OH BOY!
With some, anything with the word "chemical" is BAD,BAD,BAD
Any thing that is "natural" is GOOD,GOOD,GOOD
Some have taken this to such an extreme that it is just plain silly.
Boy am I going to catch it for this.......

mandolls, the reason soilless mix is used for starting seeds is because seedlings are more prone to diseases, so you want a relatively sterile environment for them. It is just like caring for babies. If you eat something wrong and get a food poisoning, you wait for a day and you probably will be fine. If it is your one-month old baby, that is a different story.

But with soilless mix, the reasoning for using chemical fertilizer as you mentioned above is true.

Exactly - I understand why soilless mixes are preferred, but I am questioning the reasoning of many people on this forum who are using soilless mix and then using fish/seaweed fertilizer. I was wondering if there was something I was missing.

I use diluted down general purpose Miracle Grow to water my seedlings in soilless mixes and have successfully grown several hundred plants that way. But don't use too much or too concentrated as you can burn your plants, I killed several batches until I learned to make sure I really dilute it down. I alternate with watering with pure rain water, which I also use to prepare the Miracle Grow solution.

I don't fertilize the seedlings I start in soiless mix. For one thing, depending on what you buy, the mix may contain all you need to get them through transplantation. For another, I've found that generally "less is more" when it comes to the question of what and whether to add things to my seed starts. Knock wood, I have very good, consistent results with healthy plants.

mandolls, sorry I misread your question earlier. The reason people use fish/seaweed fertilizer on soilless mix is probably because it still works. The breakdown is going to take some time but not that long.

The soil is relatively sterile but not perfectly sterile, but you always have something in the air. If you imagine putting a little piece of fish or moist seaweed on your soilless mix, it is not likely that the fish/seaweed will not spoil because the medium is sterile. It is going to breakdown and pretty fast too.

So all you have is a slightly delayed fertilizing. I would say if you happen to have chemical fertilizer, use them (diluted) for your seedlings; if not the organic should work too....