Tropical Horticulture and Gardening carries a description of the use of horticultural carbon.
Horticultural carbon is a great new product for gardeners in the tropics. It can be mixed with clay soils to improve soil porosity. It is sterile and only half the weight of soil. Unlike sand, it is able to hold water and nutrients. It is made from sawdust, compressed into briquettes and carbonized in a kiln. The briquettes are the main product. Horticultural carbon is a byproduct made by breaking up the brickets into particles of less than 1.5 cm diam. The fine dust or soot (1 mm or less diam) is sieved away and the rest are graded by size.
I use carbon mixed with burnt clay in various proportions: one part carbon to one part clay, two carbon to one clay, or pure carbon. The mixtures work well in flower beds and big pots. In small pots, they tend to dry too fast. The best mixture is probable two carbon to one soil (by volume)
I developed this material as a substitute for soil, for growing a tropical rain forest within a shopping mall in Kuala Lumpur. The forest contains over 100 species of timber trees and palms and it is thriving on this medium.
I next used this material to grow the herbal garden of the Sarawak Biodiversity Centre in Kuching. The plants in this medium are bigger and healthier than those on normal soils.
Another project is a cactus garden on a rooftop, exposed to full tropical sun and rain. Many desert plants were able to live on this medium, showing that they do not mind the tropical rain, so long as the water drains away quickly. However, daily rain during the past couple of months have prevented them flowering.