Today I got to speak with Mike McGrath. He took my call and explained all about mushroom soil. Apparently, in addition to making sure the compost is aged for about a year, you should find out the pH of the compost and also learn if it was sterilized at the farm by formaldehyde or by steam. Also, if you want it to be best for organic gardening, try to buy it from a source that does organic mushroom farming. So, again, there are no simple answers and all mushroom soil is not the same. But the impression I got is that it is always going to be a good soil additive.