#rRichard Yates
The test I did was to see if gypsum would potentially break up the clay by dropping a ball of my soil in a glass jar of water its suppose to break up in water, but it just stayed in a ball, so apparently the gypsum wouldn't do anything, so you have to use organic material. Hence the manure.
Still undecided, but I suppose any method of getting it into the soil would be good. Thinking of leaving in a big pile to rot then mixing in sand and MPC, then spreading. Idea is to kill off any weed seed the horses have gobbled up. 
I see. Clay has very, very fine particles, 0.0002mm or smaller, and it's quite hard to get it to "dissolve". I'd try getting some small samples from here and there in your soil, to get an average, and put them in a tall thin jar (like the ones that frankfurters come in, for instance) until it's say 1/2 full, then add water, give it a good stir/prod until the lumps go, top it up, shake again, then let it settle undisturbed for a few hours. You'll see a gradient of particle sizes going down the jar, and whether any clay particles still discolour the water, and be able to approximate the constituents. You might well find it useful to add lime or gypsum, if it does not readily break up in water, that implies clay.
Adding sand might not be particularly useful if your soil has a high clay content, the grains of sand get stuck up by the very fine clay (think bricklaying mortar), organic would be better. You could add wood chips, leafy if possible (from garden chipper, tree surgeon who will be glad to get rid of it), or even sawdust/shavings if you can get it "clean", to your fresh horse muck, pile that up and let it mature a while before use. I'd certainly cover the heap, or bag the stuff up and stack the sacks. Check a heap from time to time, wet it if it looks dry. I don't worry about seeds, they hoe off easily enough, and anyway seeds come in from everywhere. Grr!
