Mushroom Compost in Muck
Spent mushroom compost is available in bulk or you can purchase bags from garden supply stores. University of Florida horticulture research indicates that mushroom compost may not be the best choice for some of your plants because of high pH. Check the packaging to see if any information is available on pH. One indication of an alkaline product is a high pH. 7 is a neutral pH; below 7 is low pH, on the acidic side of the scale; above 7 pH indicates the product is becoming more alkaline.
Granted, the use of muck in bonsai creations is a minor part of the mix. However, muck is often used to anchor small trees that do not lend themselves to using wire to hold them in place. The muck holds the tree in position, bonsai soil is added to the pot and in time the roots move out of the muck ball into the soil. If the plant material is an acid lover (lower pH) you may see adverse results using muck containing mushroom compost.
"Typically spent mushroom compost contains a high degree of lime and as a result is usually on the alkaline side. It has its place as a soil amendment but probably should not be used as a soil amendment where acid loving plants are to be planted. This would include common plants like centipedegrass, blueberries, azaleas, camellias and gardenias. It would be best to know the soil pH before using mushroom compost so that you don’t end up with a pH that’s too high (too alkaline) for the plants you intend to grow." (Read the article by Larry Williams, Okaloosa County Extension Director/Horticulture Agent at <http://okaloosa.ifas.ufl.edu/pdfs/horticulture/larry_column/2005/Comments%20on%20soil%20amendments.pdf>
To be successful with bonsai, you have to use all the knowledge you can find about this living material we work with. Azaleas, camellias and gardenias are three popular plants in the Southern garden. They also make beautiful bonsai and they all are acid lovers. Keep the pH in the low numbers for these plants. If you have a choice, composted cow manure might be best to include in your muck.