An interesting article about mushroom compost in the Daily Herald expounded upon the benefits of mushroom compost, but is failed to tell the whole story.
What is mushroom compost?
No, it's not compost made from mushrooms. It is the specially-prepared medium in which mushrooms - typically those white button mushrooms you see in the supermarket - are grown.
It starts with a mixture of straw, manure, gypsum, peat moss, and almost any organic material. The bacterial activity causes the material to heat up - just as with normal compost - to about 160 degrees, killing any weed seeds or pathogens that might have been present.
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 pasteurize at about 140F. This pasteurization is vital to mushroom growing process because it kills any surface disease-causing organisms and pests. The downside however is that it also kills off all those wonderful micro-organisms that are so important to your garden.
Mushroom compost is normally used to grow three crops of mushrooms and is then considered 'used up' by the mushroom-grower.
It is typically steam-pasteurized once again and then sold off to nurseries, landscape supply firms and general contractors
Mushroom Compost Is Not Like Regular Compost
Adding microbiology to your soil is one of the primary advantages to compost, and that microbiology is vital to the disease prevention and and supplying nutrients to your plants. Spent mushroom compost simply does not provide these benefits.
Even though the mushroom compost does not come with the biology present, it is still provides a good substrate and food source for that biology. One tactic is to let it sit and 'age' for a season before using it. Other ways are to mix in a batch of compost tea made from regular compost, or simply mixing the mushroom compost with regular soil.
Here are some additional links about mushroom compost:
http://extension.oregonstate.edu/news/story.php?S_No=28&storyType=garde
http://squtch.quiet-like-a-panther.org/story/mushroom_compost-34
http://www.taunton.com/finegardening/how-to/qa/mushroom-compost.aspx
