Soil-forming factor 4: Living organisms
All plants and animals (from microorganisms to humans) affect soil
formation. Living organisms add organic matter - a key component
of soil - through the breakdown of litter and the decomposition of
roots. Microorganisms, such as fungi and bacteria, facilitate chemical
exchanges between roots and the soil to produce essential nutrients.
Both animals and plants allow moisture and gases to seep into deeper
layers along burrows and root channels. Humans can impact soil
formation through land management practices that disturb natural
processes and change the chemical and physical characteristics of
the soil. Cultivation practices and burrowing animals mix soil from
different horizons, especially from the organic-rich surface layers.
The nature of biological activity in the soil is governed by climate,
topography and soil characteristics such as depth, texture, structure
and chemistry (e.g. pH, salinity).
Ecoregions are classified by the presence of biomes, which are
major plant communities determined by rainfall and climate.
Forests, grasslands (including savannah and shrubland) and
deserts are distinguished by climate (e.g. tropical, subtropical,
temperate) and water conditions. In addition, forests are divided
into conifers, broadleafed or mixed.
Biomes
Tropical and subtropical moist broadleaf forests
Tropical and subtropical dry broadleaf forests
Temperate broadleaf and mixed forests
Temperate coniferous forests
Tropical and subtropic grasslands, savannas and shrubland
Ecoregions and biomes
Flooded grasslands and savannas
Ecoregions can be defined as relatively large units of land or
water containing a distinct assemblage of natural communities and
species, with boundaries that approximate the original extent of
natural communities prior to major land-use change. The limits
of ecoregions generally follow continental boundaries or major
barriers to plant and animal distribution (such as the Himalayas
and the Sahara). Africa south of the Sahara Desert, including the
island of Madagascar, lies in the Afrotropic ecoregion. With the
exception of Africa's southern tip, the Afrotropic ecoregion has a
tropical/subtropical climate. The Sahara and northern Africa lie in
the Palearctic ecoregion.
Montane grasslands and savannas
Mediterranean forests, woodlands and scrub
Deserts and xeric shrublands
Mangroves
Lakes
Above: Map showing the major biomes of Africa as defined by the
World Wide Fund for Nature (WWF). The continent is characterised
predominantly by deserts and savannah. (WWF/JRC) [15].
Below: Land cover map of Africa. This map was produced for the year
2000 using data collected by sensors on satellites. (JRC) [16, 16a]
Land cover
The term 'land cover' is used to describe the vegetation
covering the surface of the planet (which can also be
bare ground or unvegetated). It is important to
distinguish between the terms ‘land cover’ and
‘land use’. For example, a land cover of mixed
shrubs and grass could be a park, an orchard
or in an African context, savannah.
This map on the lower-right shows the
principal land cover types of Africa for the
year 2000 as mapped by sensors on-board
satellites orbiting the Earth [16]. The map
shows that the central regions are covered
by extensive tropical forests (dark green)
which merge to the north and south with
open woodland and grasslands or savannah
(brown and orange). In between, the light
green areas denote a mosaic of cropland and
forest that indicates the human alteration of natural
vegetation patterns. With increasing aridity, the
savannah grades into open and sparse grasslands (yellow
colours). Where water is more plentiful, rainfed and irrigated
agriculture exists (pink). These areas are particularly evident in
Zimbabwe, around Lake Victoria and along the Mediterranean,
East African and South African coast. The map clearly depicts the
enormous extent of the vast Sudd wetlands of southern Sudan
(light blue), the Okovango Delta in Botswana and the swamp forest
of the Congo Basin. The distinctive red patches are the evaporative
salt flats and pans of the desert margins.
Land cover
Land-cover classes with
a dominant tree layer
Land-cover classes with
a dominant shrub or grass layer
mixed with agriculture
Dominant agriculture
Closed evergreen forest
Open deciduous woodland
Rain-fed agriculture
Degraded evergreen forest
Closed shrubland with sparse trees
Irrigated agriculture
Closed deciduous forest
Closed shrubland
Swamp forest
Open shrubland with sparse trees
(>65% tree cover)
(5-15% tree cover)
Mangrove
Open shrubland
(<5% tree cover)
Bare rock
Mosaic forest – croplands
Closed grassland
Salt hardpans
Mosaic croplands –
other woody vegetation
Open grassland with sparse trees
Water bodies
Closed deciduous woodland
Open grassland
(>65% tree cover)
(>65% tree cover)
(40-65% tree cover)
(15-40% tree cover)
(5-15% tree cover)
Orchards
(<5% tree cover)
Other land-cover classes
Bare soil
(5-15% tree cover)
(<5% tree cover)
Sparse grassland
Swamp bushland and grassland
Introduction | Soil Atlas of Africa
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