If your plant needs water, simply water the soil evenly until you've reached 10-15% runoff, this will ensure the soil is nicely saturated without overwatering the plant.
Water soil, but don't water until runoff. Runoff in organic soil will flush the nutrients out of the soil.
Keep in mind that if your soil is extremely dry, it won't absorb water right away. Giving small amounts of water at a time as opposed to drowning your plant will ensure the water is completely absorbed and that it didn't just pour down the sides of the pot.
Cannabis plants appreciate when the soil dries between waterings but not to the point where the roots become dehydrated.
If the roots get too dry, they will reach a point where they are nonviable and the root system will be compromised. Leading to symptoms appearing as deficiencies or permanent damage to roots.
If underwatering signs persist, try transplanting into a bigger pot, or lowering the temperature in your grow area.
* Severely affected leaves usually do not recover from damage even after correcting the problem.*