How Spent Mushroom Compost is Made
Mushroom compost is normally made in a hot composting process with straw, animal manure and gypsum. Sometimes leaves or ground corn cobs are used in place of the straw. As you probably noticed, this is a much simpler recipe than you will find in most home compost piles.
There are usually other nutrients added either while composting or after composting. Many of them are organic in nature, such as blood meal, or cottonseed meal, but sometimes there are inorganic additives such as urea.
The pile is allowed to heat up to about 160 degrees and turned several times, just as you would do with your home compost pile, except on a much larger scale. The mushroom farm down the street from me uses those giant straw bales that are shipped one per semi trailer.
Now, unlike your home garden pile, they don't let the compost sit and age at this point. They take the compost, load it into planting beds and then they steam pasturize everything!
That's right, they kill off all those wonderful micro-organisms that will continue the composting process and are so important to your garden!
Of course, they aren't thinking about your garden, they are thinking about getting rid of the competition for the mushrooms, and there are a lot of strains of wild fungi in that compost.
Then they top the bed off with a couple of inches of peat moss, and innoculate the bed with the mushroom spawn. They get their first harvest in about a month, and continue harvesting for the next 6 to 10 weeks.
After they are done, they steam sterilize the bedding again, and sell it as spent mushroom compost.*
The upshot being I personally don't think it's worth it to use Mushroom compost. All the good bacteria is steralized out of it and most of the nutrients used up in growing mushrooms. It's finer and can cause crusting problems as well. If you have the time you could let the pile sit after adding a "starter" to it like compost tea or other product marketed for this sort of thing. However when I pick up a load of compost I usually have a plan for it right away - so that's not really an option for me. The other thing that I keep reading about that bothers me are the actual raw materials for the compost to begin with.
This compost is made by mushroom producers from material such as hay, straw, corn cobs, poultry and horse manure – or any combination of organic material that is 1) inexpensive and 2) readily available.**
Hay, straw, and corn cobs do not have an abundance or variety of minerals and nutrients to begin with. Horse manure is a little better when the horses themselves have been fed a variety of foods other than grasses (apples, oats, etc) and chicken poop maybe the best with a varied diet of protein (bugs) and seeds which offer the densest concentration of nutrients, plus the occasional berry and etc. (Assuming the manure source comes from "organic" free range chickens.) However since we don't know exactly what the Mushroom compost's original recipe is, or where their materials are sourced from, it's hard to tell how well it would perform even if it wasn't sterilized twice and the bulk of the nutrients used on growing mushrooms. Whereas at least with "regular" compost there's no steam sterilization, and a wide variety of original ingredients will bless the pile adding a diversity of minerals and nutrients and microorganisms. I'm all for that. Plus it's cheaper.