that Miracle-Gro soil is OK
Im not disagreeing with you about the soil I havent tested it.
Ive seen nothing but trouble with MG soils and seedlings
a seedling thats "unhealthy... with smaller roots".... can easily be burnt even with trace amounts....given the necrotic areas now beginning to show I wouldnt rule out the soil yet

I see that the nitrogen content on the label , isnt "urea orgainc based" which is less likely to burn" its ammonical & nitrate which does burn plants.

Miracle Grow has just re-formulated thier Seedling Soil Mix within the past month , it now contains some of the Urea "organic type" nitrogen (0.10% urea nitrogen organic non burning type) the ammonical type nitrogen they coat for a slow release & less burn

maybe they old formula was burning too many seedlings, or why the change adding Organic non burning nitrogen

just my 2 cents
Ganja Guerrilla Reviewed by Ganja Guerrilla on . Seedling is Yellowing! The only survivor of the damping plague is now yellowing. He is in different soil from the others which is maybe why she's survived this long. Now she's starting to yellow can anyone help diagnose the problem. Rating: 5