I was doing a little research at the college library when I ran into this ditty. This is an exerpt from Organic Gardening Magazine. Full citation below:

Miraculous mushroom compost

If you live in or near eastern Pennsylvania, you can tap the area's rich supply of mushroom compost--the medium used to grow mushrooms commercially. Mushroom compost (also called mushroom soil) is a blend of one-third horse manure, one-third spoiled hay and one-third ground corncobs with a 1.4-1-1 NPK value and near-neutral pH, making it an excellent soil amendment!

"We've been adding it to our garden each year for the past 11 years and now our soil is dark, loose and loamy," says Lee Shrawder of Hawk Valley Farm in Kempton, Pa., a noted local supplier of organic mushroom compost. A bulk load of his mushroom soil "created" editor McG's raised beds--and cost McG less than $100 for 16 tons, delivered! (Most places will also deliver in bulk or fill up a pick-up truck with mushroom soil for a mere $5 to $10.)

Organic mushroom compost, by the way, is what you want. "Ours is steam-treated to kill weed seeds and competing fungi and molds; some other mushroom composts are chemically treated," explains Shrawder.

Kennett Square, Pa., is known as "the mushroom capital of the world," but mushroom growers are scattered throughout other regions too. Check with your local cooperative extension office for names of nearby growers.

Mattern, Vicki. "Organic riches you can reap in your region! Recycle these locally abundant organic "wastes" into soil-renewing humus!." Organic Gardening Mar. 1994: p26.