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.