The box I cobbled together from old pallets, dirt is a mix of potting soil and local dirt spiced with chicken poo.
For the more observant viewers, you may notice that the closest 'V' is slimmer than the upper square -- the pallets were a little short. I plan to plant smaller vegetables in those areas.
CCA is banned for lumber designed for ground contact construction. Pallet wood has no restrictions. Pallets use in shipping means you don't even know what country it came from.
Stuff like telephone poles still use it I believe. At least in Canada. Railroad ties too likely.