Yellowing of lower leaves on young plant

Well my thought is some type of deficiency, probably nitrogen, possibly a pH issue, but what do you guys think?


http://img201.imageshack.us/img201/7586/img0522fq3.jpg
http://img179.imageshack.us/img179/8567/img0525ez9.jpg


Its White Widow, and about 3 weeks old, using Scott's Potting Soil, fertilizing with some liquid 10-15-10 solution.