Indonesian soil profiles to show how agriculture affects the soil

Shifting cultivation in Kapuas Hulu

How does it affect the soil when tropical rain forest is converted to paddy field or to palm oil plantation? The World Soil Museum is putting together a collection of soil profiles from the Indonesian island of Borneo that will illustrate the differences in land use.

In Indonesia it happens frequently: farmers raze a stretch of  rain forest, fertilize the barren and acidic soil with the ash and subsequently use the ground to grow food crops such as rice or vegetables. The farmers move up a bit every time the ground has been exhausted (shifting cultivation) and the forest reasserts itself on the vacated land. But in some places forest regeneration is not possible because the land has been used for a palm oil plantation.

It seems logical that the soil composition changes through all these stages. The less lush the vegetation, the more susceptible the soil to erosion. ‘In particular, the proportion of organic matter in the topsoil will change,’ says ISRIC researcher Stephan Mantel. To test this theory, he and the Indonesian research institute CIFOR last year excavated soil profiles in central Borneo (Heart of Boneo) beneath lightly disturbed primary rainforest and under deforested and burnt rainforest for shifting cultivation. Over the coming years the partners also plan to excavate profiles from beneath virgin rain forest and palm oil plantations.

The museum already houses soil profiles from East Kalimantan, excavated in the 1980s from beneath primary rainforest and from rainforest that has been chopped down. On show are orange-coloured soils with some grey humus in the upper layer. These are known as Acrisols, clay-rich soils in a tropical rain climate that often support rain forest. The acidic substances excreted by the tree roots erode the clay minerals and coloured iron and aluminium aggregates are formed. As a result of the heavy rainfall, the clay is washed out of the topsoil and settles in the deeper layers. Plantation vegetation such as oil palms are often a better use for such land than food crops such as grain, vegetables or fruit. This is because food crops leave the soil bare for part of the year.

ISRIC-researcher Stephan Mantel helps Indonesian partners digging a profile of a clayey-soil (Acrosol) under shifting agriculture.

‘The profiles don’t always show the differences in the composition of organic material,’ says Stephan Mantel. ‘But we also detect the weight in kilograms of organic matter per cubic metre.’ The researcher predicts that the proportion of organic matter will be greatest below mature rain forest (primary or regenerated) and lowest beneath clear-cut forest and young palm oil plantations. In addition soil management such as fertilization will also (positively) affect the proportion of organic matter.

Knowing the proportion of organic matter in a specific soil is important in climate modelling. The more organic matter, the more carbon in the soil. Soil bonds around a third of all the earth’s carbon and as such it forms a key buffer for the greenhouse gas carbon dioxide. What’s more, soils that are for a large part composed of organic matter are better able to retain water and nutrients. This in turn benefits soil hydrology and promotes vegetation and tree growth.