I have two avocados in two of the same type pots, with the same soil, and the same watering schedule. I let them dry (about a week or so) and then deep water them till the water comes out of the bottom of the pots. The thing is one of them has it's leaves curling while the other one has healthy leaves.
The only difference between the two is that the one that's not healthy was grown from a pit that was not organic. I got the pit at Whole Foods, so I don't think they could do too much harm to them. The healthy one is organic and was also bought at Whole Foods.