High ganzigunnu,
Generally, the discoloration or yellowing of a cannabis plant is caused by a macronutrient deficiency.
There are three macronutrients, which are sometimes referred to as primary macronutrients, are nitrogen, phosphorus, and potassium.
So the cause of this dificiency could be not enough macronutrients. Or too much macronutrients what cause a lockout with the same result as not enough.
OR AND I DO THINK THAT THIS IS YOUR PROBLEM. YOUR pH VALUE IS NOT IN THE RIGHT RANGE. KEEP YOUR pH VALUE 6.2 !!!
To measure pH in soil, just follow these steps. Your pH should be around 6.2.
1. Place soil sample about ¾ full in sample jar and add distilled water to cover soil.
2. Cap the jar and shake the soil vigorously a few times.
3. Let mixture stand 10 minutes to dissolve the salts in the soil.
4. Place the pH tester into the wet soil slurry.
5. Measure pH and record measurement.
When your pH is right and close to 6.2 you need to check the EC value of the nutrients that you give. In this stage of a grow your EC value should be 1.8 (do not give more then EC 1.8).
Goodluck!