Boxes of Potatoes anyone?

Not an offer for free potatoes I'm afraid! For those who don't know the square foot gardening methodology, I'll give you a brief background. The basic tenets are that you should be able to reach the middle of the box from all sides (so basically a box should be no more than 4 foot wide, or only 2 foot if only accessible from one side), and that the box only needs 6 inches of soil.

The theory being that if you have 6 inches of top quality soil (and the soil in my box is 1 third vermiculite, 1 third compost, and 1 third peat (or alternative)), then you will not need any more than this.

However, some plants just really need some extra soil, and potatoes are one of those plants. So to compensate for this, I have built some square foot boxes out of plywood, to act as high-rise homes for them. They are only 4 inches high, but this gives me a net height of 10 inches in which to grow my potatoes. Hopefully this will be enough?

Here's a quick snap of the boxes freshly made in my garage.

And here they are in position. I've covered one plant with each, and then filled them with a mix of vermiculite and home made compost. My plan being that when the potatoes are harvested, the extra compost will help raise the level of the soil in my box, as it's a bit low.