Growing organic celery is by far the best practice. Traditional farming methods use a ton of fertiliser and pesticides which remains within and on your plants throughout their lifecycle. Just think about how much unwanted residue you'll find after 4 months of spraying.
Celery needs a lot of sun, approx 5 hours would be great but if you are getting temps in the mid 20's it's a good idea to move your celery container into the shade during the afternoon to protect it from the harsh afternoon sun. Try and water your plants daily as the soil needs to remain moist.
When planting out your celery aim for 25cm apart, this will shade the stalks sufficiently through their last few months of growth. Too much sun on celery stalks can create an unpleasant bitter taste. If shading isn't working out for you then you can 'blanch' the stalks three weeks before harvest by making a cardboard or brown paper collar for your plants. Tie your collar loosely with twin.