I always start feeding at low dosage around week 4 or 3 defiency on size of plant. If you're using a heavily fertilised soil, leave until week 4 to 6. Regarding your humidity issues, I have the same problem and it can mean life or death... Cover the young plant with some kind of plastic dome with air hole/s in and keep spraying inside with water this helps keep humidity high and watering less. 🌱👍