I'd try fertilizing half of them, see what that does. I always just go ahead and soak the roots with my diluted fertilizer mix; my understanding is, as long as you have the right ratio of water to fertilizer, the amount of fertilizer+water that you add doesn't matter. For example, I fertilize with fish emulsion. I measure the tablespoon or whatever of emulsion in the gallon jug of water, mix, and then I pour it in a seedling flat and bottom water as I normally would. Once the cups are good and heavy/full of water, I take them out. I usually rotate several flats of seedlings through that one batch of fertilizer+water until it's all been soaked up (in the course of a couple of hours, I don't let it sit around or anything, stinky!!). Hope that helps, someone else chime in if I'm incorrect in my thinking.

Sleepy is right on the money.

I use 1/4 of what is stated (on the fert. conatiner) what I first start fertilizing. so if they say one tbsp per gallon I would use 1/4 once a week. I do not pre-water and then add fert. It is one process once a week and if they need water between that, I use plain water. I also use a 1/8 mix all of they time (everytime I water) with great results. The fert. is already in my watering lines in the greenhouse so it is easier to remember weak everytime than once a week stronger.

Most of my seedlings are mature now so they get full strength once a week

Keriann~