Clay soils in the tropics are not the ideal medium for plant growth, because clay particles stick together to impede penetration of water and air. On lawns, clay soils get compacted with time, starving the grass roots of oxygen.
Where plants have been growing naturally for a long time, leaves, bark, roots and other plant parts decompose and make the top layer of soil porous. Such top soil is the most fertile of natural soils, but it takes a very long time for new topsoil to form.
Chinese gardeners discovered long ago that the underlying clay soil, if burnt over a wood fire, becomes porous, friable and non-sticky, allowing for excellent water and air penetration. We do not know who discovered the process. For a description, we have R.E. Holttum's account in his book, Gardening in the Lowlands of Malaya, first published in 1953. Holttum was Director of the Singapore Botanic Gardens and Professor of Botany at the University of Malaya.
"The earth is broken up into fairly large pieces, and piled up on top of some old wood and other refuse, into a heap some four feet deep and six feet or more across. A cover to keep off rain is necessary. The stacking of the pile is of course a matter requiring skill. It must allow just enough circulation of air to keep a slow fire burning. The fire is allowed to burn for two days or sometimes more. When the operation is finished, the earth is changed from its original condition of clay to a porous granular state. It will consist of a fair propertion of large pieces, which should not be broken up too small, grading down to dust. The dust is not good for plants that require a particularly well aerated soil, and can be removed if necessary by the use of a fine sieve. The essential properties of the burnt earth are that it will absorb water without becoming sticky, that the lumps do not readily break down when the soil is watered; and that it is sterilized, all pests and diseases and harmful micro-organisms being destroyed. "
Yesterday, I visited a place in Sungei Buloh, just outside Kuala Lumpur, which produces burnt soil. There were about 10 kilns on the site, on leveled ground cut out from a hill side. Each kiln is a square enclosure with sides about 6 ft long. A post is erected at each corner, to support a roof of galvanized iron sheets ('zinc sheets') 6 ft or 7 ft above the ground. Planks are nailed to form walls on three sides. The fourth side is left open for stacking the wood and soil. First, wooden billets are closely stacked on the floor, to form a layer one foot thick, with some gaps for ventilation. Then lumps of clay soil, cut out from the exposed hillside, are laid on top of the wood to form a 2 ft layer. Then another layer of wood is laid on top, followed by another layer of clay. Then the roof is fixed, and the fire is lit. The fire burns for one week, and the pile takes another four days to cool down. The burnt soil, mixed with wood ash, is then passed through a sieve, the purpose of which is to hold back the larger lumps that need breaking down. There is no attempt to remove fine dust.
Burnt soil is used extensive in and around Kuala Lumpur and retails at $1.20 per bag of about 3 kg. At the kilns, it is sold for 80 cents a bag. In Ipoh, 3 hours drive away, it is sold for $2.00. Burnt soil is also produced locally in Ipoh but is said to be of lower quality. The difference, apparently, is that the clay soil in Sungei Buloh contains a good proportion of sand, which makes the particles more friable.