Posted: 16/06/2016 at 17:31
A humus rich multipurpose compost has plenty of organic matter to break down. What you are lacking is something like a good loam to mix with it. What you need are a soil based compost or top soil by the cubic metre.Some garden centres have a compost labelled for veg growing which is loam based.
If its any help this is what I did when I took over gardening next door. We made a raised bed area where the pigeon shed was. They has scraped down to subsoil which was clay and shale.
I put a six inches of well rotted farm yard manure on it ( I get a local farmer to tip me a load every now and then. It is a mix of cow manure bedded on straw and horse manure bedded on wood shavings.)
I topped that with a thick layer of rock dust (verve available from b &Q) a dose of 4 oz per square yard of blood fish and bone. I had some loam from a turf stack where I had skimmed a lawn off, and we topped it with that, then mixed it all in.
Its easier to buy good loam ready made and add some blood fish and bone.Just mix it all up well. The worms appear by magic.