I use Black Gold castings! Its awesome, I use the 20lbs bags with white handle built into the top and bottom I use 1.5 bags ( 30lbs ) for a half batch of SS.
SEE
Here is the Black Gold Castings
Epsom Salt, Azomite, Humic Acid & Garden Lime ( Prilled ) so i used the coffee grinder to powderize it
Blood Meal, and Happy Frog Bat Guano 0-5-0
Fish Bone Meal and Rock Phosphate
3 bags Roots Organic, 1 Bio-Bizz Light Mix ( for half batch )
Then I mix very well with a garden rake and a shovel on top a 10x10 tarp, and store then mix in two of these 55gal RubberMaids, water it with about 1.5gal of RO or Distilled water re-mix, right the mix date on the top with a sharpie and use in 30-40 days.
HOPE THIS HELPS BRO! HAPPY GROWN!