Ah, the Kai lan, yes very tasty. I know it as Gai lan. Yum, save some for me!
My friend gardens in southeast Asia and says that vermiculite is not available where she lives. She told me about using burned rice husk/black chaf which is used as a soil amendment and for starting seeds; please, when you say 'burned wood' what exactly are you using for that part of the soil?
I agree with the others that as a beginner gardener it would be best for you to purchase a commercial soil-less mix if one is available in your area. If the pre-mix is not available it may be best to use less ingredients in your home made mix.
Yes, the plastic pots may not be the best in your very humid environment; are you able to find terra cotta/clay pots? Another thought is a fabric plant pot; please us the key words 'Brown Boxer Root Pouch' to see one type of plant pot which is made from recycled plastic. The pots 'breath' and the roots are 'air pruned' as the plant grows.
Since you are growing on a window ledge and have not enough sun, perhaps you could place an old mirror (or other reflective material) inside the house to direct some of the available sunlight toward the plants?
