Hi there -

I would say that only 12" of soil is needed.

If that is the case and your box is 32 square feet, you will need exactly 32 cubic feet of soil. This is just about one cubic yard.

That is a lot to fill - so I would highly recommend checking to see if you have a county compost facility. Getting a big quantity like that is usually much cheaper - or even free. Buying in bags for $1 or more that only do 0.5 cubic feet would cost you over $64.

As an example - I had two cubic yards of compost delivered to my house last week for $60; $20 for the delivery fee and $20 for each cubic yard.

Here is a link that might be useful: BsnTech Gardening Blog

I'm contemplating the best/easiest/cheapest way to get the soil here. I know I need three parts (peat, vermiculite & compost), but I'm not sure I can get small enough loads of the three of them. Plan to call around tomorrow to see if anyone will cut me a deal and drop off enough of the 3 to fill my space.

Or did you mean I should just use compost???

Oddly enough, I'm attending a county composting class tomorrow night. Won't do me any good until at least fall though.

I am assuming your box is 4ft x 8ft x 16 inches deep. To save money, you could fill the bottom 10 inches with 1 cubic yard of bulk compost which is 27cu ft. Then fill the top 6 inches with Mel's Mix. It would take approx 16cf to fill the top 6 inches (18cf including 7% settling) which means you will need approx. 5-6cf of each of the 3 ingredients. As the compost ingredient, I would use 5 different sources of compost than you used in the bottom since you will already have so much of that.

Here is a website where you can calculate quantity to fill your box ... http://www.soilbuildingsystems.com/CubicYard.php

You will get settling so if you go down 4" from top by fall you will be 6" from top. IMHO far better to fill as much as possible to have as much soil volume as possible to hold water.

Dan