If your picture is where they are actually growing, and I don't see a normal flash shadow, it looks partially shaded. I would suggest it is water to sun imbalanced. Congratulations on having a curious eye that actually leads to doing an experiment. The pot looks very clean, perhaps you are doing this experiment inside, then you might be using tap water that has chlorine.

This post was edited by hishouse on Sat, Aug 23, 14 at 12:13

The pot is in full sun most of the day. Today is a cloudy day.

The potting soil sucks. I used it earlier this year when I winter sowed a bunch of flowers and veggies and got the exact same result as you. Slow growth and yellow-y plants. That was the first and last time I'll ever use it.

Rodney

The material looks like it has a lot of woody material in it. This will have to be broken down before the nutrients will be available for plants.

If you want a seed starting mix, I recommend Pro-Mix BX.

Here is a link that might be useful: Pro-mix BX

That looks like wood chips to me. Below is a link to some HD customer comments about this product.

Here is a link that might be useful: HD customer comments