If any ,what goodness is there in this growing medium.?

[Happy]

First i will say I am English and live in Vietnam, I am a keen gardiner but have had difficulties with soil and coco and have just not been happy with my growing medium. There is a garden centre nearby and they grow some lovely plants in what i thought [until today] was soil. But i had a friend come with me who could speak Vietnamese and I found out that its not soil, what they use is all the waste after processing white rice,it is then set on fire leaving you with this black soil like substance that is very light in weight. I am interested if anyone would know what sort of goodness if any there would be in this, I am a keen Hibiscus grower and wonder if it would be ok. Thank You.