Soils in the humid tropics tend to be highly clayey. Clay particles stick together to impede passage of water and air, and this is detrimental to root growth. Without sustained effort keep clay soils open and porous, tropical soils rapidly become unproductive. Growers resort to many different methods of farming on clayey soils. For example, vegetable growers till the soil after each harvest and pile up the loosened soil to form raised beds. During each watering session, water soaks in and drains out easily, thereby simultaneously renewing the supply of water and air in the soil. A good soil is analogous to lung tissue in that both have large internal surfaces to hold moisture and air. Unfortunately the effect of tilling lasts only for a few months.

Clay soil may be burnt over a hot fire, in the process of which it becomes crumbly (Holttum 1953). Burnt soil maintains its crumbly structure for up to one year, and such soil is often used for container gardening.

However, the most favoured soil for horticulture is garden black soil, which goes by the Malay name of tanah hitam (black soil). Black soil originated in household backyards where domestic waste was dumped and periodically burnt. The black colour was due to the accumulation of charcoal and soot in the soil over time.

Tanah hitam in Malaysia seems to be very similar the soil in the Amazon known in Portuguese as terra preta (black earth). Terra preta soils are very fertile and contain a high content of carbon (about 10%). They occur on sites that appear to have been permanent native settlements for centuries before their populations were wiped out by diseases brought in by the Europeans. It would have taken centuries of firewood burning on the same sites to have produced black soil in the vast quantities, to 2 m deep in some sites. The discovery of terra preta sites has created a lot of discussion in the Internet about its origin.The development of horticultural carbon

Open burning has been prohibited for many years in Malaysia, hence black soil is no longer available. Needing a large volume of good soil to establish a rain forest in the ‘1 Utama’ shopping mall in Kuala Lumpur, I decided to make such a soil by mixing charcoal particles with soil. We made this soil by mixing normal clayey soil (mostly subsoil) with charcoal and coconut fibre in equal proportions by volume. The charcoal was conventional charcoal produced by the kilning of mangrove wood. This came in large hard pieces that had to be broken up mechanically. The resulting particles were irregular in size and difficult to mix with the clay and fibre. I then found a much better source of charcoal in the factory of a charcoal briquette manufacturer. Charcoal briquettes are made by compressing sawdust into standard-size briquettes for kilning. The briquettes, meant for the barbecue market, can be easily broken into particles, sieved to remove dust and graded into the desired sizes. We refer to the product as horticultural carbon (Ng, 2006). We use two sizes: 1 – 4 mm particles for potting mixtures and 5 – 12 mm for garden beds.

We have found that a mixture of equal parts horticultural carbon and clay soil is good for general purpose horticulture. A mixture of three parts carbon to one part soil is better for cacti and succulents that need exceptionally well-drained soil.

Horticultural carbon is half the weight of soil, so the mixtures we make are lighter and more porous than ordinary garden soil. The reduction in weight was an important factor in my next project, a garden on the roof of the same shopping mall, seven floors above the ground. This garden, known as the Secret Garden of 1 Utama is now open to the public at weekends.

The porosity of soil mixed with horticultural carbon greatly reduces the labour of weeding because the weeds can be pulled out easily. However horticultural carbon only holds half the amount of water that an equivalent mass of clay soil will hold. Its lower water-holding capacity, together with its porosity, means that horticultural carbon dries out much faster than clay soils. The drying of the soil medium can be very damaging to the roots of plants, hence we find it necessary to keep our medium kept moist all the time. This can be arranged in various ways, for example, by watering twice a day. In pots, we would recommend placing the pots on shallow trays to hold water.

Horticultural carbon does not contain nutrients, hence fertilizers have to be applied regularly. Initially the carbon and clay particles remain separate though mixed. Gradually the carbon wears down and becomes integrated with the clay, with consequent settling of the soil mixture. The soil level drops and is topped up with pure carbon.

The performance of plants on horticultural carbon

Our most extreme experiment was to grow rice on 100% horticultural carbon in plastic basins. The basins, about 20 cm deep, were three-quarters filled with carbon particles and topped up with water. Rice seeds were sown direct on the surface. Our Indonesian workers, rice-growers in their former lives, all had a good laugh because “everybody knows that rice only grows on tanah liat (sticky clay soil)”. Well, our rice grew and produced a heavy crop of grains. We have now grown three successive crops. The roots form very dense mats. After each crop, the roots have to be dried out before the carbon particles can be shaken out and recovered..

For cacti and succulents, we use a mix of 75% carbon to 25% burnt soil in elevated beds. Some species thrive, but some still find it too wet, and rot when it rains daily. Nevertheless ours is the only decent-looking cactus bed exposed to tropical rain in Kuala Lumpur.Begonias, calatheas, and aglaonemas grow well in 50:50 mixes on raised beds provided 50 - 75% of the sunlight is cut off using shade-nets.

Of temperate plants and montane plants, we have managed to grow apple, peach, plum, Magnolia grandiflora, Magnolia liliiflora, arabica coffee, azalea, camellia, day lilies and Platanus. It has been hypothesized that in the tropics, the high night-time temperatures raise the night-time respiration rate to a level that temperate plants cannot adapt to. We think a high carbon mix allows air (oxygen) to get to the roots more easily, making it easier for temperate plants to adapt. However the flowering patterns of temperate plants are disrupted by the lack of seasons. Some species do not flower at all (e.g. day lilies), some flower infrequently and sparingly (e.g. apple and plum), and some flower all through the year (e.g. Magnolia liliiflora and arabica coffee).

Where to see horticultural carbon in use

In Malaysia, the Secret Garden of 1 Utama in Petaling Jaya, occupying 0.25 ha of flat roof top 7 floors above the ground, is the largest display open to public view. Here are grown over 500 species of plants, including palms, orchids, temperate plants, flowers, spices, rice, cacti, climbers and grasses. Also in 1 Utama but on the lower ground floor, is a rainforest with some 50 species of timber trees growing on a horticultural carbon mixture. In Sarawak, the Laila Taib Ethno Garden of the Sarawak Biodiversity Centre at Semengok, Kuching, displays a good range of native herbs grown on horticultural carbon, most of them larger and healthier than in their original rain forest habitats.

Horticultural carbon in carbon sequestration

Since the Industrial Revolution, the amount of carbon dioxide in the atmosphere has increased significantly, to bring about global warming. The increase in carbon dioxide in the atmosphere is due partly to the extraction and burning of coal and petroleum and partly to the clearing of forests, which reduces the amount of organic carbon stored in forests. Proposed measures to control global warming include reduction in consumption of coal and petroleum and the planting of trees and forest to convert atmospheric carbon dioxide into organic carbon. However, reduction in consumption has proven to be difficult, and trees and forests fix carbon efficiently only when they are in active growth, i.e. during their juvenile phase. When trees die, organic carbon is converted back to carbon dioxide through the normal processes of decay.

Holttum, R.E. 1053. Gardening in the Lowlands of Malaya. Staits Times Press, Singapore.