Celery yellowing

Celery is 6 weeks from sprouting. Has been thinned, transplanted, and fed a light (half recommended) mixture of 6-4-4 fert. Under a 1000w HPS on 18/6. I lowered them to approx. 5' from 28" thinking they were getting burned

They were about 6-8" tall when they started yellowing and getting crispy. That's when I added the nute. Recently the new growth from the center of the stalk has also started yellowing on the leaves (stems remain green). I'm thinking still N deficient, I've also heard maybe over/under watering... Although, I don't want to over-fert and the soil seems to be still moist even after 5 days. Any other suggestions?

1 - Do a complete soil flush or water change with PHed water to see if its a PH problem
2 - It could be from the HPS that your using. Some plants might not like being under a HPS.
3 - If you don't have one run your light in a cool tube.
4 - Add a fan or 2 in there if you don't have one in there either.
5 - Check the temp in the room its growing in. It might be to hot for what the plant likes.

In the end I always try to deal with the soil first since that can be the main problem.