I wish it was that simple. :)

Instead there are some variables to consider. What age "seedlings"? What mix is in use - seed starting or a growing mix? What is in the mix? Was any rooting supplement used and if so you need to examine the root development. What type of seedling? Different plants have different needs. In your zone I'd be surprised you have seedlings already. What are the growing conditions provided? Container size and watering regimen? Overly small containers concentrate nutrients while over-watering washes nutrients out of the soil

So keeping in mind that many seedlings grown at the proper time for transplanting do not require any feeding, a VERY general guideline - once 'most' seedlings have developed 2 sets of true leaves (become a plant rather than a seedling) then a weekly (assuming standard watering regimen) feeding of a well diluted (1/8 to 1/4 strength), well-balanced liquid nutrient supplement will not harm them and may be of benefit.

But there are always exceptions so with seedlings, use any fertilizer with care. It is very easy to force top growth with fertilizers. But the focus needs to be on the roots and it is important to know if the root development can support all that top growth.

Dave

No seedlings yet... just planning ahead.. a Jiffy seed starting mix of peat and vermiculite, i think. 3 inch square Jiffy peat pots will be used (yes, i have read the warnings after the fact). no rooting supplement will be used for perennials and garden vegetables.. so, wait for true leaves then very dilute once a week