, Leslie. Reduce the amount of clay soil as much as it is practical. Hydrangeas like soils that drain well so the more you can do now, the better. Your mophead plant selections will tolerate some local clay soil if you keep the content low. But something like an oakleaf hydrangea would probably have a fit.

Fertilize once, in June, since you are in the northern half of the country with 1/2 a cup of manure, cottonseed meal or a chemical fertilizer (slow release, general use, about NPK 10-10-10, just apply the proper dosage per label once in June). For more information, check out the link below.

Does that help you?
Luis

Here is a link that might be useful: Hydrangea Planting Information

Yes Luis, I think it'll help. I didn't know rather to add sand to the clay or not. I think I should/will. Now, about the 1/2 cup manure, I have a whole bag (40 lbs) of compost and manure. Should I just add a 1/2 cup of this?
or a bucket full along with the sandy loam and the clay soil I dig out of the hole?
Leslie