I'd had over watering issues lately which looks similar to nitrogen toxicity which makes the leaves curl downwards. I recognize your issue (nitrogen deficiency) from my last grow before I added nutrients the leafs were always going yellow. In flower its normal or the leafs to go yellow and fall off, just keep that in mind. The only nutrient I use is a solution that contains seaweed.

Check out this website, great guide to identifying the problem: http://www.growweedeasy.com/marijuana-symptoms

Also make sure you remove the leaves from the top of the soil. Could attract insects and other pests. Maybe its time for a big bucket and new soil the roots grow very quickly.