Looks like Nitrogen deficiency. Could be a pH lockout issue. Here is more on the deficiency: http://www.growweedeasy.com/nitrogen...iency-cannabis
Here is the soil nutrient pH level chart:
You basically need to have your pH between 5.5 and 7.5+ to avoid a lockout with Nitrogen. If your pH is 5.4 for example, the Nitrogen won't be absorbed.