Soil Erosion, Desertification and Farming in Southern Africa
Andrew Thomas and Andrew Dougill
Declining crop yields in African drylands are widespread and often associated with soil
erosion processes. An interdisciplinary case study from Southern Africa shows that soil
chemical degradation on arable fields (soil nutrient depletion or soil acidification) is the
more pertinent concern. Farmer interviews reveal that drought, poverty, links to commercial
farming systems and a lack of casual labour all contribute to degradation. This case study
highlights the relationships between people and their environments. It provides an example of
how combined environmental and social geographic studies enable a deeper understanding of
the processes and causes of an environmental problem.
Desertification is seen as one of the most serious environmental issues facing the world today,
affecting over 1 billion people who depend directly on the land for their survival. The most
widely reported process leading to desertification is soil erosion. The problem is particularly
acute in dryland Africa where the United Nations estimates that as much as 75 % of
agricultural areas are degraded. As over 70 % of Africa's estimated 500 million people are
subsistence farmers, maintaining soil fertility is fundamental to addressing rural poverty and
food insecurity.
Desertification in Southern Africa
On the margins of the Kalahari, a combination of low and variable rainfall, a high dependence
on subsistence agriculture and nutrient-deficient soils make degradation a real threat. Further,
political changes over the last decades have disrupted traditional communal land ownership
patterns, social networks and farming practices. The recent HIV / AIDS pandemic is also
particularly acute in rural communities.
One region where this complex mix of factors is evident is the Molopo River Basin, which
forms part of the border between South Africa and Botswana (Figure 1). Here farmers use a
mixed farming system, keeping goats, sheep and cattle as well as growing crops. This
strategy is designed to reduce the potential impacts of crop failure and/or animal death by
reducing sole dependence on either. The Molopo Basin is characterised as moderately- to
severely-degraded by the United Nations and a national survey in South Africa. The National
Botanical Institute of South Africa highlighted that degradation problems are focused in the
former black communal homelands, established by Apartheid policies.
How can Geographers help?
To understand the complexities of desertification requires identification of the numerous
potential causes and their associated processes (see Inset 2), together with the methods used to
classify its extent and severity. This demands an awareness of both environmental processes
and farmers' decision-making.
Geographers are ideally placed to provide such an
interdisciplinary perspective and our studies have involved both land degradation assessments
and farmer interviews.
Environmental Evidence for Degradation in the Molopo Basin
Questioning the link between erosion and degradation
The World Atlas of Desertification uses indicators as measures of degradation extent and
severity. One widely adopted degradation indicator is the occurrence of trapped sediments
around bushes, which develop to form nebkhas dunes. These can be useful indicators because
they are highly visible and are associated with erosion. The extensive dunes in the Molopo
Basin are aligned with the predominant dry season winds and therefore, show there is wind
erosion and deposition. Analysis of the dune sediments shows that they form through the
local redistribution of fine sands. The nutrient and organic content of dune sands is greater
than fenceline deposits (where there is no vegetation cover) and local fields (see Table 1).
Table 1. Average nutrient concentrations and organic matter in cultivated fields, interdunes, fenceline
deposits and nebkhas dunes in the Molopo Basin.
Sample Environment
Total-Nitrogen
Total-Phosphorus
Organic
(g g-1)
(g g-1)
Matter (%)
Arable fields
186.8
94.4
1.9
Interdunes
253.5
86.0
1.9
Fenceline deposits
144.9
71.0
0.8
Nebkha Dunes
356.2
92.0
2.8
As the wind-blown fence deposits contain only small amounts of organic matter and nitrogen,
the enrichment of the dunes sands occurs after deposition (through inputs from vegetation)
and not because of the selective erosion of nutrient rich sediments. This shows that wind
erosion is not leading to high losses of vital nutrients from the surrounding fields. Thus,
whilst dunes do demonstrate the occurrence of wind erosion, degradation (i.e. a reduction in
the land's agricultural potential) is not occurring because of erosion.
Soil acidification
Although declining productivity is not occurring through erosion, farmers are becoming
increasingly reliant on inorganic fertilisers. These fertilisers lead to a rapid increase in soil
nutrient levels, which can lead to soil acidification (see Inset 2). Soils where inorganic
fertilisers have been added for many years are now characteristically very acidic. Further
problems are also occurring due to very low soil organic matter. Consequently, the need to
replenish organic matter and raise soil pH is essential to prevent soil chemical degradation
and to improve crop yields.
Farmer Case Studies
To understand the underlying causes of declining crop yields, farmers were asked about their
decision-making processes and soil management practices.
The following case studies
highlight the range of threats to and opportunities for sustainable farming.
1. Arnold Lebatse, Mathatheng, South Africa - Poorer farmer suffering soil nutrient depletion.
Arnold has farmed in Mathatheng for over thirty years. Traditionally, Arnold used organic
manure from his goats and sheep to maintain soil fertility and crop yields on his two maize
fields and his wife's garden vegetable plot. After occasional good years, he was able to
replenish soil fertility by adding subsidised inorganic fertiliser.
However, in recent years he has lost most of his smallstock in severe droughts related to ElNiňo events. He also sold some of his goats to meet schooling costs for his children. Restructuring of Government co-operatives also means he no longer has access to subsidised
fertilisers. As a result, he can no longer retain annual nutrient inputs and yields are declining
due to soil nutrient depletion. Recently, he has shifted production to groundnuts after advice
on its nutrient fixation potential and the establishment of groundnut co-operatives. However,
this crop has also failed (due to the low phosphorus content in soils) and he is now
contemplating leaving Mathatheng to seek casual employment in Mafikeng.
2. Christina Moleele, Logageng, South Africa - Wealthier farmer experiencing soil
acidification.
Christina's family has farmed in Logageng, since 1968, when they were relocated from their
ancestral home of Modisa, under Apartheid policies. Christina has relied on annual additions
of inorganic fertilisers to maintain maize and sunflower yields. This stems from her earlier
experience on a commercial farm and also from the limited availability of organic manure.
The lack of manure results from a village decision to stop the nightly paddocking of cattle,
due to the limited availability of casual labour. However, increased soil nutrient levels have
been accompanied by soil acidification. This is leading to stunted crop growth and means
that she no longer earns any net income from farming. Her family income is now retained
from her father's pension and from money her eldest daughter sends from her job in
Johannesburg.
3. Isaac Morule, Mokatako, Botswana - Innovative farmer using integrated nutrient
management to maintain crop yields.
Isaac farms two fields at Mokatako in Botswana. Of 15 farmers interviewed, he is one of
only two who follow an integrated nutrient management strategy, involving both inorganic
fertiliser and organic manure inputs. His yields are dependent on annual rainfall, suggesting
that soil conditions remain suitable for crop growth. Furthermore, Isaac is experimenting
with the use of household waste, ash and groundnut shells. The local Government extension
officer is monitoring his progress to assess the potential for such targeted nutrient additions
elsewhere. This shows the potential for farmer innovators being the initiation of locally
appropriate, and thus more relevant, agricultural extension advice to farming communities.
The three farmer case studies demonstrate the range of opportunities and constraints on
farming in this infertile, dryland landscape. They highlight that less visible processes of soil
degradation (whether nutrient depletion or acidification) are more important than soil erosion.
They also demonstrate the relative roles of drought, poverty, extension advice, social changes
and Government policies in farming. It is only with such a holistic, locally appropriate view
of farming that geographers can offer suitable development advice.
How important is soil degradation to farmers in Southern Africa?
Soil erosion has typically been viewed as the main form of desertification affecting African
drylands. However, our case study questions this link. Studies in the Molopo Basin suggest
that whilst soil erosion is seen by farmers as an inevitable consequence of farming, soil
chemical degradation processes are rapidly becoming a major constraint on farming. Many
farmers are now leaving farming and rural areas, adding to social problems associated with
rural-urban migration.
Interdisciplinary geographic studies offer the opportunity to better understand the nature and
causes of environmental problems. In the Molopo, soil degradation results from chemical
degradation processes and is caused by poverty, drought, links to commercial farming
systems and labour constraints. Improving locally appropriate agricultural extension advice to
smallholder farmers, by using the findings of farmer-led studies is, therefore, an important
initial step in enabling sustainable agricultural-based development of African dryland regions
and reducing Southern Africa's food insecurity problems from within.
Further Reading
Middleton, N. (1999) The Global Casino. An Introduction to Environmental Issues. Arnold
Publishers. Chapter 5 - Desertification.
United Nations Environment Programme (1997). World Atlas of Desertification. Arnold
Publishers.
NBI (2001) National Review of Land Degradation in South Africa. http://www.nbi.ac.za/landdeg/
United Nations Convention to Combat Desertification - http://www.unccd.int/main.php
About the Authors
Dr Andrew Thomas is a Senior Lecturer at the Department of Environmental and
Geographical Sciences, Manchester Metropolitan University. He has been conducting
research on soil fertility and land degradation in Mediterranean and African drylands since
1992.
Dr Andrew Dougill is a Lecturer at the School of the Environment, University of Leeds where
he co-ordinates B.Sc Degree Programmes in Environmental Science and Environmental
Conservation. He has researched land degradation issues in the Kalahari for over ten years.
INSET ONE
DEFINITION OF TERMS
Drylands refer to areas where rainfall is distinctly seasonal and there is a negative balance between
precipitation and evapotranspiration.
Land degradation is defined as a reduction in the productivity of agricultural land and can involve
deterioration of water resources, vegetation or soils.
Desertification is land degradation in drylands.
Drought is often cited as the cause of desertification and has many definitions all revolving around
reductions in mean annual rainfall.
INSET TWO
CAUSES AND PROCESSES OF DESERTIFICATION
Dryland farming will almost always lead to soil erosion. Removal of vegetation cover in preparation
for planting or through grazing, coupled with soil disturbance, increases soil erodibility and the
likelihood of wind and water erosion. Erosion processes are selective, preferentially removing finer
and organic-rich particles. This can significantly reduce soil nutrient levels and adversely affect soil
structure. Dust storms may be a problem in areas where there is a significant amount of wind erosion
and can seriously damage crops.
There are also non-erosive degradation processes. Salinisation occurs when salts concentrate in the
topsoil as evaporative pressures draw moisture to the surface. Acidification is common where
inorganic fertiliser has been applied. If crops can not use all the added nitrogen, soil ammonium
(NH4+) increases and eventually oxidises, releasing nitrate (NO3-) and free hydrogen (H+) ions, which
are responsible for acidity. Bush encroachment is a problem where indigenous woody shrubs and trees
become the dominant vegetation cover on former grasslands as a result of intensive grazing.
Key Points

An estimated 75% of Africa’s agricultural areas are degraded

Around 350 million people in Africa depend on dryland farming for their survival

Erosion is often assumed to be the main degradation process in drylands but this is not always the
case where associated soil nutrient losses and structural changes are minimal

Less visible processes such as soil acidification may be leading to declining crop yields

To understand the causes and processes of land degradation requires an interdisciplinary approach
to investigate, social and economic as well as environmental processes