Ph levels are different for hydro and soil, i usualy have mine around (6.3-6.5) for hydro and a higher alkaline for soil (6.7- 7.1) Usually if you are getting yellowing of the leaves and are using a high nitrogen mix duing veg growth , you will have nutrient lock out from improper PH levels casueing the plant to lock out certain nutriants, only way to fix this is to make a heap of flush water with correct ph levels and no nutrients and flush ur pots