Seedlings turning purple

[ddrsheden]

This is the 4th year I have started my own tomato plants. But having problems for some reason. The underside of the leaves are turning purple, lower leaves turning yellow with purple veins. The are planted 100% Schultz Premium Potting Soil Plus. The soil was bought over a year ago. I have been feeding plants water from melted snow that I collected this past winter. I have the same problem on my super duper hot peppers. I posted a thread on a hot pepper website and someone posted this link to help diagnose and it looks like nitrogen deficiency. There are pictures of other nutrient deficiencies on it so I thought i would post the link here. Have to scan down to the last half of the page to see the pics.

http://4e.plantphys.net/article.php?ch=t&id=289

Or if you have any other ideas...let me know