What is causing this?

A question from a fellow grower:

What is causing my plants to first yellow then turn brown on the tips of my plants leaves i will attach pictures. My temps are a little warm range between 85 F and 80 but getting cooler now so that will come down my humidity in ranges between 30 to 70, i see they say 70 watt hps not the greatest need bigger i am using a 70 watt do you think that light would be sufficient? I havent got any reflectors around on the walls but i am going to put shortly that could help with light. They also 5 weeks so is it slow or am i just not patient seems a bit on the slow side
Let me know what you think thanx

Nutrient burn , to much nitrogen and not enough perlite in potting soil .