The biggest
boost I have found for berries is to innoculate their soil with a bit of spruce forest litter. This is where berries thrive in my
local wild areas, and so I figure there is some symbiotic relationship between the conifer duff and the berries.
I would also add that plastic containers arent really the ideal setting for berry roots, who want lots of air in the soil, and mositure but not wetness. With plastic containers, it is easy to have wet or dry, IMHE, but evenly moist seems more challenging. This is another reason why the conifer
compost mulch is helpful I would guess, keeping the soil moist but not wet.