We are planting our first raised bed this year for organic vegetables/herbs. I purchased a cedar kit from Natural Yards that is 4x4 and 16.5" high. I got the taller height because don't want to amend the soil underneath due to likely contaminated soil from old railroad ties. I've read a few articles about using newspaper or cardboard on the very bottom, but the lady at Natural Yards indicated that I should use a bottom layer of gravel in my bed, then add my soil. Does anyone have any advice here about what to use as a barrier that will still allow proper drainage and what type of gravel I should use? Thanks in advance!
You should take a look at "Lasagna Gardening" by Patricia Lanza. In her book she describes how to garden with no digging, no tilling and no weeding. She lays down layers of newsprint, etc... right on top of the soil as a barrier.
We tried this with our potato beds and in our concrete wall planters that surround our pool and it works beautifully!
Is the barrier for soil contamination, weeds or gophers/moles? I would get a soils test done by an ag lab to make sure it's contaminated. They are usually reasonably priced and can save you the headache of not knowing what exactly is in your soil. The gravel won't really stop anything because what eventually happens is that the soil will work it's way into the voids in the gravel, making it useless. Newspaper and cardboard will breakdown over time.
What we always did under our raised beds was put down hardware cloth to keep gophers and moles out, and then weed fabric (we had bermuda grass really bad), but for you if weeds aren't a huge issue, I'd go with nonwoven filter fabric (not weedcloth) instead. That will keep the soils from coming into contact with each other, but also allow for drainage.