Hi,
Thanks for your question.

Since you're planning to grow in a box versus in the ground, you'll need to pay more attention to moisture retention and drainage. You'll need drainage so roots don't become waterlogged, but the container's limited depth and exposure on four sides mean the soil will dry out much more quickly than soil in the ground with the same amount of watering. 

To facilitate drainage and avoid the soil becoming hard and compacted, use a mix of compost and soilless growing mix (potting soil) in a 1:1 ratio. If your box is at least 16" deep, you can add 20% by volume topsoil (topsoil is a mineral soil). Some county compost sites offer free finished compost to residents - I'm not sure if that's the case in Crow Wing County. 

For garden boxes or containers, the recommended depth of soil for plants with shallower roots (greens, beans, cucumbers) is 8"; for other plants (peppers, tomatoes, squash) the recommendation is 12-24" deep. (see scenario 4)

I hope that's helpful information for you.