In Singapore, soil in the ground is too clayey for use in container gardening. Instead of adding a range of soil amendments to improve its structure, local gardeners often choose to use a granular and porous material called ‘burnt earth’ for growing plants in pots.
Large chucks of burnt earth are preferred as they do not compact together and help to provide drainage.
Burnt earth is essentially heavy and sticky clay that has been heat-treated to change its structural properties. According to R.E. Holttum’s book, Gardening in the Lowlands of Malaya (first published in 1953), burnt earth is made from clay soil where large chucks of it are first stacked on top of wood refuse which served as a fuel source for burning. The pile is then set alight to kick start a slow burning process that goes on from two days to a week under shelter.
Today, this process of producing burnt earth is considered by some individuals as an environmentally-unfriendly practice. Burning produces carbon dioxide which contributes to global warming and the digging up of soil material, a non-renewable resource, from the ground can affect the natural environment adversely.
For the organic portion, I choose to add mature compost to the burnt earth portion to confer moisture-retentive properties to the mix.
After burning, what results is a sterilised material that does not become sticky when wet. Due to its porous nature, burnt earth is able to absorb water and because of its granular structure, it also drains well and provides aeration to plant roots growing in it. Burnt earth is believed to contain very little nutrients and plants grown in it would require regular feeding using a liquid fertiliser or slow-release pellets.
Lumps of burnt earth are preferred for use in the garden. Burnt earth in the form of a fine powder is usually discarded as it tends to settle and form a cake at the base of the pot. This is why most gardeners would choose to sieve a newly purchased packet of burnt earth to separate lumps from the fine powder.
A growing mix made up of 1 part of burnt earth and 1 part of mature compost – ideal for growing most plants. Burnt earth portion can be increased for growing plants that need better drainage.
Burnt earth is seldom used on its own. For potting plants, it is often combined with mature compost which forms the organic, water-retentive component of the growing mix. For plants that need to be grown in a well-drained and more open mix, concoct a mixture that comprises coarser pieces of burnt earth and a smaller proportion of compost. Note that a plant grown in a more open mix would need to be watered more frequently when situated in a sunny and/or windy location.
Fine burnt earth powder shown on the right is usually removed by sieving store-bought burnt earth. Fine powder tends to settle at the base of the pot and cause drainage issues.
It is not unusual to see local nurseries nurturing small seedlings in an oversized pot containing a burnt earth-based soil mix, such as Celosia being raised for Chinese New Year during the rainy monsoon season near the end of the year. When used in the right proportion, burnt earth helps to maintain an open and aerated structure of the potting mix which drains well and dries out sufficiently without the dangers of prolonged wet feet. This also saves the gardener the trouble of having to transplant seedlings into larger containers when they outgrow the existing ones.
Lastly, although a burnt earth-based soil mix can be heavy, it presents an obvious advantage – a flowerpot that uses such a mix is less likely to topple when placed in a windy location due to its added stability.