Seedling is in its 2nd week of veg, using Promix B...

Hey there for weak stem what you want to do is feed micro nutrients - cal.mag basically. Uptake through the roots is efficient at 15% uptake through the leaves is efficient at 85% so if you want you can make a call mag foliar spray and spray the plant until dripping wet once a day . Once a week also add your favorite kelp extract or amino acids. They stimulate the auxin hormone which is the plants growth hormone. Hope this helps ! 🚀