Shouldn't this thread be in the plant problems area?

Are the plants in flower stage? It is perfectly normal if so. Fan leaves yellow, brown, and even die in the flower stage. Nothing you can do about that.

Otherwise, be sure to check the plant problem chart here:
114 Photos of Mineral Deficiencies in Plants (~ 1 meg) for Visual Diagnosis

and here:
Grow Marijuana FAQ, Cannabis cultivation - marijuana growing tips & photos A must for all weed farmers! Copy this to your computer!

Hope that helps a little.