my girls are in 24 ltr tubs, and have just had a watering with pk13/14, they will now get nothing but water till the end, in about three weeks time

the only problem is that they will only get watered once or twice more, and i don't want anything else in the water, and i think they could do with some epsom salts, would it be ok to spray them? the buds are well formed and i don't want to salt them as there will be no way of washing them off.

what do you guys think?