Sulphur deficiency is my guess. The yellowing starts at the base of the leaves rather than the tips.
I'm gonna say K or Mg. But this is not multiple guess, so I guess I won't get marks for that! It's an early stage anyway. I wish I was better at spotting these before they become a problem. I gotta say, though, your pictures are really great and if they are accurately identified could be more helpful than those godawful blurry snaps in the growfaq!