Mushroom compost can be great stuff. I had a friend use what you're describing to start a garden and it did very very well.

Generally compost will be about neutral, and there is no reason to suspect mushroom compost will not be. Without a test it's impossible to say, but I wouldn't worry about it.

Thank you, toxcrusadr, for the info. I tested my neighbor's soil mix - it's less alkaline than my pH 7.7 native soil. I once ordered a pile of dirt only - it was sticky clay, and gunked up like rocks when dried out. It's nice to have mushroom compost mixed in so the soil is fluffy, rather than gunking up.