Leaves Curling Up (Magnesium Deficiency)
This is an issue that many marijuana plants can face and it can definitely harm the dope plants severely if not taken care of. Usually the main thing which can cause the leaves to curl up would be a deficiency of magnesium. This can usually make the marijuana leaves turn to a shade of yellow or brown and the aged leaves may see a case of interveinal chlorosis.
The leaves will likely start to curl around the margin (or tip) of the leaf and curl inwards in the middle of the veins. There is a good chance that there is still a minor shade of green on the leaves when they turn yellow or brown. The tip of the leaves might twist as well. The dope plants are just begging to get some magnesium so they can be healthy again.
To fix the problem all you have to do is give the affected weed plants a teaspoon of Epsom salt for every gallon of water supplied. Before you fix the lockout of nutrients you can use the foliage/foliar feeding method if you want to make sure the leaves get fed effectively. This will allow the marijuana plants to receive enough nitrogen and magnesium to be healthy. If you choose the foliage/foliar feeding method then you can supply a half a teaspoon for every quart of water.
There is a chance that there is a lock up of magnesium caused by the dope plants receiving a large amount of calcium, ammonium nitrogen, and a few other nutrients. The marijuana plants need to get a fair share of nutrients to stay healthy so giving too much or too little magnesium can lead to a health issue with your plants.