OVERVIEW OF THE IMPACT OF MINING ON THE ... - IIED pubs

Soil formations across the Zambezi basin reflect the influence of parent rock material, climatic features and
biological activity. The dominant soil types in the basin are acidic, leached out tropical soils of low fertility known
as tropical red soils (ferrasols) that are derived from geologically old or ancient rock types.
More fertile soils are located in areas where younger geological strata form the underlying parent rock or where
humic materials are found. In general, the average fertility of the basin soils is considered to be low (Chenje,
2000). However, several areas of high quality vertisols (such as the type known as “black cotton soils”) are
found in the basin. These form important areas of agricultural activity wherever sufficient water is available and
appropriate fertilizer added.
Deep layers of wind-blown Kalahari sands cover large areas of the southwestern, western, and northwestern
portion of the Zambezi basin. Whilst agricultural production is severely limited in these areas and small-scale
subsistence agriculture seems the only option, these areas are often well timbered with economically important
hardwoods.
The valley-bottom soils of the middle Zambezi region are generally alluvial or colluvial in origin and support
extensive agriculture. This contrasts with those soils on slopes and river valleys immediately to the south and
north of Lakes Kariba and Cahora Bassa; these are fragile, shallow, stony soils with little agricultural potential.
The soils of the lower Zambezi basin are variable and predominantly sandy of sandy loams, overlying
weathered red ferralitic soils, and are considered to be amongst the most fertile in the basin. In the Malawi
sector of the basin, (the Lake Malawi and Shire River drainage system), the soils are deep sandy loams and
extremely fertile (Chenje, 2000).
Large areas of vertisols occur on the floodplains of the Kafue Flats and are usually associated with underlying
basalt and mudstone or shale formations. Similar soils are found on the Zambezi floodplain in Barotseland.
Hydromorphic soils with a high clay content, blocky nature and marked propensity to swell and shrink with
wetting and drying are found in areas subject to seasonal waterlogging. These are the characteristic “dambo
soils” of Angola, Zambia, Malawi, and parts of Zimbabwe. During the rainy season, the water table is located at
or close to the soil surface and these soils are intensively used for subsistence agriculture throughout the basin.
Small areas of dispersive sodic soils are found in areas where the parent rocks have a high sodium content and
are subject to intensive mechanical weathering.
3.1.2 Climatic features
Because of its geographic position, the climate of Zambezi basin is influenced by prevailing wind systems,
including tropical cyclones from the Indian Ocean. One of the most important rain-bearing systems is the Inter-
Tropical Convergence Zone (ITCZ) that moves seasonally along a north-south axis. In the southern portion of
the basin, south-easterly wind systems bring rainfalls from the Indian Ocean (Chenje, 2000).
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