White widow . . . 1st grow ever

I am growing 1 white widow plant using seeds i got from http://www.cannabisseeds.com/ 12 for £10.

i started by germinating the seeds using the paper towel method and surely enough after 24 hours the seeds were sprouting, i then planted the seed in a pot using ordanary potting soil and watering regularly when the soil dries up, the plant has been growing for three weeks now under a 100 watt light bulb in the grow box i made.

The plant is healthy but there is a brown discouloration to the lower leaves which i think is due to nutrient defficency as i
have not yet started adding nutrients as they are still in the post.

and help would be much apreciated on identifying the problem

thanks

<< here are sum pictures of my growbox and plant after 3 weeks , any comments welcome >>

Ya have to start with more then 1 plant at a time. It could be a male,or just not grow well. What size pot is it in?

i had two but one died and it seems to be growing ok i will upload some pics, thanks. it is in about a three litre pot at the moment i recently transfered it from a plastic cup