Mapping the Range of Proven Soil Management Practices in Africa

Using Digital Maps and Case Studies

by Peter H. Freeman

Development Ecology Information Service

 

September, 1999

Development Ecology Information Service

619 Upland Place, Alexandria, VA 22301, USA

703 836 0058 hansfree@aol.com

Based on a report prepared for USAID using DEVECOL/Africa digital information

resources. DEVECOL/Africa is an information resource under preparation by

Development Ecology Information Service with support from USAID’s Africa Bureau

and Bureau for Humanitarian Relief. DEVECOL/Africa will be installed in the Food

Security Resource Center of the Food Aid Management Consortium and will be

distributed as a CD-ROM in 2000.
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

Contents

Abstract ..................................................................... 1

Effective soil conservation and restoration practices ................................... 1

How can the diffusion of soil and water conservation practices be assisted? ................ 2

Using maps to find the potential range of soil and water conservation practices ............. 3

Digital Maps as Information Resources ............................................. 3

Information in the base maps ............................................... 4

Information on soils ..................................................... 4

Information on agroclimate ................................................ 6

The Case Studies .............................................................. 6

Analysis of the case studies in relation to mapped soil and climate patterns ................ 7

Tahoua region, Niger ..................................................... 7

Yatenga Plateau ........................................................ 11

Extrapolating the range of the practices ............................................ 13

Agroclimate factors ..................................................... 13

Terrain and soil factors .................................................. 15

Extrapolation ................................................................ 16

Conclusions ................................................................. 19

ANNEX 1 

NOTES on the CD-ROM Digital Soil Map of the World ........................ 22

Annex 2 

Average values for two classes of African soils, calculated from soil profiles ........ 24
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

Figures

Fig. 1. Sample base map ........................................................ 5

Fig. 2. FAO soils map of Africa .................................................. 5

Fig. 3. Location map ........................................................... 7

Fig. 4. Tahoua-Maggia Valley area, Niger ......................................... 10

Fig. 5. Yatenga Plateau base map ................................................ 12

Fig. 6. Yatenga Plateau area with soil map ......................................... 12

Fig. 7. Length of growing period zones and planting pit case study sites .................. 14

Fig. 8. Potential range of soil water conservation with planting pits ..................... 17

Fig. 9. All eutric regosols, petroferric phase, within the 75-day to 120-day LGP Zone ....... 18
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

1

Mapping the Range of Proven Soil Management Practices in Africa

Using Case Studies and Digital Maps

by Peter H. Freeman

Development Ecology Information Service

Alexandria, Virginia

Abstract

Soil, terrain and climate factors related to the outcomes of soil and water conservation

practices in Sahelian Africa were examined using GIS programs, digital maps, and case

studies assembled in a single information resource. Documented experiences in soil and

water conservation in Niger and Burkina Faso were examined against a backdrop of these

maps, specifically base maps derived from the Digital Chart of the World, the

FAO/UNESCO Digital Soil Map of the World, and the FAO Agroecological Zonation

Map (length of growing period zones). Use of GIS programs to display the recently

digitized maps makes it feasible to use analytical procedures that would be impractical

using the same materials in paper format. The analyses helped identify at a

reconnaissance level the potential geographic range of one of the proven water harvesting

techniques now being adopted spontaneously on degraded upland soils in Burkina Faso

and Niger.

Effective soil conservation and restoration practices

In the drier regions of Africa, many externally funded projects were launched in the late 1970's

and 1980's to deal with the environmental degradation triggered by drought and the extension of

crop land onto areas of fragile soils. Outside influences or “change agents” combined with

material support(food, tools, etc.) served to stimulate adoption and spread of practices.

Now many sites in Africa where soil conservation or restoration practices have been proven

effective, in both research and actual practice, have been identified and documented. Some

practices, such as demi-lunes, bunds, and planting pits (zai in Burkina Faso or tassa in Niger),

have yielded impressive results in terms of increased crop production on degraded or

unproductive land and are spreading spontaneously or with technical assistance.

This paper examines several practices introduced into degraded uplands, and the potential

geographic range of those that have proven effective in restoring soil productivity. The purpose

of the exercise was to inform strategic planning within USAID’s Africa Bureau aimed at finding

ways to assess the ultimate range and the impact of such practices, eventually leading to

appropriately designed assistance.
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

1

Shaikh, Asif et al. 1988. Opportunities for Sustained Development in Africa. Vol. II. Case

Descriptions. Washington DC: US Agency for International Development ;Critchley W., Chris Reij,

and A. Seznec. 1992. Water Harvesting for Plant Production. Vol. 2. Case Studies and Conclusions

from Sub-Saharan Africa. Washington DC: World Bank (World Bank Technical Paper 157); Veit, Peter

G. 1995. Lessons From the Ground Up; African Development that Works. Washington, D.C.:World

Resources Institute. 75 p.; Reij, C., I..Scoones and C. Toulmin (eds). 1998. Sustaining the Soil.

London:EarthScan.

2

How can the diffusion of soil and water conservation practices be assisted?

Case studies, many published in the last ten years or so1, document the successes or failures of

soil conservation in different parts of Africa. They reveal the myriad factors and influences that

come into play in the eventual adoption and spread of different techniques. They provide the

human and socio-economic context as well as environmental backdrop for considering the

extension of a particular technique, such as a demi-lune, a stone bund, a terrace or a combination

of techniques. Usually undertaken to inform policies and strategies, these studies have not been

 examined on a regional or continental scale against backdrop of soils and climate information

and as a consequence do not attempt to identify other areas where practices might be applied.

Rather, they focus attention on various conditions and factors that contribute to a practice’s

success or failure, without reference to their potential geographic range.

Among the many factors influencing the choice and outcome of soil and water management

practices, physical environmental conditions are paramount, i.e. climate, terrain features(if hilly,

flat, subject to flooding etc.), and type and status of soils. The farmer must work directly with

physical environmental resources – the climate and soils – of his land, and in management of

soils, hours of hard work and skillful application are indispensable. Terrain can be altered to

some extent, e.g. terraced or drained, if soil productivity and rainfall justifies the effort. Soils can

be made more productive or restored, but only to the extent determined by their origins and state

of development. The many hours of hard work invested must be rewarded in better yields.

Differences in climate, terrain and soils will obviously determine what techniques or practices

should be attempted. The same technique applied to different soils in the same climate will have

different results. By the same token a soil may be so extensive as to occur in a range of climates,

e.g. the case of arenosols in the Sahelian zone, and the effectiveness of a particular method

developed for one arenosol may not be attained on another with a different climate. In

considering strategies for extending soil and water conservation practices, one should first

examine the likelihood that there are climate/terrain/soils combinations in other places that are

comparable to those where effectiveness of practices has been documented. 

By combining our growing knowledge of what is working well for African farmers, revealed

through case studies with our knowledge of African soils and climate revealed on continental

maps, we should be able to locate and map, at least on a reconnaissance level, those physical

environments where effective, documented techniques could be further extended. 
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

2

FAO. 1996. Digital Soil Map of the World and Derived Properties,(Version 3.5, Nov. 1995);

1994. Digitized Maps of the Major Climatic Divisions and Lengths of Growing Period Zones for the

Developing World(Release 1.1).. Rome: Food and Agricultural Organization of the United Nations. 

3

Freeman, Peter H. 1998. The use of GIS for Development Information Management in Africa. 

Revised version of unpublished paper presented at the 1st International Conference on Geospatial

Information in Agriculture & Forestry, Lake Buena Vista, Florida, 1997. 

4Attributes refers to information about a given map object, such as a bounded area representing a

soil unit, that is contained in a table linked to the map object. 

3

Using maps to find the potential range of soil and water conservation

practices

Maps of soils and agroclimates of Africa prepared by FAO can now be displayed over base maps

using GIS programs. Prior to 1995, the maps were available only in limited number as hard

copy. They have been digitized by FAO and are available on CD-ROM.2 The base maps now

available as the Digital Chart of the World became available around 1995. These maps and sites

have been assembled, along with base maps prepared from the Digital Chart of the World, within

a map-based information management resource (DEVECOL/Africa)3. Numerous case studies

documenting effective soil, water, agroforestry and other environmental resource management

experiences have been geo-referenced and included in the DEVECOL/Africa information

resource. The analyses which follow are based upon the examination of these maps and case

studies.

The analytical question posed by this exercise was: 

Can available digital maps of soils and climate (and the associated “attribute”

information4 ) help to identify soil/climate environments where known soil and water

conservation (SWC) practices could be effective and could be promoted?”

Practices in two different regions were examined – the Yatenga Plateau area in Burkina Faso and

the Tahoua area in Niger. Exemplary SWC practices, notably the use of planting pits, have been

documented in both areas. The soils and climate of the two regions are different, however. 

Digital Maps as Information Resources

The technique of composite overlays, using transparent materials, has been employed by

geographers and others for many years in the spatial analysis of multiple factors in the landscape.

Now the same technique is greatly facilitated by GIS programs, which can simultaneously

display and manipulate maps compiled at different scales. The information contained in the

maps and associated to the mapped areas becomes a much more accessible resource than in its

hard copy format. The various digital maps described next have been developed for display in

vector format in the DEVECOL/Africa information resource.
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

5

 Africa Data Sampler; a Geo-referenced Database for all African Countries. Washington D.C.:

World Resources Institute. 1996

6

FAO-UNESCO. 1977. Soil Map of the World 1:5,000,000; VI, Africa. Rome: Food and

Agriculture Organization of the United Nations. 299 p. plus 3 map sheets.

7 A digital map displayed in raster format, displays information in the form of square cells

arrayed on a grid. The ground dimension of the cells varies according to the detail of compilation or

precision of the information attached to each cell. 

4

Information in the base maps

Digitized from 1:1 million scale aeronautical charts (Operational Navigation Charts) these maps

include 1,000 foot contour lines, spot elevations, and streams, all indicative of general terrain

characteristics, as well as roads and towns or cities, and numerous other details contained in over

25 different “layers” or databases (Fig.1). The DCW datasets for Africa were packaged by the

World Resources Institute on a CD-ROM produced in 19965 that also includes politicaladministrative boundaries and their populations and major conservation and protected areas. 

These databases facilitated the geo-referencing of sites with documented SWC practices and

served as the base map for displaying soils and agroclimate information. The base map

information, when shown “underneath” information on soils or climate, allows the user to

navigate to different places with ease as well as visualize terrain features not revealed in the soils

map. 

Information on soils

Maps in vector and raster format were created by FAO from the 1:5 million scale FAO/UNESCO

soil map of the world6 and packaged on a CD-ROM (Fig 2). Each map unit is an association of

soils, normally consisting of two or more classes of soils, e.g. arenosol, although some contain

only one class. Each unit is designated by a unique code beginning with the dominant soil, i.e.

the soil class that occupies a greater extent than any other soil in the mapped unit. In most

instances a descriptive qualifier provides additional information on the soils’ physical or

chemical nature, e.g. luvic arenosol(clay accumulation in the lower horizon). Also, in some

instances a phase of the soil association is identified, e.g. if stony, if presence of hardpan or

indurated layer, if presence of a high water table, etc. For the dominant soil and second most

extensive soil, information is also coded for texture and slope. These maps can be displayed in

vector format .

In addition to the digital soil maps, the CD-ROM contains a number of databases with

information on mapped soil units(i.e. soils associations) and soil classes(See Annex 1).

Information on physical and chemical characteristics of soils was obtained from 1700 soil

profiles. The data were statistically analyzed and applied to the raster version of the map7. The

raster cells are squares 10 km on a side (5 arc minutes). The raster version of the soils

information was not used for the analyses reported in this paper.
Fig. 1. Sample Base Map. Elevation ranges derived from Digital Chart of the World files

reveal the pattern of uplands(darker shade) in Burkina Faso and neighboring Mali. 

 Source: FAO. Map by Peter Freeman, DEVECOL, 1999

Fig. 2. FAO Soils Map of Africa.

The digital version of the 

FAO/UNESCO soil map of the

world has been made available on

CD-ROM by FAO’s Agriculture

and Land Division in 1996. This

small scale map groups the 167

distinct soil classes for display at

smaller scales. At larger scales

the boundaries of the individual

classes are displayed. 
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

8

FAO. 1978. Report on the Agro-Ecological Zones Project; Vol. 1 Methodology and Results for

Africa. Rome: Food and Agriculture Organization of the United Nations. 158 ps.

6

Lastly, the CD-ROM contains data for creating maps that categorize soils according to their

suitability for grain crops, beans, and for irrigation. A table with the percent of the mapped soil

association suited to these different purposes was calculated and the maps can be displayed in

vector format. 

Limitations of the FAO soils association map for this kind of exercise are several. The soils on

old landscapes in Africa show great heterogeneity within a small area. Many practices are

tailored to local catena conditions(foot slopes, mid slopes, top slopes); soil texture, type of clay,

and micro-relief are important determinants of the capacity of soils to hold water and nutrients.

None of these are revealed by the mapped soils associations. Other limitations as well as

advantages are further discussed at the end of this paper. 

Information on agroclimate

FAO recently digitized the agroecological zone maps compiled almost 20 years ago8 at the 1:5

million scale, as an input to the study on population supporting capacity in the developing world. 

Rainfall and temperature during the growing period were used to identify zones with different

numbers of days suited to rainfed grain crop production, or length of growing period (LGP)

zones. Length of growing period (LGP) is defined by FAO as the total number of days in a year

during which rainfall exceeds one half potential evapotranspiration(Figs. 7 and 8). The

agroecological zone maps also distinguish a number of temperature regimes during the growing

period. The temperature regime of the entire region examined in this study corresponds to a zone

with temperatures greater than 20EC during the growing season. Therefore the mapped

temperature regime is not a geographic variable used in this exercise.

These agroclimatic interpretations used data available as of the late 1970's, and don’t average in

the drought years of the 80's and 90's. They are rather imprecise(due to the small scale of

compilation) for extrapolating local experiences. Also rainfall is variable in the drier areas. 

However, keeping in mind the inverse relationship between variability and amount of rain,

especially at 500 mm/year or less, maps showing average values, such as the FAO

agroecological zones map, can provide a useful overview of roughly similar agroclimates. 

The Case Studies

A major feature of the DEVECOL/Africa information resource is the assembly of a large number

of site-specific, documented experiences relating successes, and sometimes failures, in soil and

water conservation, forestry, agroforestry, biodiversity conservation and more. They are georeferenced and accessible in the form of a database record as well as in full text in the

DEVECOL/Africa information resource. 
Fig. 3. Location Map showing the extent of the two types of soils associations 

 and the case study sites.

Associations with Luvic Arenosols as the dominant or only soil are shown in grey. Eutric regosols with ironstone (lateritic) crust are

shown in orange. The associations with the cases analyzed are shown in slightly darker tones.
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

9

Shaikh,Asif, E.Arnould, K.Christophersen, R. Hagen,J Tabor and P. Warshall. 1988.

Opportunities for Sustained Development: Successful Resources Management in the Sahel. Vol. III, Case

Studies.

10 Reij, C., I.Scoones and C. Toulmin (eds). 1996. Sustaining the Soil. London:EarthScan.

11Hassan, Abdou. 1996. Improved traditional planting pits in the Tahoua Department, Niger; an

example of rapid adoption by farmers. In. Reij, C., I.Scoones and C. Toulmin (eds) Sustaining the Soil.

London:EarthScan.

8

Several cases were examined in depth. These were documents describing various soil restoration

and water harvesting techniques on the Yatenga plateau of Burkina Faso and in the Tahoua

region of Niger and published in two volumes: Opportunities for Sustained Development:

Successful Resources Management in the Sahel ( referred to hereafter as the “Opportunities

study”)9 and Sustaining the Soil 10 . The first, published in1988 for USAID, includes 70 case

studies of various kinds and scales of renewable natural resources management in Mali, Niger,

Senegal, and Gambia. The second published in 1998, records the results of over 25 studies of

soil and water conservation in Africa, focusing on indigenous techniques or farmer adaptations of

introduced practices.

Analysis of case studies in relation to mapped soil and climate patterns.

Several map views were generated to allow analyses of case study sites against the soil map

background and associated base map information. Two areas were examined in depth (see

location map, Fig. 3). The first, in the Tahoua region, is shown in dark grey on the location map

and corresponds to an association in which luvic arenosols are the dominant soil.(Characteristics

of these soils are described below.) Other luvic arenosol associations with slightly different

composition or terrain are shown in light grey. Four case study sites are located on this soils unit;

two are examined in this paper. A third case study reviewed here documents practices on the

Yatenga Plateau, on degraded lateritic soils(eutric regosols), shown in a somewhat darker brown

color than other eutric regosol mapping units.

Tahoua region, Niger

The case studies summarized below record the results of soil restoration practices supported by

foreign assistance projects in two areas southeast of Tahoua and found within a single FAO soil

mapping unit (Ql12-2ab)(Fig 4). Most notable are the planting pits, or tassa, and demilunes(crescent shaped bunds on sloping land), supported by an IFAD project in the Illela district11

in the southeastern part of this soil map unit. They have been spreading since the drought year of

1990, when their effectiveness in achieving better yields methods was observed. By 1995, an

estimated 6000 hectares of degraded upland soils in the Illela district had been treated with

planting pits and planted in millet. The tassa are also being used to restore degraded uplands in to

the north of Illela District. In recent years USAID technicians in the field have
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

12 Personal communication with Mike McGahuey, USAID, Africa Bureau, Washington D.C..

9

noted the spread of tassa on degraded soils along the road to the south and southeast of

Badaguichri12 within the same FAO soils association.

When the soil mapping unit within which these experiences are located is displayed against the

base map its occurrence in uplands over 1,000 feet above sea level is apparent. The uplands of

the eastern portion of the Illela district where this project is active fall within the soil association. 

The association also embraces the Keita district experience, described later.

Tassa are shallow pits approximately 30 cm wide and 10 to 15 cm deep dug on crusted soils of

gently sloping or flat areas at the rate of 10,000 to 15,000 per hectare. On sloping areas,

excavated materials are placed on the lower edge. The pits harvest runoff from the surrounding

crusted soil, and capture rain- or wind-driven silt and organic matter to the benefit of millet

planted in the pits. Germi-nating millet in the pits is also protected from the dessicating or cutting

action of the wind.

Millet yields in two years (1993, a year of low and irregular rainfall and 1994, a year of “good “

rainfall) were recorded on 470 farms in the Illela district with fields that were prepared with tassa

and demi-lunes. Yields were measured from a variety of treatments: no treatment, tassa only,

tassa with manure, and tassa with manure and superphosphate fertilizer. Technical analyses of

the soils were not reported. Yields in plots treated with tassa were triple those of untreated areas. 

In 1994 tassa- treated plot yields were almost double those in 1993. Yields of millet ranged from

400 kg/ha (tassa only in 1994) to almost 1000 kg/ha (tassa + 0.5 kg manure). Triple superphosphate in 1994 further boosted yields to almost 1500 kg/ha. Millet yields from plots with

demi-lunes were comparable to those of tassa-treated plots.

Northeast of the Illela district in the Keita district, an Italian government-funded effort has

supported a number of techniques to treat the plateau surfaces and watershed slopes in Keita

within the same soils association. The “Opportunities” case study team, which included a soil

scientist, documented the nature of the terrain and soils. The upper watersheds in Keita consist

of hills and side slopes with limestone-derived soils having high water holding capacity, high

fertility, and high pH. Undertaken in the late 1970's as an integrated rural development effort,

the project sought to treat plateau areas, slopes descending from plateaus and hills to the valley

areas, and the sandy soils of valleys. A watershed approach was followed, with treatment

beginning at the higher elevations. 

Trenches 3 meters long, 0,6 meters deep and 0,6 meters wide were dug by hand on these slopes

at the rate of 600-800 per hectare and planted with native and exotic trees. Notwithstanding the

scant 300 to 350 mm/year rainfall, the captured run-off helped trees grow to 2.5 meters in 18

months. On the gently sloping, hardened plateau surfaces, tractors were used to rip the crusted

soils and build contour bunds, in order to slow and harvest runoff. Similar methods were used to 
Fig. 4. Soil association boundaries (dark brown) superimposed over the base map of the Tahoua region. A number of soil and water

conservation case studies have been documented within this soil association. The dashed grey lines are district boundaries; thin red

lines are roads. Dashed brown lines are drainage ways.
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

13Ouedraogo, M. and V. Kabore. 1998. The Zai, a traditional technique for the rehabilitation of

degraded land in the Yatenga, Burkina Faso. In: Reij, C., I.Scoones and C. Toulmin (eds) Sustaining the

Soil. London:EarthScan. 

11

build contour bunds on valley soils. In both places, surveying preceded bund construction. The

results for millet yields on these soils were not recorded in this case study, but it was observed

that farmers refused to buy triple superphosphate for these soils due to the risk posed by low

rainfall. Donors have ceased supporting works built with tractors since they weren’t kept up.

The soil map unit within which these two experiences are located differs in important ways from

adjacent luvic arenosols. It is an association of three soils with only 50% being luvic arenosol.

This is an inherently low fertility sandy soil with clay accumulations at depth. The rest of the unit

is composed of regosols(30%) and lithosols(20%). Regosols are poorly developed soils that

reflect the nature of the subsurface parent material(rock or other) from which they developed. 

Lithosols are very thin, rocky soils. 

The Tahoua soil unit is characterized by the presence of a phosphate-rich limestone - the basis for

nearby Tahoua’s phosphate fertilizer plant. This is not the case for adjoining luvic arenosols

which have developed over Continental Terminal rocks: sandstone, shale, marl, sand and clays

according to the FAO monograph. Thus, the soils of this unit that overlie limestone or have

developed over limestone-derived deposits (probably the regosol portion of the map unit) are

benefitting from the phosphate as pointed out in the “Opportunities” case study on the earlier

Italian-funded soil and water conservation effort. This may explain the good millet yields in the

tassa-treated land of the Illela district, but it is not mentioned in the Illela case study. 

Because this soil map unit is unique owing to the presence of P-rich limestone parent material,

the yields or tree growth noted in the case studies may not be attainable on similarly classed soils

developed over less fertile materials. However, a search, using GIS techniques of “all regosols or

lithosols developed over phosphate- rich limestone” is not possible. The information on

lithology(rock at the surface) of the map units is not contained in the database linked to the

digital soil map.

Yatenga Plateau

The planting pits, or zai, which are reported to have spread over the past ten years on this plateau

after being promoted by the Oxfam/UK funded Projet Agroforestier, are now well documented13,

and have become something of a model(Fig. 5). The area was visited by farmers from the

Tahoua region, under the aegis of the IFAD project cited above. 

OXFAM/U.K. initiated soil restoration actions on the plateau almost ten years ago, and the

techniques evolved to the presently used zai, or planting pits, which harvest runoff from the

surrounding hardened ground as well as fine materials, including organic materials, deposited by 
Fig. 5. Uplands over 1,000' in elevation are clearly seen in this view of the Yatenga Plateau area

prepared from the Digital Chart of the World files. Plains and valleys below 1,000' are light yellow.

Fig. 6. Zai are reclaiming degraded plateau soils in Yatenga province that are classified as eutric

regosols with lateritic surfaces (shown in orange). The FAO map of these soils was originally

compiled on a 1:5 million scale base. Shown over the 1:1 million DCW base map the soil

boundaries do not closely follow plateau borders. If compiled over the more detailed base map the

map unit boundaries would probably be more congruent with the terrain patterns.
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

14 Konate, M. 1984. Climate of the Sorghum and Millet Cultivation Zoness of the Semi-Arid Tropical

Regions of West Africa. In: ICRISAT.Agrometeorology of Sorghum and Millet in the Semi-Arid Tropics;

Proceedings of an International Symposium, ICRISAT Center, Patancheru India, November, 1982. Pages 101-113.

13

wind as well as water action. Sorghum or millet planted in the pits provides yields of 500 kg/ha

to 1000kg/ha depending upon rainfall. Varieties planted and nature of the soil, other than its

degraded state, were not mentioned. 

The soil association on this plateau is characterized as follows: Eutric Regosol, petroferric phase,

coarse texture, flat terrain (Fig 6). This signifies a soil with a cuirass or laterite, on the surface or

near the surface. These soils have developed from Pre-Cambrian metamorphic rocks: quartzite,

schists and – notably – greenstone (metamorphosed basalt),according to the FAO monograph. 

Greenstone contains the calcium-rich mineral epidote, among others, which may explain the

“eutric” descriptor (that signifies high base saturation.) applied to the regosol. This is probably a

factor in the productivity of areas that have been reclaimed using planting pits.

. 

Extrapolating the range of the practices

Areas with terrain, soils, and agroclimates generally similar to the uplands where the successful

practices are reported were displayed and examined, using GIS.. The uplands areas of this part of

Sahelian Africa are extensive, and the soil maps and agroclimate maps allows a selective view of

those portions where soils and climate are similar. Several qualified interpretations can be made. 

The soils of the Yatenga plateau and those of the Tahoua uplands are similar in that both are in

flat to rolling upland areas, both are reported to be degraded and manifesting a hardened surface,

and both have areas that had fallen into disuse before being recovered by planting pits. Also both

areas are typified by regosols. The two soils are very different in their origins and their climates,

however. 

Agroclimate factors

The length of growing period map superimposed over the soils map shows that the two soils

areas with documented planting pit sites are not in the same agroclimate (See Fig. 7). The

Tahoua region, with less than 90 days of growing period, is much drier than the Yatenga plateau

region which is largely within the 120-150 LGP zone, although the case study area is at the dry,

120-day LGP, limit of the zone.

This FAO agroclimatic map is of limited use for extrapolating the yields reported for areas

treated with water harvesting techniques. Rainfall is highly variable in this part of Africa, with a

coefficient of variation 20 to 25%14. In the Illela district, where tassa have proven so effective,

rainfall averaged 368 mm/yr during 1980-87 whereas the 1950-89 average was 
Fig. 7. Length of Growing Period Zones and Planting Pit Case Study Sites(purple)

The two soil associations within which the planting pit case studies (purple squares) are located are shown in orange (Yatenga

Plateau) and grey (Tahoua area). The 1 to 75-day LGP is generally too dry for rainfed cropping and is mainly a pastoral zone.

Raainfed millet and cowpeas are crops suited to the 75 to90-day LGP zone. Millet also does well in the 90 to 120-day LGP zone,

along with short season sorghums. Sorghums grow best in the 120 to 150-day LGP zone. 
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

15Hassan, Abdou. 1998. Op. Cit.

16 Chemonics, Inc. 1996. Seeds for Disaster Mitigation and Recovery in the Greater Horn

of Africa. Washington D.C.: USAID, Office of Foreign Disaster Assistance.

15

448 mm.15 Comparable averages were not documented for the Yatenga Plateau, which falls in

the 500 to 700 mm per year average range, and enjoys somewhat less variability. 

In general rainfed sorghum grows best with at least 120 days during which rainfall is greater than

one half potential evapotranspiration. Millet is indicated in areas of 75 to 120 days of LGP.

However, faster maturing varieties of sorghum such as IS-76 (85 to 95 days to maturity) and

drought tolerant sorghums such as Kari/Mtama 1 (90 - 100 days to maturity) and others have

been bred16. On the Yatenga plateau, both millet and sorghum were reported to be planted on

land treated with planting pits while in Tahoua only millet was reported. This is consistent with

the LGP designation of the area where the case study is located. Varieties planted weren’t noted.

That Tahoua region planting pits should have yielded such good results at the apparent limit of

millet cultivation is remarkable, but rainfall during the season of reported yields should be the

first factor to be examined. For the years in which yields were recorded, rainfall amounts were

not mentioned. Also rainfall will be slightly more effective at 1,000 feet above sea level, a factor

not manifested in the LGP isolines, which were compiled over a smaller scale base.

The LGP zonation provides a reasonable view of the geographic distribution of rainfall and

might be used to roughly map areas of comparable agroclimates where the results of soil or water

conservation methods for crop growth might be achieved. However the LGP boundaries 

probably have an error of plus or minus 50 kilometers or more depending on the particular year

and they don’t show the influences of lesser elevation changes on rainfall efficiency.

Terrain and soil factors

The DCW base map includes 1,000-foot contour intervals, which, at the 1:1 million scale, reveal

the major terrain features, i.e. valleys, uplands or mountains, areas of steep slopes. This terrain

information revealed in the DCW base map provides important additional information about the

mapped soils. The FAO map does not identify uplands, but when the two maps are

superimposed(contour intervals and soil map units) the pattern is readily seen(Figs. 4 and 6). 

The “blocky” appearance of the soil unit boundaries in Fig. 6 reflects the smaller compilation

scale(1:5 million) compared to that of the base map (approx.1:3 million). 

A fairly large area in the northwest sector of Burkina Faso is uplands with soils classed the same

as those in and around Ouahigouya and the Yatenga plateau area, as shown in lower map on page

12. Within the identified area reclamation of degraded soils using planting pits could be a

fruitful practice, worth supporting. Condition of the soils, land tenure, nearness to markets,

traditional landuses, and other factors would have to come into play, of course, in any assessment

of an area-wide approach. However, the area so identified would serve to 
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

16

delimit the region where these factors should be studied. The case study, which was written

without the benefit of the FAO soil map, remarks on the fact that zai are “hardly used at all” in

the provinces of Passori, Bam and Sanmatenga, where it observed that upland soils are equally

crusted and population densities are high. It is clear from the soils map that the eutric regosols

typifying the case study area are found in parts of these provinces, but not everywhere.

In the Tahoua region the luvic arenosols which name the soil association unit where the planting

pits have been documented, also occur in adjacent soil associations. But the latter have no

associated regosols and are not derived from limestone. While adjacent upland soils associations

may also be degraded with surfaces sufficiently indurated to allow run-off capture, the absence of

limestone could make a difference in the fertility of soils that accumulate in the planting pits as

well as in the root development of plants, needed to reach moisture at depth. Thus the role of

phosphate -rich limestone as a determining factor in the success of tassa seems clear (but needs

to be confirmed in the case of the Illela tassas), and its importance for tree growth in Keita was

noted.. It appears that the regosol-over-limestone combination is the key to understanding the

success of water harvesting efforts in this soils association. 

The GIS analysis of potential ranges of this practice on other luvisols (or regosols) is limited

because lithology is not an attribute in the databases that are linked to the mapped soils

associations. Consequently, the extent of soils developed over limestone cannot be mapped using

GIS. (A review of the lithology descriptions in the monograph revealed no other soils in Niger

developed over limestone. )

Extrapolation

In the case of the experiences in the Yatenga Plateau, the results achieved with planting pits are

likely to be achieved on other eutric regosols with a laterite crust where rainfall is sufficient to

make water harvesting worthwhile (Fig. 8). The degraded state of flat to gently sloping surfaces

is an antecedent condition of course; it is more likely to have occurred in this drier zone, where

regeneration is more difficult. This is the agroclimate with a 75 to 120-day LGP in which both

millet and sorghum cultivation would be more feasible by virtue of water harvesting (Fig. 9). 

Within this agroclimatic zone, the total extent of lateritic eutric regosols is 3.2 million hectares.

In the 120-150 day LGP zone, where large areas of this kind of soil are also found, rainfall is

higher and degradation of upland soils may not have been as severe. However, where surfaces

have been denuded, zai could prove effective. Fig. 8 shows that within the 120-150 LGP zone the

largest extent of eutric regosols, petroferric phase occurs in Burkina Faso; large areas also occur

in Senegal Oriental, and Nigeria. 

In the Tahoua case, the unique combination phosphate-rich soil parent materials and very low

rainfall, is not found elsewhere and argues against spatial extrapolation of the experiences. 
17 LGP, or length of growing period, is the total days when rainfall exceeds one half potential evapotranspiration.

Fig. 8 Potential range of soil water conservation with planting pits

The yellow colored LGP17 zones span the 75-day LGP to 120-day LGP – agroclimates of low and variable rainfall where soil water

conservation could increase or assure yields of millet and sorghum. Areas of degraded eutric regosols within these agroclimates could

be made productive with soil water conservation and harvesting methods such as the planting pits reviewed in the case studies. 
Fig. 9. All Eutric Regosols, petroferric phase, within the 75-day to 120-day LGP Zone

All soils associations with eutric regosols, petroferric phase in Africa within the 75-day to 120 day LGP zone are displayed in this

view. They are found principally in Burkina Faso, with smaller amounts in Mali and Niger. However, although eutric regosols are the

dominant soil, they do not occur over the entire mapped areas; their total extent is 3.2 million hectares, or 60 % of the 5.4 million

hectares occupied by this particular soils association within this agroclimate
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

19

Conclusions

Potential range of planting pits

The success of planting pits on the Yatenga plateau holds promise for other areas with degraded 

eutric regosols having laterite crusts. However, only in Burkina Faso, Mali, and Niger do

similarly classed eutric regosols fall within the drier agroclimates (75-day to 120-day length of

growing period) where water harvesting would make sense if degradation has rendered the soil

denuded and unproductive. A total of 3.2 million hectares of such soils are found, though all are

not degraded. (Crusted soils other than eutric regosols in these zones may also respond well to

planting pits.)

The combination of terrain, soils, and climate contributing to the good results of planting pits in

Tahoua are, on the basis of the materials examined, unique and not found elsewhere. In

particular, the phosphate-rich limestone over which some of the soils have developed is not

found elsewhere in Niger, and elsewhere in Africa it is found only in small areas. 

The differing outcomes of the extrapolation analyses for these two instances underlines the

significance of lithology for understanding the inherent soil fertility of regosols and for

understanding the effectiveness of soil restoration methods for these soils, where effectiveness is 

measured by yield increases.

As regards the technique of geographic extrapolation of documented site-specific experiences in

soil management, the analytical process can begin with digital versions of FAO maps of soils and

agroclimate, but it does not end with them. As a first approximation of the geographic

occurrence of areas worthy of attention for extending a method recorded in a site, the maps are

very useful and areas can be quickly delineated. In subsequent analysis and planning of areas

identified by this means, more detailed soils and climatic information must be examined, and

demographic, cultural and economic factors relevant to the introduction and sustainability of

such practices must be studied. The agenda of such studies, now well established in the

sustainable development approach, is spelled out in the volumes from which the case studies

featured in this exercise were drawn. The reconnaissance level geographic analyses illustrated in

this paper illustrate how to identify areas that merit the cost of the additional studies.

The utility of digital maps

The small scale maps used in this exercise were originally assembled in order to help

characterize the general environments of site-specific studies as a means of finding similarly

situated studies. However, they proved far more useful in assessing the potential range of

documented experiences than their scale or precision would suggest. This is partly explained by

the ease with which maps at different scales and different combinations of maps can be examined

using GIS programs. It is also due to the supplementary information associated with the FAO

digital map, their attribute tables, and related documentation. 
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

20

The cross-referral of site-specific documentation with mapped information related to soils and

terrain was synergistic and increased the total information available for the analysis. In one of

the cases studied(Yatenga plateau) the FAO maps contained more soil information than the case

study documents. It is not unusual for case studies of development experiences to omit technical

information about soils, climate and other physical environmental features. In contrast, the Keita

case study contained more soil information than the FAO map; a soils scientist worked with the

study team in this instance. 

The graphic display, using GIS, of continental scale soils and agroclimate maps, on digital base

maps together with site-specific information on actual experiences, makes possible a

reconnaissance level analytical process and visualization of spatial relationships that is not

possible using actual paper maps and reports. To the author’s knowledge the use of small scale

digital resource maps for analyzing the potential range of practices documented in site-specific 

studies for the purposes stated has not previously been attempted. This kind of analysis greatly

facilitates the understanding of geographic patterns and the posing and answering of questions

about site-specific experiences. General characteristics of soils and climate where these

experiences are located can be examined in light of the experiences and their possible range of

spread. However, the limitations and advantages of the various maps must be kept in mind. 

Advantages and limitations of the FAO soil map

This digital map makes possible examinations of soil association patterns and occurrences that

greatly facilitate the procedure of posing and answering questions related to soil influences and

their geographic extent. The attribute information contained in different databases with links to

the soils association polygons contributed to an understanding of the soils factors at play in the

documented cases. 

The potential for soil restoration in areas mapped as regosols appears to be strongly related to the

minerals in the surface rocks. The digital soil maps don’t include lithology as an attribute and

can’t serve to map this relationship. Other sources would have to be examined; in this exercise

the FAO monograph accompanying the paper version of the original FAO/UNESCO soils map of

Africa held useful information, and the case studies provided complementary information.

Surface geology maps would be valuable for better understanding regosols. 

Given the scale of compilation, the FAO soils map does not, and could not, generate local

information on field capacity or CEC, organic matter and other chemical characteristics useful to

understanding both results of experiences as well as their potentials elsewhere. The tables with

such data, based on statistical results of the 1700 profiles analyzed in 1992, are useful for

understanding the nature of different soil classes. However, they are of limited use for

understanding the individually mapped associations.

Lastly, the confidence with which geographic extrapolation of site-specific experiences can be

interpreted varies from one soils association to another according to its constituents. The more

soils in an association, the more qualifications there will be to its usefulness in identifying other

areas with soils comparable to those of the site. Thus, as the Keita area analysis demonstrated, it

was the hillside soils, not the plateau soils, that were described as fertile and derived from
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

21

limestone. These were interpreted to be the regosol portion of the association, i.e. not the

dominant soil, which is a luvisol. Tassa weren’t tried on the plateau soils in Keita, but were used

successfully on plateau soils in Illela, and are reported to be spreading on other degraded

(crusted, bare) plateau soils. Whether or not the plateau soils are regosols derived from

limestone can’t be known from the maps or case studies, but the good responses to tassa in Illela

district support the deduction that they are. 

Advantages and limitations of the Digital Chart of the World base map

This map is essential for locating site-specific studies and related features. It is an essential

complement to the soils and agroclimatic maps, particularly as a source of information on terrain

and drainage. The DCW base map does not include all populated places, i.e. villages of various

sizes, and consequently will not serve to locate many named places, including some villages cited

in the above-referenced case studies. However, the same case studies often mention distances and

directions from larger town and cities which are featured on the base map, which along with an

examination of other terrain features, such as streams and roads, makes possible an approximate

location. 

Advantages and limitations of the Length of Growing Period map

Although the length of growing period map delineates the general areas where a documented

technique might be effective in respect to total moisture availability, the map does not lend itself

to projecting or estimating different outcomes as measured by yield of a particular experience in

the semi-arid zones. In these zones, average yields, like average rainfall, are of less predictive

value than the coefficient of variation. Also in extrapolating the influence of a particular water

conservation method on crop yields that has been recorded in a study, the precipitation values

for the year(s) reported are an essential piece of information. This is all the more true for the

interpretation of soil water conservation results. Likewise, in the interpretation of agroclimatic

factors that explain soil erosion control experiences, results must be examined in light actual

rainfall and rainfall events during the period reported upon, e.g. the duration and intensity of

rains and the time of year of the most intense rains. 

Advantages and limitations of working with maps of differing compilation scales

Both the soils and the LGP maps were compiled over a 1:5 million scale base map, which was

the American Geographical Society map of Africa. When depicted over the more detailed 1:1

million scale DCW base map, incongruities related to the influence of elevation and terrain on

soils and climate are evident. Thus the soils boundaries corresponding to plateau or uplands land

forms will not coincide well with the DCW delineation of these land forms. Also, the somewhat

cooler temperatures to be expected at high elevations compared to temperatures in lower

elevation plains or valleys should affect rainfall efficiency and by extension length of growing

period. Elevation differences of 1,000 feet can be shown using DCW data, but the LGP isolines

do not reflect the possible influence on evapotranspiration in this part of Africa. (Elevation

controls on temperatures in East Africa modify LGP patterns but, again, with much less precision

than would have been possible had the DCW base map been used.). An awareness of the

influence of elevation and terrain, as depicted on the DCW maps, on soils and agroclimate can

help in the interpretation of the FAO maps.
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

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September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

27

ANNEX 1 

Notes on the CD-ROM Digital Soil Map of the World

The CD-ROM contains both vector and raster files for all the regions of the world. Only the vector

files were used for the present report.

The FAO taxonomy identified 25 classes of soils, however these are further divided into “sub-classes”

by descriptors, resulting in a total of 167 classes in Africa. These in turn occur in different combinations

with other soils to form associations, which are the mapped units displayed on the map. For Africa

there are almost 6,500 unique associations or polygons.

Each mapped soil association is identified with a code, beginning with a letter symbol for the dominant

soil class(S1), which occupies a larger portion of the association than any other soil – usually 50% or

more. Associated soils are also identified in the code. Some units have associations of three or four

soils, especially those with lithosols as the dominant soil. Others consist of only one class of soil. 

For individual mapped units the following parameters are available and can be analyzed and displayed

on the map or accessed in an information “window” from the map image:

Phases of S2 And S2

Hectares of S1 (dominant soil)

Hectares of S2 (next most extensive soil)

Texture and slope of S1 and S2

% of S1 and S2 not suited to different grain crops, beans or irrigation

Other data have been prepared in databases that, although not congruent with the map units, afford

information about the soils classes that is helpful in understanding the potentials and limitations of

different classes of soils.

Texture and terrain are presented in a separate database that aggregates associations that are similar

into approximately 4632 units with the following data presented for each unit:

% coarse texture

% medium texture

% fine texture

% flat

% rolling

% steep

Information on soil chemistry is presented in a database (Sustxt.txt) derived from a statistical analysis

of 1700 soil profiles in Africa, taken from a 1992 survey by FAO as well as from USDA data on

African soils. It contains statistically derived average values for the following parameters, arrayed for

each of the 167 soil classes found in Africa. 
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

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%sand topsoil(0-30 cm.....hereafter referred to as top)

%sand subsoils(30-100 cm ....hereafter referred to as sub)

%silt top

%silt sub

%clay top

%clay sub

pHwater top

pHwater sub

....and so on (top and sub)for the following and for each of three texture categories(sandy,

medium, clayey):.

OC (organic carbon)

N (nitrogen)

BS (base saturation)

CEC (cation exchange capacity)

CaC03 (calcium carbonate)

BD (bulk density)

C/N (carbon/nitrogen ratio)

Annex 2 displays the values for the two soils described in this report.

These data also are presented in two tables, one for the top soil and one for the subsoil, that

include statistical interpretations of the 1700 soil profiles for the above parameters, e.g.

maximum, minimum, mean, standard deviation, and the like. 
September 23, 1999 Mapping the Range of Proven Soil Management Practices in Africa

25

Annex 2 

Average values for two classes of African soils, calculated from soil profiles 

 Source: FAO CD-ROM DSMW, data table sustxt.txt.

% sand topsoil(0-30 cm)

% sand subsoil(30-100 cm)

% silt top

% silt sub

% clay top

% clay sub

pH water top

pH water sub

OC (organic carbon)top

OC sub

N (nitrogen) top

N sub

BS (base saturation) top

BS sub

CEC (cation exchange

capacity) top

CEC sub

CEC clay top

CEC clay sub

CaC03 (calcium carbonate)

top

CaCO3 sub

BD (bulk density) top

BD sub

C/N (carbon/nitrogen ratio)top

C/N sub

 Luvic

Arenosol 

93

92

3

3

5

5

6.3

6.1

0.2

0.12

0.02

0.02

74

72

3.5

3.2

58

57

 

0.4

0.4

1.4

1.5

10

5

 Eutric

Regosol 

68

72

15

15

17

13

6.4

6.8

0.2

0.45

0.06

0.06

83

76

 

10.2

6.7

44

42

1.6

6.4

1.4

1.2

9

7