I added to many to everything year before last. I think everything I planted with them heaved out of the pots or ground. Clearly a few go a long way. I cant see them in an earth box tho, it should be moist in there all the time anyway.
If you follow Earthbox* instructions they say definetly not! The genuine EBs retain water providing you use a quality container mix w/o moisture control granules and keep the cover on it at all times. Never use anything that says "soil " or you will have root rot.
I have another question. Until I can get to the States and buy coir I am using shredded palm trees which are like thin straw. The water doesn't percolate up very well. I've thought of using an old nylon stocking with pea and perlite as a "wick" from the resevoir to up towards the roots.
I'm making my own e-barrels, so I'm going to try the cornstarch type of crystals. They are much, much softer, and didn't heave the small grow bags I had with them in it. Since I'm making my own, I know I will need to experiment with how wet/dry they get - that's why I'm adding some of those crystals (quench). It's easier to add in a drainage wick later if I need it than it is to water more frequently. If you are using more of the straw like material, I'm guessing it won't wick quite as well. In that case, a small amount of stuff that does wick better, or some of the softer crystals, may be a good way to go. How well does that stuff drain or draw water? you may want to put a pile in a bucket, with 2 inches of water, and about 1 ft of the palm straw, and get an idea of how far up that water will travel.
I tested the clear crystals next to the cornstarch crystals. when filled with water, I can easily imagine those clear ones pushing stuff around. The softer ones didn't.
The cotton cord does wick great - I used it to make a vacation watering system, but in an HEB, they may not last long enough. It likes to break down quickly. That was why it was no recommended to use as a drainage wick for overwet HEB's or raised beds.