Here's what Tom hill uses...I'm sure you could scale it down though.
"...I've played with a lot of different soil recipes over the years, 1rst year new soil mixes, as well as yearly additives. Here's a very simple mix that is well proven and I am comfortable recommending for those large outdoor containers. It gives about 50 cubic feet or just over 300 gallons (dry U.S), and fills a 6ft diameter container to a depth of about 18inches - perfectly.
25 bags black gold potting soil (1.5cf ea)
4 bags stutzman farms chicken manure (1 cf ea)
1 bag perlite (4 cf ea)
1 bag (50 lbs) bonemeal (steamed, not precipitated)
1/2 bag gypsum (aprox 1/2 cf) - Edit -> 1/2 of a 40 lb bag (20lbs).
Mix well, water thoroughly, let rest for 2-3 weeks minimum, transplant, and stand back
Best Regards,
Tom...."
