N Toxicity?

Too much water ?

Probably stressed by growing at 23/1, (if you are doing the same as in your other diary). Cannabis does best at 18/6, - which is THE optimum light/dark cycle. Are you sure you are not over watering? Are you letting things dry out between waterings or are you watering daily, as you have indicated in your other diary? Are they sitting in saucers of water for any length of time? They look drowned to me.

If you're not giving them any nutes, and this is indeed a nitrogen toxicity (I question that) then yes, whatever is in the soil is the culprit. I purposefully do not grow in any soil that has any nutrients in it because I want to be in charge of what nutes they get and when they get it. Now, just to explain why I question your conclusion that it's nitrogen toxicity - your soil look awfully dry... this COULD be just a case of not giving it enough water and they're needing more. You should be getting some runoff after each watering... and I like to catch that runoff in a saucer under the pot and let it sit for at least 15 minutes before dumping it out because often, if the soil is really dry, the water will just run right through and the soil doesn't really get time to absorb it... so it's a little like bottom watering ... If, after 15 minutes, there's still water in the saucer, dump it out because you don't want sopping wet roots! Good luck!