I've watched The Martian and I've read the book, yet I still don't understand one thing.
If he ran out of horizontal space for crops, why didn't Mark Watney start digging downward?
In the book he used part of the Hab's floor canvas to make a "mars tent" for sleeping and working (which was dropped completely from original release and present as pretty big 100% Earth-made tent in directors cut), so he could make a "hatch" without losing the Hab completely. Digging downward through soil, rocks, etc would be a problem at first, but Mark is creative, he could make some kind of pickaxe. In a few month he would have an underground area for planting more potatoes (plus in the book he had peas and corn, yet he chose potatoes only... for some reason), which would give him way better chances of survival till Ares 4 (and a stronger hit, when the Hab actually blew up).
So, why didn't he dig?
P.S.: I'm not asking this on scientific communities of Stack Exchange, because, frankly, considering Martian radiation, very weak sunlight and pretty bad (although not completely unusable) soil, the book/movie is not really a hard science... Not hard enough, at least.