this is Chlorosis, which is a lack of the green pigment chlorophyll, generally happens due to iron deficiencies. The veins of the leaves will keep their green, but the thinnest part-turn yellow. ... For soil growers, an iron deficiency is hard to fix at the source; your soil is already either overloaded with nutrients or has a high pH. Hopefully, I answered your question and remember " Happy Growing and Free your mind one puff at a time ".