Soil Quality and Soil Productivity in Africa

Hari Eswaran, Russell Almaraz, Paul Reich, and Pandi Zdruli ¹

¹ Director, International Programs, Soil Scientist, Geographer, and Visiting Scientist, respectively. US Department of Agriculture, Natural Resources Conservation Service, PO Box 2890, Washington DC 20013.

Summary

More than four decades of research and development work in Africa have not resulted in the 3-5% annual increase in agricultural growth necessary for most African countries to ensure sustainability of agriculture and the promise of food security in the next decade. The present study evaluates the quality of the soil resource base of Africa and also the risks to sustainable agriculture and soil productivity on a continent-wide basis. Fifty five percent of the land in Africa is unsuitable for any kind of agriculture except nomadic grazing. These are largely the deserts, which includes salt flats, dune and rock lands, and the steep to very steep lands. Though these lands have constraints to sustainability, about 30% of the population or about 250 million people are living, or are dependent on these land resources. About 16% of the land has soils of high quality and about 13% has soils of medium quality. This 9 million km² of land in Africa currently supports about 400 million or about 45% of the people. There are compelling reasons for African nations to return to fundamentals in terms of research and developmental initiatives. Those countries with subsistence agriculture have had minimal inputs in acquiring and managing information about their resource conditions. The green revolution succeeded in Asia in those countries where there was a serious effort to match technology with resource conditions and where advances in development and use of high yielding cultivars was accompanied by appropriate soil, water, and nutrient management.

Soil Quality and Soil Productivity in Africa

Introduction

More than four decades of research and development work in Africa have not resulted in the 3-5 % annual increase in agricultural growth (Badiane and Delgado, 1995) that is necessary for most African countries to ensure sustainability of agriculture and the promise of food security in the next decade. Sluggish or zero growth is likely because of the cumulative effect of many factors but with strong bearings on soil productivity. Agriculture production is not merely the managing of the biophysical resources; it is also strongly controlled by the socioeconomic milieu. The opening of national markets to world trade has induced new stresses in the on-farm socioeconomic situation. The resource poor farmers of Africa have few options today to enhance their agricultural productivity.

The challenge to African governments and the international community is to enhance the farmer's ability to effectively participate in the national and global economy and a prerequisite is the improvement of the productivity of the millions of small farms. The traditional low-input agriculture practiced by many of the farmers in the absence of replenishment of mineral nutrients, is slowly reducing many of the soils to almost inert systems (Stoorvogel and Smaling, 1990). As many of the soils also have low resilience, future corrective measures may be exorbitantly expensive.

The study of Oldeman et al. (1991) indicates that soils on about 5 million ha of land in Africa are degraded to a point where their original biotic functions have been fully destroyed and resilience reduced to such a level that rehabilitation to make them productive may be economically prohibitive. This empirical assessment based on the judgment of many persons and often made in the absence of supporting data, points to the magnitude of the problem. With reliable resource inventories and monitoring of the resource base, better assessments and projections can be made. Such knowledge is as important as helping national planners and farmers to enhance their agricultural productivity.

The purpose of the present study is to evaluate the quality of the soil resource base of Africa and the risks to sustainable agriculture and soil productivity on a continent-wide basis. Similar to other assessments, the task is fraught with difficulties stemming from an absence of reliable databases and almost a total lack of studies evaluating the state of a nation's resources at any time in its history.

Soil Quality

Quality is the essential character, distinguishing feature or property of an object. It identifies that feature which makes the thing useful or perform a task in a beneficial way. Most persons refer to soil quality in a similar way and look for attributes that enable the soil to perform its functions in an acceptable manner. Larson and Pierce (1991) view soil quality as the capacity of a soil to function within its ecosystem boundaries and interact positively with the environment external to that ecosystem. They link soil quality to the four sustainability objectives of Lourance (1990) -- agronomic, ecological, micro- and macro-economic sustainability.

A more formal definition, of Larson and Pierce (1991) and many others is, " the capacity of the soil, as an integral part of the ecosystem to perform the functions of:

• enabling life to thrive in or on it;
• acting as an ecosystem source, sink, and a filter reducing contaminants affecting water and other resources;
• providing the foundation for buildings and other structures, and space for rooting and support for plants;
• buffering the life support system against thermal, chemical, gaseous, and/or other stresses; and
• regulating microclimate through its hydrological function of controlling water flowing over or in it."

Soil quality has emerged as a unifying concept to address the larger issue of sustainability of ecosystems in general and agriculture in particular. USDA Natural Resources Conservation Service (NRCS) is utilizing an "Ecosystems Based Approach" for its technology development and transfer program with soil quality being one of the basic criteria for the many decisions that have to be made with respect to sustainable land management. From the point of land use and land management decision making, soil quality

• provides a quantitative basis for evaluating different land use options and impacts of technology;
• furnishes parameters for quantifying ecosystem interactions;
• evaluates status and impacts of soil conditions over given periods of time;
• provides a basis for targeting conservation programs;
• enables environmental assessment, or specific assessments related to biodiversity, chemical loads, bio-contaminants, etc., as it is a measurable component of the environment;
• serves as a critical parameter for evaluating sustainable agriculture and forestry;
• serves as one of the tools for evaluating conservation programs, levels of compliance, effects of conservation practices, and targeting high risk areas for conservation reserve activities, and
• provides a basis for identifying tension zones and serves as a triggering mechanism for implementation of mitigating strategies (when indicators suggest that critical soil quality thresholds are exceeded or ecosystem collapse is imminent).

As the present study is of a continent, we are focusing on the last item which deals with identifying tension zones for sustainability. Again, due to the nature of the study, it is at best approached in an empirical manner, recognizing that assessment of soil quality is a function of time and space. Soil properties change with time and changes are accelerated through management or mismanagement. Parameters, methods, and threshold values have still to be established to evaluate and/or monitor these changes and the sensitivity of the variables or indicators, which is an important consideration, must be developed and validated. Spatial microvariability is of great importance in soil assessments and one that is not easily evaluated, particularly at small scales.

Defining the need

Countries want many things from their soil and water resource base. The basic elements include food and fiber of high quality and at low price, a healthy environment, a high standard of living, and an improved quality of life. To achieve these goals, decisions must be made about how best to match future demands of food, fiber, water, fuel, and other agricultural products with the needs for a healthier environment. These decisions must consider the social, economic, and political structure of the country as well as the conditions of the natural resources. They must be based on accurate and adequate information, appropriate technology, and a cost-effective delivery system. The new paradigm of sustainable agriculture of AGENDA 21 (UNCED, 1993), calls for a major focus on environment. Pragmatic considerations, particularly in the context of African countries, clearly suggest that attention to the environment will only occur when agriculture is economically viable, with levels of input acceptable to the society, and stable enough to make long term commitments.

Finally, there is the question of technology and its utilization by the resource poor farmers of Africa. Greenland et al (1994) suggest that technologies are not adequately verified, validated, and more importantly, certified for field use. Technology transfer, assuming there are technologies appropriate for the site, has always been a daunting exercise in Africa. Reasons include both the reluctance of the farmer to accept and the mechanisms of transferring technologies to the farmers (Eswaran et al., 1997b). The non-adoption of technologies often arises from the mismatch between the sophistication of the technology and the expectation of the farmer. Many transfers fail because they are not attuned to the socioeconomic conditions of the farmer and the biophysical attributes of the farm. There are very few countries in sub-Saharan Africa with farm-level resource information. National-level information is equally scarce.

Method

The Soil Map of the World (FAO, 1971-1981), digitized and made available by the Food and Agriculture Organization (FAO) is the starting point for the assessment. The office of World Soil Resources of the USDA Natural Resources Conservation Service (NRCS/WSR) has a pedon data base with more than 400 pedons from Africa and this, together with published national soil survey reports, provide the recent information for the translation of the legend of the Soil Map of the World, from the FAO legend, into Soil Taxonomy (Soil Survey Staff, 1996). The FAO soil map of Africa at a scale of 1:5 million has over 6,000 polygons. The FAO soil unit designator for each polygon is systematically converted to a Soil Taxonomy unit and for this process, only the dominant unit in the association is considered. A minimum map delineation size of 25,000 hectares represents the smallest polygon which can be made at this scale without reducing legibility (Soil Survey Staff, 1993).

An associated database of average monthly rainfall and temperature of more than 25,000 stations was assembled and, using an unpublished model of NRCS/WSR, modified later by Van Wambeke (1982), estimates of the soil moisture and temperature regimes (SMR and STR) are made for each station. The basic data set is from Wernstedt (1972). For some countries, this data set was substituted with more recent data where available. Thus the data set is not chronologically uniform. An evaluation was done at a few locations by on-site visits and in most cases by comparisons of the computed SMR and STR with other climatic and vegetation maps. Geographical patterns were the main criteria for assessment, and if within a given area a station has an aberrant rainfall and/or temperature condition, this station was eliminated from the database. Each polygon on the FAO map is then assigned a soil climate code. This is necessary because the Suborder and Great Group categories of Soil Taxonomy include soil climate information. The validation of the final Soil Taxonomy map of Africa, produced at a scale of 1:12 million, is made using the pedon database, published information, and the field experience of the authors.

Eswaran et al. (1997a) used the above approach to develop a soil map and a soil climate map of Africa. The soil name of Soil Taxonomy incorporates many soil properties from which many attributes may be derived (Beinroth et al. 1994). A combination of soil property and soil climate information is then used to make a general assessment of soil quality, employing the principles of land evaluation (FAO, 1976). Soil quality is used in its widest sense ­ the ability of the soil to perform its function of sustaining agriculture. Sustainability is strongly dependent on the level of input determined by the socioeconomic milieu. In this first assessment of soil quality, a low level of technological input is considered as a measure of the inherent capacity of the soil. Increasing the inputs, through management, enables one to utilize the soil at different production levels. The sustainability goal is to operate at a sufficiently high economically profitable level while having minimal negative impact on the ecosystem. In Africa, with the exception of very few countries, this is still theoretically possible.

With this consideration in mind, Eswaran et al. (1997a) developed a map showing the soil potential for sustainable agricultural development. They ranked the lands in Africa as "prime land, high, medium, and low potential land, and unsustainable land" and determined that prime land occupies about 9.6 % of Africa and the lands with high potential occupy an area of about 6.7 %. The resilience (Eswaran, 1994) of the high potential lands is not expected to be as good as that of prime lands and may thus be permanently damaged through mismanagement. The medium and low potential lands, which together occupy 28.3 % of the surface, have major constraints for low-input agriculture. Resource-poor farmers who live on these lands have high risks and generally the probability of agriculture failure is high to very high. The remaining 55 % of the land consists of deserts or other lands with major constraints even for low-input agriculture.

The study of Eswaran et al. (1997a) is used as the basis for the following evaluation of soil quality in relation to sustainability of food production systems. Table 1 gives the framework under which low, intermediate, and high input conditions are assessed for the evaluation of sustainability under different levels of technology. Only the biophysical resources are considered in this assessment. The socioeconomic conditions are not considered because of unavailability of suitable databases. The quality of the soil and the current population density are considered as first level variables that determine sustainability. To arrive at this, the soil quality map was overlain with an interpolated population density map of Deichmann (1994). The total land area in each population density class for the respective soil quality class is then computed. This is used to make the assessment of risk for sustainability. It is important to point out that only the potential for agriculture is considered. It is recognized that there are multi-purpose uses of land and the partitioning of the land for these purposes, such as wild-life sanctuaries, forestry, mining and other industrial uses, or urban settlements and recreation parks is a subject of another analysis, which is best performed for national land allocation programs (FAO, 1976).

Evaluating Risk Zones

Land quality distribution

Table 2 provides the land area in each population density/soil quality class. Fifty five percent of the land is unsuitable for any kind of sustainable agriculture apart from nomadic grazing. These are largely the deserts, which includes salt flats, dune and rock lands, and the steep to very steep lands. The constraints for managed agricultural systems are so great that such lands must be maintained in their natural state. Though some soils in desert areas may have favorable attributes for irrigated agriculture, their quality in this general appraisal is ranked low for two reasons. First the high initial and continuing investments needed to assure the required performance of the soils, and secondly, the rapid build-up of salinity and/or alkalinity that is a continuing problem in this environment. Though agriculture on these lands is considered unsustainable, about 30 % of the population of Africa or about 250 million people are living on or are dependent on this land. A large portion of the people are on the desert margins of the Sahara and Kalahari regions and along the larger rivers that traverse the areas, such as the Nile and the Okavango Delta.

About 16 % of the land has soils of high quality and about 13 % is of medium quality soils (table 2). About 9 million km² of land with these qualities, support about 400 million people. The soils are relatively free of major constraints and rainfall is usually stable and adequate for one major crop. Moisture stress ranges are minimal and when present, confined to the dry season. The zones with adequate rain during the year and generally with a dry season of less than one or two months, have some form of plantation agriculture or are under forests.

The soils with low quality are those which have serious limitations. The limitations include high soil acidity, impermeable layers in the soil, frequent water-logging, propensity to accumulate salts, or some attribute which requires major investments to correct and manage. Such soils occupy about 16 % or 4.7 million km² of land and currently support about 200 million persons (23 %). The quality of these soils may be an inherent property of the land or may be due to human-induced degradation over long periods of time (Oldeman et al., 1991). The present analysis does not make this distinction.

Tables 2 and 3 also show that there are significant areas of good, high, and medium quality soils which currently have low population densities. In principle, such lands offer an opportunity for expansion of agriculture but this has to be in the context of the competing uses mentioned earlier. For example, large areas of Tanzania and Kenya have soils of medium to high quality but these are also the last vestiges of wildlife zones. Competition for these lands by an expanding population will surely threaten the wild-life habitats and national strategic developmental plans, should also ensure that sufficient significance is attached to these other uses.

At the other extreme, there are lands which already have a high population density. Some of these may support higher densities but most will not. The high density areas (tables 2 and 3) may be considered as tension zones for social problems, particularly when natural calamities in other zones attract population to the better quality lands. The recent crises in Somalia, Rwanda, and Burundi, are perhaps illustrations of such processes.

Risk zones

Given the distribution of soils of different quality and the existing population, an important concern, as posed by Conway and Barbier (1988), relates to the level of risk faced by the people with respect to sustainability. For this analysis, risk is considered in terms of the quality of the soil and the level of inputs. This is similar to the approach of FAO (1982) in their study of the population supporting capacity of countries. A low quality soil under traditional low input systems poses a very high risk for the farmer. However, it does not imply that a soil with high quality but under similar low input system, has a lower risk. Such a soil may be initially productive but with mismanagement, productivity may be quickly lost and thus, risk of agricultural sustainability is still high. In this study, risk of sustainability is evaluated in a qualitative way which can be significantly improved when better and more reliable databases become available.

It is estimated that currently, there is about 22% (6.7 million km²) of land which has a low to very low risk for sustainable use. As inputs increase and management is more refined and responsible, an additional 4 million km² of land can be brought into this class. At the other extreme, there is about 54 % (16.6 million km²) of land that has very high to excessive risk. The lands with very high risk (8.2 % or 0.7 million km²) are mostly at the desert margins in the Sahel and along the eastern seaboard of Africa. There is also a large contiguous area in Zaire where the Kalahari sands have penetrated the country. About 200 million people live on these deserts and desert margins and eke out a living with low input systems.

The potential for sustainable agricultural use increases with levels of input, as long as there is a rational use of inputs. Sustainable use is also a function of the population stress on the land and in the following analysis, this is taken into consideration using the the most recent population density information. Defining the levels of inputs in a general manner as in table 1, the potential for sustainable use of the land resources are evaluated and presented in tables 4 and 5 and Figs. 1, 2, and 3. Under current conditions prevailing in sub-Saharan Africa, lands with high outputs are of local importance. Stability of production is generally high (except in the areas with highly erratic rainfall) due to generation-tested techniques of the farmers; the production level is very low and the propensity to degrade the resource base is concomitantly high. Yet with a minimal technological innovations such as providing some fertilizers to the women farmers, about 43% of the land area can be brought into a moderate level of sustainability. If input levels can be enhanced and farmers trained to use these in a judicious manner, about 11 million square kilometers or 35% of the land surface has the potential for highly sustainable agricultural use (tables 4 and 5). With medium to high levels of inputs and with the associated services and facilities, Africa's food security problem could be resolved for a long period.

Resource Management Domains (RMD)

Figures 1, 2, and 3 show the spatial distribution of the lands as a function of risk to sustainability. These maps depict areas which are thought to be most responsive to development and areas which will require different kinds of mitigating technologies to reduce degradation and enhance sustainability. These kinds of maps, or more detailed maps of regions and countries, have important applications in designing research and developmental strategies when combined with the socioeconomic characteristics of the people living on these lands. If these continent level maps are to be used for research and developmental thrusts in Africa, each of the classes represents a resource management domain (RMD). A RMD, in its simplest form, is a unit of land that has similar management requirements for the same kind of land use and by extension, would need similar research and developmental initiatives. Fig. 3 can be envisaged as a goal for African development, with Fig. 1 representing the current situation.

For this approach to be applicable to national decision making and to assist donors in promoting meaningful developmental programs, each country should undertake an assessment of its natural resource endowments. A soil map of the country is the basis for making such an assessment. Assigning the quality of the soils to their productivity level under defined levels of management must be done realistically, taking into account not only the socioeconomic conditions of the regions but also a realistic appraisal of the potential of the soils. Often national scientists, not privileged with wide experience gained through international travel, may not be aware of the real potential of the soils. With availability of computer technology there is a tendency to use complex models which eventually prove unnecessary or misleading because they do not have the supporting data, or have not been validated for the local situations, or were developed for a totally different purpose. Pragmatic approaches relying on the experience of the field scientists and farmers provide suitable information. However, the need to continually develop good, reliable databases cannot be over-emphasized. As good data come on-line, the approach can be made more sophisticated.

The RMDs can be as broadly or as narrowly defined as determined by the objectives. RMDs are geographic areas with utilitarian purpose and have natural physiographic boundaries. However, a RMD for a national fertilizer program may have political units as entities if these provide the most efficient way to distribute fertilizers. A sorghum growing area may form a RMD by itself or may be subdivided into sub-RMDs if these can be defined with specific management criteria. The classes of soil quality, as used in this study, may serve as a first delimitation of RMDs in a country. Within a given soil quality RMD, sub-zones may be defined based on cropping systems or management systems.

Concluding Remarks

There are compelling reasons for African nations to return to fundamentals. It is not a coincidence that every country with highly productive agriculture also has a long tradition in resource assessment programs with continuing research and development work. Those countries with subsistence agriculture have had minimal efforts in acquiring and managing information about their resource conditions. The path to sustainability taken hitherto by many sub-Saharan countries, partly lured by the funds supplied by international financial institutions, and credibility ensured by the self-serving interests of donors supporting international agricultural research has been one with pitfalls and many times have not delivered the dramatic expectations envisaged. The "green revolution" had minimal impact in Africa and one reason for this stems from the assumption that the formula for Asia is applicable to Africa. The green revolution succeeded in Asia in those countries where there was a serious effort to match technology with resource conditions and where advances in development and use of high yielding cultivars were accompanied by appropriate soil, water, and nutrient management. It is being realized by African agriculture that high yielding cultivars alone is not the path to sustainability; research and development must be holistic and structured on expected risk levels and based on criteria such as those in table 6.

Borlaug and Dowswell (1994) state that agriculture in sub-Saharan Africa, more than in any other part of the world, is in crisis. The low-input low-output systems of agriculture which maintained Africa at subsistence levels is no longer able to feed the people. In addition, there are the associated problems of land degradation accelerated by low-input systems which in some instances has exceeded the resilience threshold of soils. Naturally low quality and human-induced low quality soils now characterize much of the African landscape, however there are areas where high levels of productivity are still possible.

The challenge of African agriculture is not only of enhancing production to meet the increased food demands of the expanding population, but also the judicious use of soils so that their productivity is sustained in the foreseeable future. This study shows that continent-wise 55 % of land area in Africa is unsuitable for agriculture and 16 % of land area has high quality soils which can effectively be managed to sustain more than double its current population. These soils are spread among many countries making it difficult to develop a continent-level strategy to equitably help all countries. Africa has more than 8 million km² of land with rainfed crop potential, however much of it has not been used for this purpose. This potential land reserve needs to be carefully evaluated so that rational policies can be developed for their exploitation. A large part of the potential land reserve is currently under forest but is increasingly being subject to slash and burn agriculture. The fact that there is a potential land reserve should not lull decision makers into reducing research and developmental activities for other parts of the country. Enhancing effective land area by increasing cropping intensities instead of increasing land area under cultivation is an important policy consideration. Reducing population pressures on stressed ecosystems and keeping marginal land as 'set-aside land' for nature development, are also policy concerns that affect intergenerational equity.

The challenge of enhancing the productivity of well endowed lands and reducing the pressures on the fragile ecosystems is the solution to putting Africa on the path to sustainability development. With appropriate capital infusion and support services, the efficiency of resource-poor farmers of Africa can be raised and the seeds to another Green Revolution sown.

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