That's thinking out of the box to get them started. I use a lighter soil ( FF LightWarrior) as well in top third of my 5 gal pots to get good tap and lateral root development before it hits my dry granular fertilizer amended mix. Not for nothing @TheNuttyProfessor, I utilize fabric as well and...
According to the 09/13 GWE newsletter "What makes leaves turn yellow". I reckon you can narrow it down.
Poor watering practices
Heat Stress/Cold Shock
Not Enough Light (Seedlings)
Bugs or Pests
I amend my soil with an organic dry granular slow release fertilizer at the recommended dosage of 2 tablespoons/gal of pot size, or 10 tbs/5oz.. per my 5 gal pots. Works out to 25 cents per plant. But I grow autos. I'm sure price would easily double with longer time frame photos.
Cost per 40lb...
As an effective preventative for soil-based larvae that may be included in my bagged medium of choice, I use cold
pressed orange oil, 2oz./gal water when I pre moisten soil before transplant with great success and reapply at 2-3 week intervals. Organic soil auto grower.
I gave you misinformation on the Spinosad soap. It's for aphid and mite control. You're spot on with the
3% H202 at a 1:4 ratio /water. @ Bush Doctor 77 method is effective as well. My bad, I'm an old and had a senior moment.