I have to agree that its probably not pH, but I also think that if it was heat, the plant would turn its edges up. I burned a few big fan leaves on my last grow in the first few days and the edges curled up. I have had at least one plant but usually more like 4 or 5 curl up in every grow so far even when heat was not an issue, so I am 90% sure it was because they were rootbound. My other 10% is reserved for root rot in my case, but since you're in soil I'd say they are probably rootbound. Get those girls into some bigger pots and watch them explode! They look great!
The above post is fictional and should in no way be taken as truth.
If I were to grow cannabis, I would only do so to supply myself and my close friends. Doing this would take myself and my friends out of harms way by knowing the source of our cannabis, as well as take hundreds of thousands of dollars off the black market annually. It would also lower the demand of black market cannabis, and would ultimately help keep cannabis out of the hands of minors.