Wholly shit...
Since you included no nutrient levels, pH levels, heat or humidity information, etc, etc, it's going to be pretty difficult to get this correct. Fortunately the solution to many problems like this is similar.
Flush your media with an excess of water. Take that plant to the bathtub or shower and just clean the dirt from around the roots. I'm talking 15, 20 gallons of good clean water.
Then, you cut off all of that dead, fucked up, leaf that is left on there. It could be some mold or something so get as much of the dead out of there as you can.
Give the plant another week before you feed anything else. When you do feed again I suggest you use 1 gallon of RO filtered water, 5ml of Grow Big, 10ml of Magical, and mix up enough of that solution to get 1/2 of what you water to drain out the bottom. It is important for me to stress that the pH of this solution is no lower than 6.5, so you need to test your solution somehow.
If you are working in soil top dress the planter with 2-3 Tbsp. of crushed dolomite lime to help balance the pH and provide much needed micro elements.