Plant cupping with PIC... HELP!??

am using a 90w UFO grow light, supposed 400w output, and right now i have 2 sour diesel plants i started from seed underneath it about 4-5 inches below the light. This light does put off SOME heat, its very minimal and it seems like 4-6 inches shouldnt be too close. They are in a dark soil i dug out of the ground under a leaf compost pile. I KNOW THE SOIL ISNT THE BEST CHOICE. It was my only option at the moment they needed an emergency transplant from the small 1pint container they were started in. The leaves directly on top are curling, and the second node down are doin the same while all the other nodes down from that point are healthy. but they are all green, waxy and thick leaves, my curled up edges are NOT crispy or discolored. and its only on one plant. what are the potential diagnoses and solutions to this problem? IM ABOUT TO START MY 12/12 cycle TONIGHT, i need to know ASAP

That soil looks like it gets no drainage. What's your humidity like? Sometimes if there is low humidity, they try to retain moisture by closing the leaves up.

i dont know the humidity actual number.. but its fairly high here where i live at this time of summer, i doubt humidity is the problem unless my UFO lights minimal amount of heat is drying my growbox