As you know Jim fertilizing seedlings and how is hotly debated. Many don't feed them at all until transplanted to the garden and they do just fine. Others prefer to feed 1/4 strength every time they water or 1/2 strength every 7-10 days. So opinions will vary widely.

My approach is bit different in that I prefer to focus on root development rather than top growth during the seedling stage. That is what is vital to the future health and production of the plants - great roots not tons of dark green leaves. So many other factors come into play.

How long until transplanting to the garden? Soon - skip feeding. Weeks - feed.

Potting mix - ProMix (depending on which one used) may need a mild P&K supplement but I don't feed N, those that use MG mixes with ferts added may not need to feed at all but if they do it sure doesn't need to be N.

Size of the container determines how often they need to be watered and governs the nutrient leach rate IF nutrients are there. So those in cell pack or 4" pots will need more frequent feeding that those in 16 oz cups or 6" pots.

Those who stage and 'pot-up their seedlings can feed more than those who refuse to do so (despite all its advantages) - even some N as their root development will be better because of the transplanting.

Those who grow seedlings on the dry side don't have to feed nearly as much as those who insist on over-watering. Those who bottom water don't have to feed as often as those who top water for obvious reasons.

Then there is the type of fertilizer used and the wide variance in performance. Organics vs. synthetics, liquids vs. granular, slow release vs. quick fix types, etc.

See what I mean about lots of variables?

So can there be a rule of thumb for fertilizing seedlings? No, not IMO. Since I am growing plants for both sale and use in my own garden they all get the same treatment. General guidelines that I follow: ProMix BX, minimal watering and bottom watering only, lots of supplemental lighting even in the GH, low to no N, well diluted P&K supplements weekly IF needed (examine root development first), liquids only, synthetics only (save the organics for the garden where they have a chance to work), go for 6-8 week transplants, not 2 foot tall 12 week old plants, and do staged potting-up.

But that's just my opinion and I'm sure many will disagree in whole or in part. Works for me.

Dave

Thanks for the good advice, Dave. There is a lot there for me to think about. Fortunately, I already do many things the way you suggest, which is reassuring.

Jim

I think Dave wrote the most complete answer already. :) I just wanted to add my thought on fertilizing, which is that it needs to have a purpose, a result that I want. Fertilizing may make already healthy seedlings bigger, but maybe not healthier. So I reserve it only for those plants which will do best growing fast and dying young- lettuce, broccoli, for example. Long lifers like tomaotes I let alone and try to match up my sowing to my transplanting times as best I can so they need no fertilizing. This year that didn't go so well, but most years it is easy enough. All this is for what goes on indoors under lights. Once out in the garden everything gets nourished with compost and mulch. Cheers!

My direct planted garden seeds (beans, spinach, cucumbers, beets, radishes, lettuce, carrots, corn, muskmelon, and watermelon) are planted into soil that has already been fertilized for this growing season. (Some are side-dressed again later).

My transplants (tomatoes and peppers) are sprouted in plastic bags in wet paper towels, then inserted into growing tray cells, and eventually into small plastic cups, then transplanted into the garden.

I see no reason why tomatoes would, or evidence that they do, need to be started without exposure to fertilizer. Those "volunteer" tomatoes that always come up in my "fertilized" garden always seem to be as, or more healthy than, those I grow inside, or could buy from a greenhouse.

I use Ferti-lome Ultimate potting mix for starting, potting-up, and also for my container plants. Have good results, do not need to use a special "starter mix" or decide whether to fertilize them before planting out.

Here is what I take on this issue. When I direct sow seeds in the garden, the soil is more likey balanced. Which means it has all the nutients to some extent. So then presence of some nutrienst in starter soil should be fine. Then the plants will decide when and how much they need to take. The only exception to this, in my mind, is excesive amount of N.

But I could be wrong there . Maybe that is why you can get better results by planting in a controlled medium. As Dave pointed out, more than anything else, you want strong roots first . That is why you feed mostly P & K early on.

I plant eggplants in nutrient-containing potting mix 8 weeks before planting out and I always fertilize with N, P, K and Mg at the 5 week stage, otherwise they begin showing deficiency symptoms especially those of N and Mg.
Before seeding, I also soak the mix with a very dilute micronutrient solution containing B, otherwise my seedlings show B deficiencies by the 2nd week.
Personally, if I was using potting mix with no added fert, I would not wait until the 4th week to fertilize.