Posted: 11/10/2014 at 09:41
Is this clay your basic soil or has it been dug out for foundations and is really sub-soil?
If it's the latter I think I'd break it up as best I could and leave it to break down in the frost, then mix with all the other things you mention. Without treading it back to a paste in the process
I wouldn't let it anywhere near a tree fern. Having said that I don't grow tree ferns here, too dry, so what do I know about them?