I've used the 3B mix in containers for years but I have run into some problems. Since they contain mostly peat they can hold a lot of water. Sometimes this can be good when plants are using a lot of water and it is hot. But it can be bad if you have a lot of rain and the mix stays too wet. (And of course you need to add approprate fertilizer)
Now I'm using more of the the Fafard Soil Condition (bark fines) which drains much faster. It might work well with the earthbox since it has the water system.
Al, on the container gardening forum, has a 5:1:1 formula which might work well.

Al, on the container gardening forum, has a 5:1:1 formula which might work well.

Laura, that mix is primarily for top watering containers, not self-watering containers. They advocate 5:2:1 for the top- covered bottom-watering containers, need a little more peat to wick the water up against gravity....

Tom

Thanks Tom...makes sense.