What is your water quality. I had similar issues using well water from the tap when I first started growing.
Switched to distilled, problem solved.
Could also be the size of your pot restricting the plants root growth, stunting overall growth.
Low nitrogen as well, although, I dont think low N would account for the size of your plant. That plant is the size of a 2 week old.. so something stunted its growth well prior to the time it would start using N beyond what was already available in the soil. Not to say it isnt Nitrogen deficient, but I'd think there is another underlying cause.
Try repotting into a bigger pot w/ a good growing soil. Itll have nutrients available in it already, so you wont need to add anything to it right away aside from plain (clean) water for a week or 2. This will cover all the "obvious" stuff, and I'm sure will resolve your issues, if the plant isnt too far gone