Before we get into this I want you to check a couple things, check the following things:
- Plants have plenty of water
- Soil is seven PH or even a little less
- Your light cycles are correct
- No pools of water
- Constant fresh air
Now thats out of the way let Mr. Ganja God diagnose your plant
Bigger leaves are turning a yellowish color while the smaller leaves are green.- Nitrogen deficiency - add nitrate of soda or organic fertilizer.
Older leaves will curl at edges, turn dark, possibly with a purple cast.- Phosphorous deficiency - add commercial phosphate.
Mature leaves develop a yellowish cast to least veinal areas. - Magnesium deficiency - add commercial fertilizer with a magnesium content.
Mature leaves turn yellow and then become spotted with edge areas
turning dark gray. - Potassium deficiency - add muriate of potash.
Cracked stems, no healthy support tissue. - Boron deficiency -add any plant food containing boron.
Small wrinkled leaves with yellowish vein systems.- Zinc deficiency - add commercial plant food containing zinc.
Young leaves become deformed, possibly yellowing.- Molybedum deficiency - use any plant food with a bit of molybdenum in it.