You should regularly check the pH value of the environment your cannabis plant is growing in. At grow shops and regular garden centres you can easily find a testing kit.
The pH indicates the acidity of a solution. pH stands for potential hydrogenium and indicates the hydrogen ion concentration of the water. A neutral water solution has a pH of around 7. A pH below 7 means the solution is increasingly acidic (down to 0) while a pH above 7 (up to 14) means basic or alkaline.
You water must have a pH between 5.8 and 6.0. Between these values nutrient uptake is most efficient.
Hard water (with a pH higher than 7.4): you’re best off adding a pH-minus powder or liquid. This acidic granule neutralises the alkaline components that are the cause of the water’s hardness. Regularly check the pH!
Soft water (pH under 7): adding pH-plus powder or liquid, baling soda raises the pH above 7.