I have bought some tea tree oil to add to my shampoo and conditioner in the hope that it will help my psoriasis.
I'm not sure how much to add though, the web suggests anything from a few drops to fifteen, but no mention is ever made about how big the bottle of shampoo is!
The assistant in Holland & Barrett was little use, she advised against me trying it as it would dry my scalp out and then went on to try and sell me Tea Tree Shampoo & Conditioner!
Psoriasis is very difficult to treat with essential oils. Ones you might want to try in addition to tea tree are:
bergamot
helichrysum
cajeput
carrot seed
German chamomile
Roman chamomile
lavender
juniper
sandalwood
tea tree
pomegranate seed CO2