Gary Brown wrote:

I am terracing a slope on our yard. I am unsure whether to put
in topsoil or just mix manure or compost into the existing soil.
Wouldn't the latter allow a higher concentration of plant
matter?

It's not a question of "either or", compost and manure will continue
to compost until all that's left in less than two years is dust... on
a slope a few good rains and it will be gone... if soil is needed use
top soil and with amendments if needed depending on the quality of the
topsoil.