Page 43 - Soil Degradation, Conservation and Remediation

28 1 Soil Resources and Soil Degradation

Fig. 1.4 Preparing land for shifting cultivation (Photo courtesy of late German biologist Dr. Josef
Margraf, with permission from his wife Mrs. Minguo Li-Margraf)

models of future climate, replacing tropical forests with a landscape of pasture and
crops creates a drier, hotter climate in the tropics. Some models also predict that
tropical deforestation will disrupt rainfall pattern far outside the tropics, including
China, northern Mexico, and the south central USA. When people clear the forests,

usually with fire, carbon stored in the wood returns to the atmosphere, enhancing
the greenhouse effect and global warming. Once the forest is cleared for crop or
grazing land, the soils can become a large source of carbon emissions, depending on
how farmers and ranchers manage the land. In places such as Indonesia, the soils of
swampy lowland forests are rich in partially decayed organic matter, known as peat.
During extended droughts, such as during El Niño events, the forests and the peat
become flammable, especially if they have been degraded by logging or accidental

fire. When they burn, they release huge volumes of carbon dioxide and other green-
house gases. When the ground surface is stripped of vegetation, the upper soils
become vulnerable to erosion. Soil is washed into rivers and streams. This destroys
the ability for the land to regenerate because of the loss in soil fertility. It also dete-
riorates water quality. In several parts of the world, entire sections of countries have
been rendered unproductive because of soil erosion.

1.7.1.2 Shifting Cultivation

Shifting cultivation or slash-and-burn agriculture is an ancient farming system usually
practiced by the indigenous people in the hilly areas of the humid tropics. In this sys-
tem, a patch of forest, usually on gentle slopes or on summits of hills, is cleared,
vegetation is slashed and burned (Fig. 1.4 ), holes are dug in soil with elementary tools,