Soil Erosion, Desertification and Farming in Southern Africa

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
(g g-1) / Total-Phosphorus
(g g-1) / Organic
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 El-Niňo events. He also sold some of his goats to meet schooling costs for his children. Re-structuring 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.

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.

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.

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.

Droughtis 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