I usually wait until a seedling is 4"-5" tall and has 5 nodes of growth before I begin introducing 1/4 strength nutrients in my soiless mixture. I would wait until your seedling is 4" tall and has 4 nodes of growth before introducing 1/4 strength nutrients. If you do use nutrients before your plant is 4" tall and has 4 nodes of growth I would only start with a 1/6 strength nutrients.