Yellow dying leafs.

Sound slike they are Hungry, you could have started a 1/4 strength after the first two sets of leaves, only if yu have a ppm meter to check for sure though, Also make sure you PH is not out of wack, if you don't have a meter use measuring spoons to cut your food by fractions accurately. I would bet they are hungry as hell, and don't worry about the leaves on the bottom you won't need them, the plant is getting rid of them right now because it hasn't got enough food to support the extra growth.

Peace

About how long does it take to see recovery from N difeancy?

Depends on if it's hydro or soil. With hydro you'll see the difference the next day; soil takes about a week to notice a difference. That's why I like hydro.

Thanks potpimp.
I got a soil tester and my ph is still good at 6.5. As for nutes it was really low, almost none so I gave them a good feeding yesterday.
Pretty sure I figured out the problem.

Thanks to everyone that helped out!!!