this info is from Jorge Cervantes's book "Marijuana Horticulture The Medical Growers Bible"
"environmental factors start influencing sex the moment the seedling has 3 pairs of true leaves. increasing N makes more females, lower N levels make more males. more P make more male tendencies, lower P encourages female plants. low temps more females, high temps more males. high humidity more females, low humidity more males. low growing-medium moisture increases males. blue light more females, more red light more males. fewer daylight hours (14hrs) more females, longer days (18hrs) make more males. any amount of environmental stress tends to yield more male plants when growing from seed. at the end of this it says Henk, owner of Dutch Passion Seeds was kind enough to allow us to adapt this info from his archives."
hope this helps...