Friday, January 19, 2007

Gardening on tropical clay soil

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.


anilg said...

i will b ein kl on 19th evening and will eb very keen tolearn from you about your work people's knowledge and biodiversity

is it possible to meet

Dr Francis Ng said...

Anilgb, the purpose of the blog is to share information and opinions with all those who follow the blog. Why not post your concerns on this website and see what responses you get?

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hi dr. francis Ng,
my name is Jeffrey Broadfield I have been invited to be involved in the 'salaput' building project by Norman Wong.This has come about by my long association with Richard Leplastrier with whom I have had the pleasure of building 7 of his houses & hopefully our paths will soon cross.
As I am not familiar with the Malaysian timbers & their uses in both building construction & furniture could you please recommend some books to help further my knowledge. My email is Thank you

David William said...

Dear Sir,

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Clay Soil

Anonymous said...

Hello Dr Francis,

I'm a hobbyist gardener and I have a lot of doubts regarding burnt clay soil.

To me, it seems that red clay soil is only available in abundance in Malaysia. Yet, I am unable to obtain any information whatsoever regarding this soil type.

I knew that burnt clay soil had good aeration but had no idea about the chemical properties of the soil.Below would be a list of my doubts and I hope you can assist me. Thank you in advance.

1. Will burnt clay soil slowly crumble and return to its original clay properties? If so, how long does it take for this process to occur.

2. I did some ph tests on burnt clay soil and it was alkaline. What are the chemicals that are usually present in this clay soil? I learnt that calcium, potassium, magnesium and sodium are alkaline elements. Does this mean the soil has an abundance of these elements?

3. Also, the burnt clay soil has a reddish tinge to it, does that mean there is the presence of iron 3 oxide in the soil? So does it mean this type of soil has an iron element deficit, due to its poor solubility in highly alkaline conditions as well as its chemical form in the soil?

4. Can burnt clay soil be a substitute for sand? Especially for xerophytes.

Thank you so much in advance.