Somewhere on Soma's web site he talks about how to do it.
Soma's Sacred Seeds — Official Blog
Prairie
When you start working with plants as a hobby, there is no end to the wisdom that can be gathered. I started working with the sacred cannabis plant in 1971, I now know so much more than when I first started, I can teach it to others.My main activity these days with the Holy plant is breeding seeds. I am constantly cross breeding in search of the finest of medicines. There is such a huge diversity in the genetics of Cannabis. Everything from industrial strength, to medicinal strength. Whenever two new strains are bred together for the first time, you get the hybrid vigor effect. This gives you a very potent fast growing mother plant. When creating seeds from males crossed to females, you can't help but get several phenotypes.
When making seeds from females turned male, you can keep it down to one phenotype.Through the years I have made female seeds many different ways. Light stress, heat stress, cold stress, fertilizer stress, and gibberillic acid stress. Although they all worked and gave me female seeds, I still kept searching for something better.
Then came STS solution. Silver Thiosulphate Solution. It is an equal mixture of silver nitrate mixed with sodium thiosuphate.
What happens when you spray this mixture on a female cannabis plant is hormonal stress.All cannabis plants have the capability to change from female to male. The reason they can do this is survival of the species. The inherent drive within life forms to preserve the species is all-powerful and will supercede all impulses. As an example lets say a cannabis plant gets subjected to very low temperatures when it should be warm. The plant says to itself, as an individual I feel severely threatened, I might even die from the cold, therefore I must make my species survive. The stressed female plant then manifests a male branch and with it makes seeds to make its species survive even if as an individual it perishes.When you spray a plant with STS solution, this is what happens. Within the first 24 hours all the tips of the branches wither and turn color almost looking dead. The large fan leaves stay green, just the tips cringe from the stress.
They stay that way for several days, and it will make you wonder whether you have killed your plant. Don't worry you have not killed it. The plant starts to recover by the 5th day. You can see the tips of the branches start to regenerate.Making female seeds is done in this way. First you select females that you want female seeds from. You vegetate them until you have reached the desired size and height under 24 hour light. Right at the time you are going to turn the lights down to 12 hours is when you select your one female of the group to become the male sex partner. You spray the plant away from the others carefully saturating the plant both under the leaves and on top of them, it is good to wear rubber gloves so as not to get the STS spray on your skin. You let the plant fully dry and then place it back with the others. You carefully mark down the date that the plant has been sprayed, and turn the lights down to 12 hours, or flowering light. It takes 28 to 35 days for the sprayed plant to turn to a male depending on the species. One female turned male plant can get many females pregnant as the pollen is microscopic and is in plentiful supply. You can also help spread the male pollen by shaking the plant gently over the tops of the other females.
It takes the plants 6 weeks to finish making healthy seeds that are hard and dark. These finished seeds are 100% female, they have no male chromosomes. They will also carry only the phenotype that was used, making every single plant uniform. This is the state of the art in making female seeds, and within the next 6 months I will have the whole top of my Soma's Sacred Seeds line in female form as well as standard.Wishing you much fun with your plants.