Why is my celery leaves turning yellow?

Answer 1

Here are some reasons your celery's leaves might be turning yellow:
https://www.gardeningknowhow.com/edible/vegetables/celery/yellowing-celery-leaves.htm

Low nitrogen can be one cause. Compost is a great source of nitrogen for celery. An alternative is to add 1 tablespoon (15 ml) of a 10-10-10 fertilizer next to the plant every 3 weeks, then water it in. Make sure the fertilizer doesn't directly touch the stem.