Some people dont fertilize seedlings at all. A lot depends on what the plant is and how long it has to live in a pot inside.

Organic fertilizers are best used in the garden they need all of the active soil bacteria etc to break down for the plant to use it. If you are using a soil-less mix, organic fert isnt going to do you much good. I use a little bit of non-organic fert inside, and then use organic in my garden where it matters.

Agree with all mandolls said. Fertilizing seedlings is strictly optional. First you have to make sure there is none in your potting mix. if there is, adding more stuff only stresses the seedlings.

If done it MUST be done with a well diluted (1/4 strength) fertilizer. That means a liquid. Only recommended organic liquid usually recommended is fish emulsion since it is already in an emulsion or some of the worm tea products. Most organics don't work in sterile soil.

Which is yet another example of all the mis-leading "organic" marketing hoopla out there. But that is another issue all together.

Dave

For the first time I lost some seedlings due to fertilizer. I have a large sink in the greenhouse that I use for watering seedlings from the bottom. I keep a few inches of water in it and drop the containers of seedlings in it when needed. While using the same sink for my new divisions of last years perennials, I added the normal amount of soluble fertilizer. The next time I put a pot of seedlings in the same water, the effect on the seedlings was seen the next day. The lesson is, if you must fertilize seedlings, be sure it is a VERY weak solution. Years ago I did use a compost leach-ate with mixed results. Al