OK, Deb, the "fertilizer" in the Burpee mix is virtually non-existent. So you won't have any problems fertilizing now. Shultz is a good fertilizer in my opinion, so use that. I don't think Superthrive is a fertilizer, so your Shultz is not the same. I would reduce your rate of fertilizer/water though. When I am first fertilizing my seedlings, I cut the rate by 1/2 or even 1/3. So mix 1 - 1 1/2 oz. per gal. Fertilize with this diluted solution every time you need to water. If you are growing on a heating pad, my might have to fertilize/water every 2-4 days and if not using the pad, perhaps weekly. As in orchid growing, I like to say fertilize weakly, weekly.
When you put your seedlings out in the garden/pots, you can use the Shultz full strength. I will then alternate my watering and fertilizing on a schedule of watering twice (again, perhaps every week) and then fertilizing. Since I work Osmocote time-release fertilizer into the soil just before I put my seedlings out, that will reduce my additional fertilizing (Schultz or something similar). I then would water 3-4 times for every application of the liquid fertilizer. You are basically then only using the water-soluble fertilizer to "super-charge" the slow-release Osmocote.
