Mushroom soil is the most wonderfully rich additive to any garden for moisture retention, softening of hard clay, improving drainage, and just plain addition of beneficial nutrients. I use it all over in all my flower and garden beds, and I even spread it around my yard to feed my grass. It is soft, spongy, lightweight, easy to shovel and work with, and a deep black color that screams richness, and the best part: it's cheap.
In my town, mushroom soil is sold everywhere for $10 a yard. I knew it was the spent growing medium from the mushroom industry, and I suspected from the smell that it was made from manure. I didn't know if it was treated with any kind of chemicals or if it was safe for organic gardening, so I did a little research.
Yesterday I came across an article about mushroom compost and I decided to email the author, a Penn State professor. Very graciously, he responded quickly with an answer. No, he said, mushroom soil is not treated with chemical fertilizers or pesticides, and it is commonly used in organic gardening. This was excellent news. Here is a link to the article if you would like to read more: http://www.mushroomcompost.org/faqs.htm
For anyone who is looking to enrich their soil, this is a must-try.