The pallet wood thing is smart but check the stamp on the pallets before you grab them, because some are treated with methyl bromide and that stuff is not something you want near your veggies. Look for the "HT" mark which means heat treated, and stay away from any pallet with no mark at all or one that looks like it hauled chemicals. The other thing nobody mentions is that pallet wood is usually pine, so even heat treated boards will rot faster than cedar in a wet climate unless you keep them off the dirt. Set them on gravel or a couple of concrete blocks so water drains away and you'll get way more seasons out of them. For $15 a bed you can afford to replace a board here and there, so the tradeoff makes sense as long as the wood is safe.

Did particle spread actually matter for how the coffee tastes though?