I started to post this as a comment on the "pig manure" question, in response to KBaron's suggestion about mushroom manure but decided it was sufficiently off-topic to warrant a new thread. Most of what is sold as mushroom compost is spent mushroom substrate (SMS) and its the stuff left after a few crops of mushrooms have been grown in it. The material starts out as sterile, composted animal manure, bedding (straw) and ground-up plant waste. When it is no longer useful to grow mushrooms, it sometimes gets bagged and sold as "compost". I had been curious about "mushroom compost" since we have a few mushroom operations around here and many gardeners get it from the back door of the plant, for next to nothing. Many of them rave about it. I asked the top guru in our hort society and he pointed me to a University of Guelph study that pretty much nixed the stuff. The Guelph study noted high "salt levels high enough to potentially limit the range of applications" http://www.ridgetownc.on.ca/research/documents/fleming_SMS_Final_Report.pdf It is interesting to note that the study, perhaps because it was sponsored by the Canadian Mushroom Growers Association, minimized this negative finding. Another study (I couldn't find the link, only references) done at Penn State University, shows that "good quality SMS products do not contain salt levels high enough to damage turf. " This study was limited to turf, and it didn't look at whether the SMS was actually beneficial-- only that it was not harmful. Here is the very detailed fact sheet put out by Penn State: http://turfgrassmanagement.psu.edu/spentmushroomsubstrate.cfm I personally wouldn't spend money on it, especially when less potentially damaging compost products are available. If it were free and I didn't need to pay to haul it home, I would probably use it as one of the feedstocks for my compost bins, in relatively small proportions. I'm curious about the experience of gardeners who've used it, though...
I have used SMS for many years. It is especialy good for hilling potatoes. I have also used it as a source of organic matter in new beds. In this area SMS is available free for one week during spring. For the remainder of the year, a pickup load is $5 to $10 depending on truck size. The mushroom operation here does not appear to have trouble disposing of the material. The salt they speak of is not necessarily NaCl. It would be any of the soluble salts. The EC measurement indicates that it is in the high range but the germination tests are surprisingly positive. The germination test performed on the compost yielded a value of 97% for Channel 3 (using cucumber seeds). The lowest value received was 86%. The relatively high germination percentages do not appear to support the EC results. Perhaps the composting process has some ameliorating effect. There is also sphagnum peat moss in this mix. A wet layer of peat moss covers the top. It is in this layer that the mushrooms pin out. Large clumps of peat moss held together with hyphae are commonly found in SMS piles. From this report and my own experience, I will continue to use SMS. The alternative is to send it to a landfill. The Organic movement has its origin with Sir Albert Howard at Indore, India. The title of his co-authored first book was, The Waste Products of Agriculture.
I have found mushroom compost to fairly high in pH as well as salts; it seems to be fine for vegetable beds, but I would not use it around acid loving plants.
