Nitrogen Defficiency? or So I Thought

So i have some youngins that iv had outdoors since just after they popped up.
Theres 3 of them and 2 are looking really healthy and growing steadily, but 1 is growing much faster than the other 2 but its leaves are looking yellow in between the veins.
so i was thinking it was a nitrogen defficiency, so i gave it a feed waited a few days, nothing happened, so i kept it on its regualar feeding schedule and then about a week later gave it some extra N (mind you im already on a pretty high N fert so i didnt want to give it to much)
Anyway now its been about another week and i still havnt seen any improvment! mind you the plant is still the biggest out of the lot!
Heres a couple of pictures.



Now im a bit of a n00b so i wouldnt be surprised if i have mis diagnosed and it wasnt a nitrogen defficiency at all, so if someone could point me in the right direction thatd be awesome

if im not mistaken N deficiecy usually just shows on the lower leaves until it gets bad. Your new growths look to me like they are sharing the deficiency... is it alot worse down low?
the biggest plants always show the deficiencies because they are growing the fastest/strongest they require more food

yeh its mainly on the lower leaves and its worse down there. So im probably right about the N deficiency but why didnt it start to show any improvements ??

I'm pretty sure your diagnosis is correct, but I could be wrong considering feeding didn't help after 2 weeks

N def shows on lower leaves yellowing thats not a N def, could be a p or k il have to do some
Reading up to be shure

It looks like iron to me