Part of me is concerned that such slow growth at this early stage could stunt their future growth, but it sounds like you're suggesting that wouldn't necessarily be the case.
If I am right ( and they just need fertilizer), then yes, they only thing you have lost is time. They will grow just as fast now as they would have earlier with the proper fertilizer.
It's good that you ered on the low side of fertilizing, But I did mean half strength of the regular dose. No worries.
I just looked up your link to Foxfarm Big Bloom. There is no nitrogen! Nitrogen is the limiting nutrient in mineral soils like yours. And at 0-.5-.7, this seems really low compared to normal things like Miracle-gro (24-8-16). But Miracle-gro mixes at one tablespoon per gallon water. What is your mixing ratio?
Rick Rodich zone 4a. Annual precipitation ~24 inches
near Minneapolis, Minnesota, USA
The recommended amount on the label for "Seedlings and Transplanting: 1 tablespoon per gallon of water every other watering." So I mixed 1/2 tablespoon in a gallon of water and watered the pots with that.
When you say "mineral soils," you're referring to the gravel mixture I put together?
That's interesting that there's no nitrogen, and I see it's 0-.5-.7, as you say. I usually like Foxfarm products and wrote to them asking for a recommendation for seedlings I'm growing indoors (not Penstemon) in cells under grow lights and they recommended Big Bloom. I felt a bit wary as I wouldn't think something called "Big Bloom" would be ideal for young seedlings just starting out life with a few sets of leaves, but out of convenience I did use it on my indoor seedlings in seed trays, and then yesterday on my Penstemon in gravel.
Near Boston, MA, zone 6b. Average annual rainfall = 44 inches
Yes, a mineral soil has very little or no organic matter, like your mix.
You really need nitrogen in the fertilizer. If you want to stay natural, you could use fish emulsion, or I'm sure there are other options if you look around. I don't think you will notice a difference in growth with the no nitrogen fertilizer. I always use a liquid form, but I am not picky about the brand. (Of course, Miracle-gro granules is a liquid becuase you mix with water before application.)
"Chemical" fertilizers still get their nutrients from natural sources, but have been processed to get higher concentrations and/or better overall marketing value. Natural fertilizers are always relatively low in nutrient concentration, because that's just how nature works. I think it is pretty incredible that Foxfarm recommended a half dose for seedlings, becuase that would be like 30 times less potent what Mircacle-gro recommends.