check this out........from dutch passion seeds.....
follows:

From literature and our own findings it appears that the growth of a male or female plant from seed, except for the predisposition in the gender chromosomes, also depends on various environmental factors. The environmental factors that influence gender are:

1. a higher nitrogen concentration will give more females.
2. a higher potassium concentration will give more males.
3. a higher humidity will give more females.
4. a lower temperature will give more females.
5. more blue light will give more females.
6. Fewer hours of light will give more females. It is important to start these changes at the three-pairs-of-leaves stage and continue for two or three weeks, before reverting to standard conditions.

the link is : http://www.dutch-passion.nl/engels/female.html