I find no matter what I amend with, 2-3 cups per cubic foot is about where you want to be. Anything more than 3 cups and I’ve had issues.
3 half filled five gallon buckets is a cubic foot of soil.
And as @ReikoX tells people…Kelp is awesome. An organic gardener should always have a healthy supply of kelp and EWC (I sadly just ran out and had to buy some for my indoor garden 
).