Thanks for sharing thoughts on this; 2 questions:

1 - a recommendation for a fertilizer I could dilute?

2 - would you say that these have their second set of true leaves?

I plan to repot after the leaves are wider than the trays, which they are getting close!! :)

I had trouble with damping off, but having the ceiling fan turning now seems to help with this, and damping off seems to just be a consequence of how certain potting soils plumps ended up in the seedling trays. All of them where they died would barely absorb water when sprayed, meaning I guess the specific mixture for that cell was just too impermeable, and not able to drain well.

Any liquid or water soluble fertilizer would work - Miracle Gro, one of the Dyna Gro products, even fish emulsion.

They could be showing the second set of leaves (at least a couple) but knowing the tree species and a closer photo would help to confirm.

Here is the top right one (original photo), this is a paulownia tormentosa.

Standard 2x6 seedling tray, ~1.5 in x 1.5in x 2in (tall) per cell.

The leaves are burnt a little because I raised them for about 2 weeks under a CFL light that was apparently too strong after a certain point :(

personally .. i doubt these things could use up all available nutrients ... before you would need to up pot by fall ...

you are on the verge of loving them to death ...

i hope you are turning them once a day ... because there is insufficient light .... as noted by them leaning severely toward the window ... and lack of light is going to be a far greater problem than fert ...

think about it.. a tree converts sunlight into sugars to grow the plant.. and you want to substitute good light with fert .... something might be wrong in that equation ...

ken

" a tree converts sunlight into sugars to grow the plant.. and you want to substitute good light with fert .... something might be wrong in that equation ..."

Photosynthesis - the conversion of elements into what a plant needs to feed itself - requires a lot more than just sunlight. It also requires access to nutrients (as well as oxygen and water) and there are generally little to none in any kind of potting soil. ALL seedlings, regardless of type - require nutrient supplementation if growing in any sort of standard potting or seed starting soil.

This is not at all the same sort of process seedlings encounter when growing outdoors in the wild and one must make allowances for that. It is not a case of "loving them to death" but merely supplying what they need that is lacking under the current growing conditions.

To the OP: yes, the seedling in the last photo you posted does have its second set of true leaves.

Agree with gardengal. If you use liquid fert like miracle grow, you will be potting up within a month. A lot of growers recommend using a higher P fert (in N-P-K) ratio to stimulate roots, i.e., MG's rose fert. Don't think it's a big deal either way.

By the way, a granular 10-10-10 (or any other analysis HAS to consist of mostly filler, or we would never be able to apply it properly! A bag of full power fertilizer, if it were even possible to manufacture, would cost hundreds of dollars!

I wish ken would stop lecturing about tree nutrition, good grief.