Took all the replys out but agree with the horse manure/compost.

All I would add is if you have a sawmill or roof truss manufacturers
near you, try and get a few bags of sawdust (generally we pay to get rid
of it) and slap that on top of the ground, water well with a nitrogen
based liquid fertiliser (sawdust robs nitrogen whilst it decays) then
put the manure/compost on top of that. Then when it is looking pretty
much gone, roundabout march, do some spring digging and put the same on
again.

It`ll soon pickup if you get some organic matter in it. Clay is a devil
to deal with unless you have tons of grit to put in it all you`ll have
is clay, organic material is always the way to go in a garden situation.