I see the "Edit" function is no longer working for my original post so I'm writing a comment.

What happened is that I typically use a certain liquid product made by a popular organic fertilizer company (that I have trusted for years) for houseplants and outdoor plants to encourage blooming. It's reflected in the name of the product.

When I started these flower seedlings under grow lights I sent an email to that company asking which of their fertilizers they would suggest for young seedlings. Their reply suggested the same fertilizer I'd been using for plants for blooming. It surprised me, and I wrote back to confirm that, never got a response, and I realize now that I got mentally lazy, didn't think it through, and figured "OK, I'll just use what I have" and did use the same thing for my young seedlings.

After the true leaves appeared and I started using the fertilizer, for the next month or two I noticed slow or no growth with some leaves not looking very green and almost a chlorosis-looking veining to some leaves. I couldn't figure out what had happened. Then related to something else, someone pointed out that this particular fertilizer has no nitrogen at all. It hit me: could this have been the problem? Maybe it's completely obvious to everyone but me that it is, but my knowledge of the chemistry of seedlings is not good so I'm hoping someone here can help me think this through.

Young seedlings don't need any fertilizer. In fact you do more harm than good by fertilizing. It is not until they have a good root structure that they can take in fertilizer and fertilizer may burn the roots if given too soon. I only fertilize when the plant is getting bigger and then I use 1/4 strength water soluble fertilizer. I never use full strength until they are planted outside.

High Nitrogen is for grass not flowers and vegetables. There are 3 numbers on most fertilizer. The first number on fertilizer is nitrogen. Nitrogen puts the green in things. The second number is Phosphoruus. It promotes strong healthy roots and stems. The last number is potassium. It is good for the over-all health of a plant, For flowers and vegetables you want a high middle number and a low first number

Rick, I am going to disagree with dowlinggram. Young seedlings certainly need nutritional support as they begin to grow, especially if you are using a soilless potting medium. Nitrogen is all important (essential) to all plants, it's a component of chlorophyll and the amino acids of plant proteins....plants die without nitrogen.

Nitrogen doesn't simply support the green parts of plants, but is fundamental to the processes of photosynthesis.

Find a decent fertilizer and use it! I use a soluble fertilizer as soon as my seedlings have been transplanted to their cell packs from the germination flat, at about an inch tall. If I grew my seedlings in the same containers in which they were sown, they would be fertilized shortly after raising their little heads above the soil level.