I'm willing to bet Stinky hit the nail on the head. Every nursery I went to sold me ferted soil even though I specifically told them I needed a nutrient free soil.

I would say get a peat puck and try sprouting a seed in there and grow it out. See if you still have the same problem.

You should be able to grow a seedling in a peat puck to a few nodes (3-4); at which point it should be able to handled a *light* pre-ferted soil in the instance you cannot find a nice hummus based soil mix.. or if not hummus then peat soil mix for your baby seedlings.
daihashi Reviewed by daihashi on . Help! - Yellowing leaves! Leaves started turning yellow starting at the edges about 24 hours ago. They have started showing slight yellowing on the edges and has quickly spread to most of the leaf. Conditions: Vegging under 4-42w CFL & 1-125w CFL. All are 6500k and about 2-4 inches from plant tops. Temps from 78-85 degrees, usually around 81. No ferts yet. Water is PH balanced to about 6.8 and plants are watered about every day. Currently in 4 inch pots for vegging. Soil was checked last week at around 6.7. Rating: 5