I don't unless the need arises. The seedling has everything it needs until just after it develops its true leaves- not the first leaves you see (cotyledons), which look much different, but the true leaves. Now I do add some yummy stuff to my soil block mix- or seed starting mix. Things like rock phos, green sand, maybe some kelp meal. So my seedlings have what they need for a bit. But what if I see this...
|Borage in need of nitrogen|
In the above image is borage. Some have very yellow leaves. The three in the front have been treated with fish emulsion- which gives them a boost of nitrogen. This was taken a day after I gave the 3 in front a dose. Nitrogen is the one thing you'll read everywhere that everyone thinks seedlings need- really? Well, maybe not always. I've read on many blogs that everyone advises feeding seedlings fish emulsion automatically after the first true leaves emerge. I think this is incorrect, and I wait until I see something needs it. Right next to the borage is Hon Tsai Tsi and Kohl Rabi. It's hard to see in the pic but both are very green and don't need nitrogen. All these crops are made from the same batch of soil mix. The borage needs a little more nitro- the others don't. It's been my experience that plants that have too much nitrogen, given to green up their leaves, attract aphids- they love too much nitro- nurseries are guilty of this. So feeding nitrogen indiscriminately is a mistake, I think- and one I've paid for before, So to answer the question- no I don't automatically feed my seedlings fish emulsion unless I see a need for it- and then it's only plants that need it, not every the entire flat. Since I'm not a nursery, I have time to do this, thankfully.
Now one thing I do find that seedlings need, especially the first ones of spring, is rock phosphorus, which is why I add it to my seed starting mix. Cold soils of spring inhibit the plants ability to take up phosphorus- a nutrient needed for good root growth, something you want when you sink those new plants into the ground first thing in the growing season.
Hope this clears that up! Happy growing!