good idea and worth doing but if you toss chunks of clay and compost in
a bed it will take them a couple years to form the final soil. At any
rate it is clear there is no large difference, ad even for mineral
retention, I bet old compost is similar to clay. I will add compost
simply because it is easier for me to shovel manure than to dig clay,
plus with the manure I do not need to fertilize for a couple of years,
and in the past I have been happy with the performance of a manure bed
covered with a thin layer of wood chips (no fertilizing, no weeding).