Perlite would cost you about 100 bucks I bet for a hole that size. If you can mix in some sterile cheap (with no nutrients added) with Perlite you would have a great crop. Don't worry too much about the roots the other plants in the area will seek out your nutrients and water on their own. Once the root ball is established the plant will be fine.

It only takes about 12" to establish a good root ball. 1/3 x 1/3 x 1/3 would be an ok mix to save a buck or 10 but; "If you go cheep you grow cheep"

What you are building is a big outdoor grow pot. Treat it the same way you would treat your plants if they were in grow pots and they will grow fast for you.

Cheers.