Leaves tend to show necrosis in a soil grow when root bound too. If its a young seedling in a small container, she could be telling you that its time to repot. Before you kill the plant by tossing a shitload of nutes and mg and whatever you decide to do, repot into a larger container and wait a few days.

Plants tend to lose some of their older growth as they go. This is a natural process. Even pine trees that are green all year shed their needles.