HUMAN INTERFERENCE WITH SOILS 2 1


Dense forest vegetation intercepts heavy rainfall that would otherwise erode the soils.


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Trees transpire moisture back to the atmosphere as water vapour. This transpired water vapour supplies the ‘raw material’ for more convectional rain.


Terms to know 2


Trees provide shade for smaller plants and for animals. Plants and animals enrich the soil with vast quantities of organic matter.


l Deforestation: clearing forests without planting new replacement trees.


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Shallow-rooted trees soak up vast quantities of groundwater and dissolved plant nutrients. This nourishes the trees and reduces leaching.


l Overgrazing: excessive grazing that causes land to become damaged.


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The hot wet climate causes organic matter to break down rapidly into humus.


l Overcultivation (overcropping): excessive cultivation that causes land to become damaged.


Some of these ‘outsiders’ are land-hungry peasants who, with the


blessing of Brazilian governments, have tried to set up farms in the Amazon Basin. Some are commercial loggers who have torn through the forest in search of teak, mahogany or other hardwood trees for export. Others were businessmen who received World Bank aid to take part in massive settlement and farming projects such as the Polonoroeste Project in the state of Rondonia (see Figure 1). Many were wealthy cattle ranchers who cleared vast areas of forest in order to supply American fast-food outlets with cut-price beef burgers. Some are miners in search of the high-grade iron ore, copper and gold that lie beneath parts of the Amazon Basin. The activities of all these ‘developers’ have contributed to the clearance of


an astonishing 40 hectares of forest per minute. This massive deforestation causes soils to become more eroded and less fertile: l When trees are cut, soils become exposed to the full force of equatorial rain. The rain sometimes erodes soil by sheet erosion, which is the even washing away of the soil’s fertile upper layer.


l Gully erosion is even more common. This happens when rivulets of surface water cut deep grooves into the land.


l Without trees to soak up groundwater, leaching increases rapidly. More and more soil nutrients are then washed down below the reach of plant roots.


l The absence of trees deprives the soil of the plant litter that it needs to remain fertile.


l The ‘development’ of the selva has also resulted in the severe pollution of soil and groundwater. Pollutants range from household rubbish to deadly mercury that is used in gold mining.


l Convectional rain: usually short periods of heavy rain; associated with hot air rising from the earth’s surface.


l Biodiversity: the presence of a wide range of plants and animal species.


l Transpiration: the release of water vapour from the surfaces of plants into the atmosphere.


l Amerindian: American ‘Indian’; people indigenous to the Americas.


l Ecosystem: an interlinked system of living things and their natural environment.


l Desertification: the spread of desert conditions into new areas, usually following the erosion of exposed topsoil.


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How Amazonian soils depend on the forest for their fertility


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