A local company will sell me a truck load of 70% topsoil and 30% mushroom compost for $25. Can/should I buy a load and mix it with my native clay soil to use in my beds? If so, in what ratio? Would I need to add something else for drainage concerns? [I won't have enough soil from just the retaining wall excavation to fill the beds, hence the need for an additional source of dirt]

Also of note that although I'm in 8b (it's currently in the 30-40s), our summers are brutal with 3+ months in the 90-100s with very high humidity. I was thinking the clay would help with water retention during the hot months.

Thanks for any help!

i would suggest use all the soil you create on site, add heaps of gypsum helps for most clays as does also adding heaps of or organic matter this could simply be covering it all with a good depth of mulch hay or the like.

if you have room bring some of that other material in or simply buy mushroom compost from the farm and cover with that before mulching. i have created herb beds using on site clay.

len

I have amended clay soil in raised beds and hot summers...it's a trade-off, they dry fast in the summer, but the unamended clay around the rest of the yard is so soggy in early spring that things can drown and rot.