From what I've read, every experienced grower on this forum avoids fertilizing young plants like this one.
Misting instead of watering is an odd choice. The pot has a drain hole, right? Use it. Misting the leaves, in combination with high temps (you said 81 deg.) and being too close to the light may have caused scalding or whatever it's called.
Did you get that horrific looking soil from your back yard? I know they say it's a weed, but the reality is young plants are fragile and it's impossible to treat them too well.
Do you have any idea of pH? Your water and your soil?
Are you using tap water right out of the faucet? By law, all but a handful of municipal water systems in the U.S. have to disinfect. That means chlorine and chlorine by-products are in your water. Chlorine and CBP are NOT good for plants. Especially young plants.