Is this a nitrogen deficiency?

I'm 3 weeks into flowering (long flowering sativa) and my lower fan leaves are turning yellow. Is this a nitrogen deficiency? Should I add fish fertilizer or something to up the nitrogen or let it go?

Dh,
I've seen organic growers use worm castings as top dressing quite sucessfully. Fish emulsion should work too. They defenitely need something...if you hope to run them 12 weeks or so.
JD