It doesn't need any fert, not for awhile yet. That soil has lots of nutes in it and it looks like a good sized pot, so it's good to go for awhile, in that department. My guess it that it's staying too wet. What sized pot is that, and does it have drainage holes? Without knowing more about your grow, I'd have to suggest letting the soil dry out between waterings. That seems to be the most likely problem, right now.
How much light is it getting per day? Temps? Is it below 60* at night? Above 85* in the day? Etc...?