Is 50% mushroom compost + 50% top soil a good mixture? I am starting a community vegetable garden at my apartment, and this is the soil my landlord provided for it.
It's a bit heavier than it needs to be on the compost, but it probably won't hurt the plants. If there are raised beds involved, keep in mind that as that large fraction of compost continues to decompose, the level will sink. You may have to top them off next year.
I'm sure a lot of renters would be green with envy that you have a landlord generous enough to bring in all that good stuff!
And while my landlord does love nature, he's mostly allowing the garden to ramp up the property value.
Beggars can't be choosers, I guess...they can just be good people!
I would think it should be great, depending on the composition of the topsoil. With that much SMS in it, my biggest concern would be drainage. It should hold moisture really well, the issue is does it hold it too well and get mucky.
One of the experts here has list of about six things to test for good garden soil. Hopefully he'll come along and post his list again, and you can try them out and judge for yourself.
TXEB, what are you referring to with SMS?
SMS = Spent Mushroom Substrate (aka mushroom compost)
While both mushroom compost and top soil will provide structure and may look like good soil there is no way you will really know what is in the top soil or which nutrients are lacking that your plants require. Also mushroom manure can have high amounts of salts which may affect some plants.
A soil test might be worthwhile with this unknown 'soil'.
The people at Pennsylvania State University have not found soluble "salts" in enough quantity to be a problem for the plants growing in spent mushroom compost.
Most knowledgeable people suggest working 3 to 6 inches of compost, or organic matter, into soil depending on the amount of organic matter in the soil initially. Going beyond about 6 to 8 percent OM in soil does not have much benefit and can create, drainage, problems because the excess organic matter can hold too much moisture.
Here is a link that might be useful: About spent mushroom compost
Everything you ever wanted to know about mushroom substrate (compost).
Here is a link that might be useful: Mushroom Substrate