Ya that is fine...if you want it to grow easy don't forget to add perlite to keep the soil loose so the roots can grow with as little stress as possible.

Balance you Ph while you are at it too.

What you are planning on is similar to growing in pots so treat it the same and you will be fine. I assume you are growing organic.

It will cost some bank to fill a hole that large with good soil mix. You don't think it's better to just dig one hole for each plant and not just one big plot?