Rick, my 'go-to' for fertilizer for just about anything has been Peters all purpose - now called Jacks - for a whole lot of years. I have some on hand and use 1/4 strength for seedlings. Mix with water. I will typically use it just half strength for mature potted outdoor plants. Once in a while something heavily blooming like geranium will be treated to full strength.

If I did not have the Peters (Jacks ) I might be tempted by one of the Espoma liquid products. Those few times I've needed to fertilize mature plants in the ground, I've found their Holly Tone and Plant Tone granular fertilizers very efficient.....no burning, no instant flush of susceptible growth.

Jacks Classic by J R Peters

ETA: Whatever you do buy, be conservative with it and don't begin with the full strength directions on the package. Full strength may be more than is really needed and especially for seedlings. For seedlings, begin with 1/4 strength and see how they respond. Compact sturdy seedlings with great root development is your goal. They'll add their top growth once you've transplanted and they've matured a bit.

Thanks, morz8. I'd never heard of Jack's and will keep it in mind. I've read an article from someone I trust indicating that for seedlings it's better to have a 1-2-1 NPK ratio with twice as much phosphorous as either nitrogen or potassium. I see the Jack's All Purpose Plant Food, as well as the Miracle Gro All Purpose Plant Food I have, is a 20-20-20 formulation. I imagine that's perfect for a general fertilizer, and I'm speaking as a novice here, but don't seedlings a few weeks old do best with a different NPK ratio than grown plants?

The only part of NPK that particularly concerns me is avoiding any products higher in 'N', staying with one more balanced. I haven't found seedlings to appear lacking in anything and they develop with normal growth. I should add though that most of my seedlings are perennials, shrubs, a few vegetables and rarely annuals.

Fish emulsion fertilizer sounds like a great idea. My only caveat is that I wonder how my golden retriever would respond to the smell. He loves fish and... whoops, there go all my seed trays...:-)))

Our garden group starts hundreds of tomatoes every year and we use Osmocote after the seedling are big enough to transplant into 4 inch pots. They are transplanted when they have a set of true leaves.

Thanks, lgteacher. I use Oscomote when putting plants in the garden but I never thought to use it for my young seed-grown seedlings in seed trays or pots just putting out their first set of leaves.

I know I should never question what works for people, but the first thing I read in learning about fertilizing young flower seedlings in my seed trays was this article by Marie Ionnotti from The Spruce which I have a pretty high regard for. The author says "Seedlings tend to need a fertilizer high in phosphorous, like a 1-2-1, N-P-K ratio." In my still-few years of gardening I've come to have an appreciation for the idea that experts disagree about many things, and more importantly that things often work very well even if you don't follow any one "expert" opinion, but at the same time I want to understand what I'm doing. I think there are at least two different Osmocote fertilizers, one with 14-14-14 and the other with 19-6-12?

In any case, thanks for the note. I ended up spending way too much on this "Plant Health Care for Seedlings and Houseplants" fertilizer from Gardeners Supply Company that specifically mirrors the 1-2-1 NPK ratio recommended by Marie Ionnotti in that article. I used it a few years ago and had a lot of success with it. I notice that GSC also sells an "All-Purpose" fertilizer that's 8-10-10 but I got the other formulation instead.

Neptunes harvest. https://www.neptunesharvest.com/liquid-fertilizers.html

Thanks, ekgrows - in addition to the Plant Health Care product I also got the Neptune Fish & Seaweed Fertilizer which looks great. From what I've read, the 2-3-1 NPK ratio (higher phosphorous content) is good for young seedlings. btw, on Amazon I paid almost exactly half the price I see on the Neptune web page.

Rick - I think you'll be pleased with their products. I grow about 30,000 plants a year, and all the seedlings do well with it. My dogs have never been interested in it once it's applied. I don't buy it directly from them either - as there are cheaper sources. Glad you found one of them. :)