This is the biproduct of my hardwood saw dust substrate blocks 
Its a nutrient/fungal/bacterial rich substrate that is a great amendment to the soil. 
This will also likely produce Oyster mushrooms for you as well. 
I dump my used substrate off at other properties and they pump out beautiful mushrooms year round still, especially if you add it to some woodchips or straw like material 

 
$10 per bag or 5 for $40or come fill up your own bin for negotiated price
Bag size in picture for reference about 2'long 
 
Read more at Gardening Know How: Mushroom Compost Benefits: Organic Gardening With Mushroom Compost
 
 https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/mushroom-compost.htm