This is for tomatoes, but I imagine it extrapolates.. Nitrogen cause that yellowing, but there are other things that can be lacking. Magnesium.. I think.. is one that the veins stay green but the rest of the leaf pales.. Potassium and Zinc are two others. Either way, it isn't usually too hard to test the soil and see if that is an issue just to rule it out.
http://www.haifa-group.com/kno...
