I think it's a magnesium deficiency. http://www.overgrow.com/growfaq/1382.
It looks like this.
Fhydro
fhydro Reviewed by fhydro on . Leaves going yellow and brown? Help? (photos) Hi everyone, I'm growing my plants outdoor, and they've been growing steadily for the past two months now. Lots of water and a bit of nutrition every week. But since a couple of days I started noticing discolorations in the leaves. Not just on the bottom nodes, but in the middle and the top as well. When I looked closer I noticed most of the leaves showing a very slight discoloration. I attached a couple of pictures to this post, but since it's my first one on this board, I hope it works ;) Rating: 5