tillage methods and soil and water conservation in west africa

Soil& Tillage Research, 20 ( 1991 ) 165-186 165 Elsevier Science Publishers B.V., Amsterdam

Tillage methods and soil and water conservation in West Africa

P.O. Aina a, R. Lal b and E.J. Roose c aDepartment of Soil Science, Obafemi A wolowo University, lle-lfe, Nigeria

bDepartment of Agronomy. The Ohio State University. Columbus, OH. USA CRESEA U EROSION ORSTOM, BP 5045 F34032 Montpellier, France

(Accepted 25 January 1991 )

ABSTRACT

Aina, P.O., Lal, R. and Roose, E.J., 1991. Tillage methods and soil and water conservation in West Africa. Soil Tillage Res., 20:165-186.

A review is made of appropriate tillage methods for West African soils. Soil and ecological constraints to crop production and soil and water conservation measures in West Africa are discussed. Experimental results relating to the effects of different tillage methods on soil productivity and crop responses are described for different eco-regions in relation to the potentials of different tillage methods for soil and water conservation. The review shows that limited experimental data and diverse research methodologies make generalizations from the available information tenuous at best. Serious gaps in our knowledge of the ecological suitability of alternative tillage methods and soil and water conservation technologies are identified. The future course for regionally coordinated research in soil tillage and soil and water conservation is suggested.

I N T R O D U C T I O N

Tillage is defined as the soil-related actions necessary for crop production (Boone, 1988 ). It is an integral part of a cropping system aimed at optimizing crop production by solving specific soil-related ecological constraints to crop production. There are several specific reasons for soil tillage. Short-term reasons include optimization of soil temperature and moisture regimes, seed ger- mination, emergence and seedling establishment, root proliferation and development, minimizing weed competition and energy input. Long-term reasons are maintenance of soil productivity and sustainable management of soil and water resources.

Tillage produces changes in soil conditions which interact with physical, chemical and biological crop-growth factors. Specific soil and crop responses to tillage differ among soils and climates, and knowledge of them is important in the selection of appropriate tillage systems for optimizing crop production

0167-1987/91/$03.50 © 1 9 9 1 - Elsevier Science Publishers B.V.

166 P.O. AINA ET AL.

in a particular eco-region. The socio-economic status of the farmer also influences the selection of desirable tillage systems. In general, the suitability of a tillage system in achieving the short- and long-term objectives of crop production is judged by its effectiveness in soil and water conservation. Understand- ing this role is important in relation to the proper management of the fragile ecosystem of the tropical region. In fact, the failure of many large-scale mechanized agricultural systems in parts of the region has been attributed to soil mismanagement owing to incomplete knowledge of the potential and restraints of different tillage systems (de Wild, 1967; Lal, 1985 ).

The West African tropics has diverse eco-regions, soils and agricultural systems which manifest themselves in a wide range of soil-management problems, tillage and conservation needs (Roose, 1989). Although considerable research information has been reported for this region, there is a dearth of interregional information on tillage and soil and water conservation systems. This paper collates and reviews the available information on tillage and conservation alternatives for the different eco-regions of West Africa. It com- pares research findings from different countries in an attempt to identify gaps in the present knowledge and to consider approaches for future research in developing viable solutions to the soil-management problems.

E C O L O G I C A L E N V I R O N M E N T

West Africa lies approximately between latitudes 5 °N and 20°N and lon- gitudes 15 ° E and 17 ° W, and comprises the following countries: Mauritania, Senegal, Gambia, Cape Verde Islands, Burkina Faso, Ghana, Togo, Benin, Mali, Guinea, Guinea Bisau, Sierra Leone, Liberia, Cote d'Ivoire, Nigeria and Niger. As might be expected from its large land area of 615 million ha, there is great ecological diversity which affects the nature of the vegetation, soils, cropping systems and the objectives of soil management across the region. However, variations in climatic factors from north to south including rainfall, temperature, growing season and solar radiation are sufficiently regular for generalizations to be made with some confidence. Based on the length of the growing season, which is an expression of the combined effects of temperature and rainfall, the region can be subdivided into major climatic zones each with distinctive ecological features (Dudal, 1980).

H u m i d zone

The humid zone has a growing period exceeding 270 days and annual rainfall in excess of 1500 mm. It is characterized by a coastal strip of mangrove swamp and extensive areas of rainforest. The predominant soils of this zone are the deeply weathered Ferrallitic soils (mainly Ultisols, Oxisols, Alfisols and Inceptisols) which are characterized by low activity clays, intense leach-

TILLAGE METHODS IN WEST AFRICA 167

ing, low base saturation, acidic condition, low effective cation exchange capacity and deficiencies of major nutrients including N and P. They often occur on moderately steep slopes and therefore are prone to erosion on exposure.

Sub-humid zone

The sub-humid zone is characterized by a growing period of 150-269 days, and comprises two regions: dry sub-humid (with a growing period of 150- 179 days and annual rainfall of 600-1200 mm) and moist sub-humid (with a growing period of 180-269 days and annual rainfall of 1200-1500 mm). The sub-humid zone is characterized by savanna-forest mosaic vegetation in the south and successive east-west belts of relatively undifferentiated South- ern Guinea and Northern Guinea Savanna and relatively dry undifferentiated Sudan Savanna in the north with the succession reflecting in general, the decreasing water supply from south to north. The predominant soils in the zone are Ferruginous tropical soils (mostly Alfisols ) (d'Hoore, 1964; FAO, 1976) which cover about 58% of the zone and occur widely between 500 and 1200 mm isohyets, and the Ferrisols which occur mainly between 1200 and 1500 mm isohyets. These soils are characterized by sandy surface layers with a low organic matter content (rarely exceeding 2-3% ), low exchange capacity (1-10 meq per 100 g), compact, relatively impermeable subsoils (Ferrugi- nous hardpan) and low clay contents (less than 25%). Soils undergo a very. marked hardening and increase in bulk density during the dry season. The hardening, poor structural stability, surface compaction, crusting and suscep- tibility to erosion constitute serious constraints towards sustainable management.

Semi-arid zone

The semi-arid zone with annual rainfall of 400-600 mm and a growing period of 75-149 days has predominantly Sudan Savanna vegetation. The zone has an arid-ustic soil moisture regime which supports mainly grass, sorghum and millet production. Predominant soils are the Ferruginous tropical soils and brown and reddish-brown soils (mainly Alfisols, Aridisols, Inceptisols and Vertisols). In addition to the constraints associated with Ferruginous soils, the agricultural potential of the zone is limited by aridity and frequent drought.

A rid desert

The arid desert zone is characterized by very low, and highly annual, erratic rainfall ( 100-400 mm) distributed over a growing period of less than 75 days. The soil moisture regime is aridic. The predominant soils are the brown and reddish-brown soils (Inceptisols, Aridisols and Alfisols) formed from parent


materials of aeoline origin and some weakly developed saline, sodic and cal- cic soils. They are characterized by very low organic matter and clay contents in the topsoil, and CaCO3 concretions at depths. The agricultural potential of the zone is severely limited by the high compactability of the soils (Nicou and Chopart, 1979b) and the aridity that makes the region unsuitable for rainfed crop production.

Some common features and important constraints to crop production can be summarized for different eco-regions. With only 0.3% of the land under irrigation, food crop production is dominantly rainfed in West Africa. With the decreasing trend of rainfall from south to north in the humid and sub- humid eco-regions soil-management systems must provide means for dis- posal of surplus water in the south and conservation of water in the root zone in the north. The bimodal distribution of annual rainfall south of latitude 9 ° N allows for two cropping seasons per year in the region. Several research- ers have reported the high aggressiveness of rainfall (Kowal, 1970b; Charreau and Nicou, 1971; Adu, 1972; Roose, 1973, 1976, 1977a,b, 1978; Lal, 1976b; Kowal and Kassam, 1976). A comparison with a number of studies around the world shows the range of erosivity values for West Africa to be extremely high. Wind erosion is equally severe in the arid desert areas. These eco-regions are thus particularly vulnerable to accelerated wind erosion (Lemaitre, 1954; Guiscafre, 1961 ) and water erosion (Kowal, 1970a; Charreau and Ni- cou, 1971; Adu, 1972; Lal, 1984b) in the humid and sub-humid regions. Both forms of erosion are further accentuated by deforestation and intensive cultivation in the humid and sub-humid regions and overgrazing in the arid regions.

Although a soil survey of the region is far from complete, useful general accounts about the distribution and characteristics of the soils are available (d'Hoore, 1964; FAO, 1976). The agricultural potential of the predominant soils (Alfisols, Ultisols, Oxisols and Inceptisols) is constrained by their shallow effective rooting depths, coarse texture of the surface layers, low organic matter contents owing to high mineralization rate and high temperatures, low pH (in the humid and sub-humid regions), low inherent fertility and low plant-available water and nutrient reserves. Frequent drought (Hsiao et al., 1980) commonly observed in the region is attributed to the low water retention characteristics of the soils and the inadequate and poorly distributed rainfall. Soils are very susceptible to compaction, erosion and rapid deterioration upon use owing to their weakly developed structure. Despite low erodibility, soils are highly vulnerable to accelerated erosion (Roose, 1973, 1976, 1980). Accelerated erosion rates of 200-250 tons ha -1 year -1 (Fournier, 1967; Roose, 1967; Kowal, 1970b; Aina, 1989) are commonly observed in the region. The soil profile becomes shallower as one goes northwards. Crop yields are low and unsustainable owing to low input technology. The agricul-


tural systems are predominantly subsistence with generally small (less than 1 ha) farms, low inputs and poor market incentives.

TILLAGE METHODS

In general, there are usually several alternatives for alleviating specific soil- related constraints. The choice of a suitable tillage method depends on a number of factors including soil characteristics, nature of the crop to be grown, agro-ecological and climatic environments and the socio-economic conditions of the farmer. Tillage methods can broadly be separated into conventional tillage, reduced tillage and no tillage. Reduced and no-tillage systems are also called conservation tillage (Soil Conservation Society of America, 1982).

The major traditional agricultural systems in West Africa are the shifting cultivation and rotational bush or grass fallowing. The shifting cultivation system is more or less haphazard movement of cultivation from place to place as fertility is exhausted or weed populations become uncontrollable (Morgan and Pugh, 1969; Okigbo, 1977 ). Rotational bush fallowing is a deliberate al- ternation between cropping and bush or grass regeneration. It is usual to alternate 2-3 years of cultivation phase with a longer period of regrowth of natural forest, bush, thicket or grassland fallow. The duration of each phase of the cycle depends on soil fertility, weed encroachment and population pressures on the land. These systems are associated with a wide range of land preparation techniques used in the different eco-regions. Traditional tillage, done mostly by manual labor, is performed by native tools which are generally few and simple, the most important being the cutlass and hoe which come in many varieties depending on their function (Morgan and Pugh, 1969). To facilitate seedbed preparation and planting, forest undergrowth or grass is cleared with a cutlass and trees and shrubs left, but pruned. The cut biomass and residues are disposed of by burning in situ. This type of clearing is non- exhaustive and leaves appreciable cover on the soil, and also the root system which maintains a good structural stability in the topsoil for one to two years.

In many parts of West Africa, particularly in the region of Alfisols with gravel layers at shallow depths and the compact ferruginous soils, surface soil layers are heaped with a hoe into small hillocks (mounds) or ridges on which a range of crops are simultaneously grown (intercropped) (Kowal and Stock- inger, 1973 ). Roose (1989) showed that there are relationships of mounding or ridging with root crops, soil infertility, poor drainage and also with climate and water balance. The dimensions of the mounds and ridges vary widely. In the poorly drained soils of the humid and sub-humid regions, mounds are larger and higher than on well-drained upland soils, ranging from 2 to 4 m in circumference and about 1 m high (thus raising the soil above the water table). The main advantages of mounds and ridges would appear to be the cre-

170 P.O. AINA ET AL,

ation of a relatively deep rooting zone, the improvement of soil drainage, bur- ying of organic residues, and creating more favorable aeration and temperature regimes. At the same time, under certain conditions, mounds and ridges have been reported to accentuate drought, temperature stress, organic matter de- composition and erosion (Layenear and Hunter, 1971 ). In the semi-arid and arid eco-regions, manual tillage operations, notably ridge-splitting with a heavy hoe and ox-driven implements (e.g. ridger) is commonly used for mechanical seedbed preparation.

In general, the traditional systems of seedbed preparation are considered relatively efficient for the prevalent subsistence agricultural system. Manual and ox-tillage systems are reportedly good for soil conditions, and poorer for weed control than tractor-powered systems (Dunham, 1988 ). These systems are labor-intensive and are based on low-inputs of fertilizers, herbicides, in- secticides and machinery.

As the population pressure and the demand for food increases, the length of the soil-rejuvenating period for bush or grass regrowth fallow becomes in- creasingly shortened relative to that of cropping, thus increasing the system of continuous cultivation and mechanized tillage. In this system, mechanized clearing, usually with bulldozers, is followed by plow-based methods of seedbed preparation. The system offers greater energy use, increases arable land area per farm household, reduces drudgery and assists weed control. However, its suitability depends on the soil type and the climatic conditions. It has also been reported to be effective in some soils and to degrade soil conditions with a resultant decrease in yield in others. The poor socio-economic status of farmers in the region also appears to limit the wide adoption of this system. Mechanized tillage accounts for less than 10% of the cultivated area in West Africa. In northern Cameroon, Sodecolon observed that for 100000 ha sown to cotton/cereal rotation under intensive cultivation in 1989, 75% were plowed/harrowed and ridged by oxen traction, 10% by small tractors and 15% were cultivated manually. Boli et al. ( 1991 ) observed that plowing increases the degradation of the topsoil in 12-15 years causing rapid degradation of clay and organic-matter content.

SOIL AND WATER CONSERVATION TECHNIQUES

The high variability of rainfall in the humid and sub-humid regions, and inadequate amounts and poor distribution in the semi-arid and arid eco-regions, with consequential excess or deficit of soil water, determine the cropping patterns and the level of productivity of rainfed food crop annuals. Pe- riods of water deficits occur frequently even in the humid and sub-humid regions, and positive responses to moisture conservation techniques have been obtained in these regions despite a theoretical need to dispose of excess water. Drought stress is, therefore, an important climatic constraint to crop produc-


tion in West Africa. It is attributed to poor rainfall distribution, inadequate rainfall, high evapotranspiration, the low available water-holding capacity of the root zone of the predominant soils, poor structural stability, the high com- pactibility of soils and soil hardening resulting in low infiltration rates and high runoff and erosion losses. Appropriate soil and water conservation techniques can minimize these constraints and optimize crop growth.

Appropriate soil-management techniques for soil and water conservation differ according to soils and eco-regions. In the arid areas, the choice of soil- management system is clear; all rainfall must be retained by techniques which reduce runoff, improve infiltration and increase the water-holding capacity of the soil to benefit crop growth (Charreau and Poulain, 1963; Gillier, 1964; Birot and Galabert, 1967). In the humid areas, where a balance has to be struck between conservation of soil and water by runoff control and the avoidance of surface waterlogging and excessive leaching, the choice of conservation system is not as straightforward. However, the basic principles of maintaining the soil properties and soil-water regime optimal for crop growth are the same and include practices that: ( 1 ) reduce storm-water runoff and soil erosion; (2) prevent degradation of soil properties; (3) improve infiltration rate, soil-water storage and water use efficiency; and (4) decrease evaporation losses.

Tillage is an important soil-management component that plays an important role in maintaining the productivity of soils in the tropics. A number of comparative studies have shown that the traditional shifting cultivation and rotational bush fallow systems are low in productivity but are more ecologically sound practices of soil and water conservation than modem agricultural production systems involving mechanized tillage and continuous cultivation (Verney and Williams, 1965; Roose, 1967; Charreau, 1969; Charreau and Fauck, 1970; Greenland and Lal, 1977). The studies show that the structure of the surface soil deteriorates rapidly and high runoff and accelerated erosion occur more quickly under mechanized than under traditional manually powered cultivation. Deforestation by mechanized systems followed by plow- based tillage causes severe problems of soil erosion (Tables 1 and 2). A considerable body of research on tillage practices and their influence on the productivity of soil in the tropics has been accumulated over the years. Lal ( 1985 ) made a tentative categorization of soils regarding their suitability for different tillage methods based on soil characteristics, climatic conditions, moisture regime and principal soil degradative problems of the eco-regions. Some successful soil and water conservation practices are briefly described below.

Mulch farming

The favorable effects of mulching on water conservation and high crop yields have frequently been reported for several soils in West Africa (Nye, 1952;

172 P.o. AINA ET AL.

TABLE 1

Effects of methods of deforestation and post-clearing management on erosion and runoff ( Lal, 1981, 1984a)

Treatment Basin area Runoff Soil loss (ha) (ram) (tons ha -~ )

Forest 15 Traditional farming 2.6 Manual clearing/no-tillage 3.1 Manual clearing/conventional tillage 3.2 Shearblade/no-tillage 2.7 Treepusher-root rake/no-tillage 3.2 Treepusher-root rake/conventional tillage 4.0

Trace Trace 6.6 0.O2

16.1 0.40 79.7 9.9

I O4.8 4.8 170.0 15.7 330.6 24.3

TABLE 2

Effects of deforestation on runoffand soil erosion in Cote d'Ivoire (Roose. 1988)

Parameter Cassava Peanuts Cultivated on mounds fiat bare 1966 1967 1968

Rainfall (mm year- ~ ) 1496 1673 2084 Annual runoff (%) 18.3 25.0 24.7 Maximum runoff rate (%) 75 77.0 65.0 Erosion (t ha- t year- ~ ) 162 427 622

Lawes, 1957; Djokoto and Stephens, 1961; Ashrif and Thornton, 1965; Lal, 1975). The beneficial effects of mulching are attributed to physical effects that principally improve rainfall acceptance, reduce runoffand surface crusting and improve moisture conditions and aeration in the topsoil. In addition, there are also improvements in soil chemical and nutritional properties. In addition, there are also improvements in soil chemical and nutritional properties. For the sub-humid and humid regions 4-6 tons ha- l of residue mulch appears optimal. In Ibadan, Nigeria, Lal (1976a) reported annual saving of 32% of rainfall in water runoff for a crop residue mulch rate of 6 tons ha- Similar results from an experiment conducted in Kumasi, Ghana, are shown in Table 3. In Cote d'Ivoire, Roose (1988) reported drastic reductions in runoffand soil erosion from a mulched pineapple field on a 20% slope (Table 4). Mulching studies in the humid and sub-humid eco-regions have attributed the beneficial effects on crop yields and reduced soil losses to the protection of the soil surface against climatic elements (Fournier, 1967), lower soil temperature at the seedling stage, suppression of weed growth and improved moisture storage in the root zone (Lal, 1973) and better aeration (Lawes, 1966). Mulch also enhances the activity of some species of earthworms such as Hyperiodrilus spp. whose burrowing channels are very effective in con-

TILLAGE METHODS IN WEST AFRICA

TABLE 3

Effects of tillage system on soil loss and runoff in Ghana (Mensa-Bonsu and Obeng. 1979)

173

Treatment Soil loss Runoff (t h a - t year - t ) (%)

Kwadaso Ej ura Kwadaso Ej u ra

Bare fallow 313.0 18.3 49.8 36.4 No-tillage 1.96 9.2 3.4 0.52 Mulching 0.42 1.9 1.4 0.33 Ridging (across slope) 2.72 4.5 1.9 1.30 Minimum tillage 4.90 3.8 1.7 1.10 Mixed cropping 33.6 2.5 13.2 5.10

TABLE 4

Effects of pineapple residue management practices on runoff and soil erosion from pineapple grown on an Ultisol in Ivo~" Coast on 20% slope (Roose, 1988) ~

Treatment Runoff Erosion (% of rainfall ) (t ha - t per 16 month )

Bare cultivated (without crop ) 29.0 410 Residue burned/plowed 7.5 69 Plowed under 3.4 33 Residue mulch 0.1 1

~Rainfall was 3337 mm per 16 month cycle.

ducting water through the soil profile (De Vleeschauwer and Lal, 1981 ). Res- idue mulching saves moisture during the first stage of evaporation (Willis, 1976) owing to low soil temperature and the insulating effects of the mulch layer. These are all factors one would expect to be equally important in the semi-arid and arid eco-regions where similar favorable effects of mulching have been reported (Nye, 1952; Lawes, 1957; Djokoto and Stephens, 1961; Roose, 1975; Adeoye, 1985; Ike, 1986). Experiments with rainfall simulation in the Sahel have shown that mulching is less efficient for runoff reduction on poorly structured sandy loam soils than on clay soils.

No tillage

The main problems with mulching are the inadequate mulching material, especially under the long dry seasons of the semi-arid and arid regions. A compatible tillage system that retains protective amounts of residue mulch on the surface is no tillage. The advantages of no tillage for soil and water conservation are widely recognized for a wide range of conditions in the humid and sub-humid eco-regions of West Africa. The no tillage system with


crop residue mulch is considered the basis of conservation farming because, where found suitable, it conserves water, prevents erosion, maintains organic matter content at a high level and sustains economic productivity (Lal and Hahn, 1973; Rockwood and Lal, 1974; Lal, 1975; Greenland, 1981 ). This system seems to have a broad application in the humid and sub-humid regions. Several studies have shown that a no-tillage system with crop residue mulch may maintain the productivity of upland soils by reducing erosion (Lal, 1984a; Aina, 1988), maintaining a favorable soil temperature (Rockwood and Lal, 1974; Lal, 1986a), improving water-retention capacity (Aina, 1979; Opara-Nadi and Lal, 1986), and increasing nutrient use efficiency (Lal, 1979). Results similar to these have been obtained in other regions such as Ghana (Kannegieter, 1967, 1969) and Liberia (Lal and Dinkins, 1979).

The effectiveness of no-tillage farming in soil and water conservation and in improving water use efficiency is usually substantially improved when used in association with planted cover crops of legumes or grasses. Some important cover crops have been described in relation to their growing habits and suitability for soil and water conservation in the tropics (Okigbo and Lal, 1977; Lal et al., 1979). Growing cover crops for 1 or 2 years, e.g. Mucuna utilis, Pueraria phaseoloides and Centrosema pubescens, has been reported to improve soil structure (through addition of organic matter) and infiltration capacity (Kannegieter, 1967; Lal et al., 1979; Hullugale et al., 1986; Obi and Nnabude, 1988 ), conserve soil water (Pereira et al., 1958 ) and improve crop yields through better water use efficiency and improved weed control. Wilson and Lal (1984) reported that the dry weight of residue from cover crops can be as much as 6.5-I1 tons ha -1 year -~.

Plowing

A no-tillage system is less effective in some eco-regions, and decreases the yield of root crops, e.g. cassava (Lal and Dinkins, 1979; Okigbo, 1979; Aina, 1982 ). A no-till system is also less responsive on soils with compacted surface and subsoil layers, soils susceptible to crusting, and hydromorphic soils with poor internal drainage. Increasing costs of chemical inputs (herbicides cost a large proportion of total inputs) may also prohibit the continuous use of no tillage, thus making mechanical tillage imperative in such circumstances (Lal, 1985; Aina, 1988). Aina (1988) suggested a rotational system of no tillage with conventional tillage as a solution. Soils with these restraints are common in semi-arid and arid regions such as Ustropepts, Ustalfs, Alfic eutrustox and other Aridisols, which are characterized by weak structure, low porosity and low infiltration rates. The beneficial effects of plowing such soils have been demonstrated in the region (Some and Ovaltara, 1991 ). In the humid regions, Lal ( 1984a, 1986b) showed that chiselling in the row zone has an ame- liorative effect on initially compacted soils. Ogunremi et al. (1986) reported


TABLE 5

Plowing effects on root weight per plant at flowering stage on a ferruginous soil of Senegal (adapted from Nicou and Chopart, 1979a)

Crop Depth (cm)

Root weight (mg per plant)

Control Plowed

Millet 100-150 532 1225 Millet 150-200 91 308 Groundnut 50-100 4010 5700 Sorghum 40- 90 2370 3910

higher yields of upland rice with conventional tillage than no tillage on an Ultisol in Southeastern Nigeria.

For some root-restrictive soils, several studies have shown positive responses to plowing (Ofori and Nandy, 1969; Ofori, 1973 ). Le Buanec ( 1971 ) noted that in southern and central Cote d'Ivoire, the effectiveness of plowing depends on the conditions under which it is carried out. Tourte et al. ( 1963 ) recommended dry plowing in sandy soils receiving less than 800 mm rainfall per year but not in heavier soils.

In the semi-arid and arid eco-regions, annual rainfall is inadequate and poorly distributed. Soils which are predominantly high in the proportion of fine sand and silt fractions are easily compacted (Nicou and Chopart, 1979b ) and have low infiltration capacity. Under these conditions, mechanical tillage operations involving soil inversion and sub-soiling especially done toward the end of rainy season are advantageous for soil and water conservation. Soil inversion improves surface retention capacity and weed control and mini- mizes soil hardening during prolonged dry seasons (Nicou, 1976). The beneficial effects of plowing these soils have been demonstrated widely in Cote d'Ivoire, Burkina Faso, Nigeria, Senegal, Niger and Mauritania (Charreau and Nicou, 1971; Nicou, 1974; Nicou and Chopart, 1979; Adeoye, 1982; Maurya, 1986; Ike, 1988). The studies ofCharreau (1969) and Nicou (1974), show that the beneficial effects of deep plowing (to 25-30 cm depth) can be attributed to improved infiltration and enhanced root system development. The data in Table 5 from Nicou and Chopart ( 1979b ) show drastic increases in root weight due to plowing.

Ridge tillage

Simple cultural practices like the ridge-and-furrow system, especially when the ridges are aligned across the slope, can be useful as soil and water conservation methods. This system is widely practised and has the dual purpose of erosion control and surface drainage. Christoi (1966) reported from Burkina


TABLE 6

Crop yields and soil and water conservation effects of tied-ridges in Burkina Faso (Fournier, 1967 )

Tillage Runoff Erosion Yield (kg ha-* ) method (% rainfall) (t ha- t )

Groundnut Millet

Tied ridges 0.9 1.4 846 729 Sloping ridges 6.3 6.1 479 376 Ploughed ridges 12.2 13.2 658 352

Faso that for gently sloping ferralitic sandy soils with rapid infiltration rates, erosion was highest with unridged cropping systems, less with systems where the ridges were across the contour and almost nil where the ridges were par- allel to the contour lines. Fournier (1967) observed that the less steep the terrain, the greater the advantages of ridging. In Cote d'Ivoire Fournier noted that ridging reduced soil erosion by sevenfold on a 7% slope and by fivefold on a 4% slope. Fournier (1967) reported a 13-fold decrease in erosion in Burkina Faso caused by ridging as measured in 100 m 2 runoff plots (Table 6 ). Roose and Piot ( 1984 ), in a 7-year comparison on 5000 m z peas on grav- elly ferruginous soil near Ouagadougou, showed that neither ridge tillage along or across the slope, nor diversion ridges and motorization decreased runoff (average or maximum) or erosion in comparison with Mossi traditional zero- tillage. Only tied ridging had a beneficial effect on runoff and erosion but not on yield in sorghum-peanut rotation.

Ridge-tying is an improvement over the traditional ridge-furrow system and is an effective soil and water conservation measure in the semi-arid and arid eco-regions. It has been successfully used in Nigeria (Kowal, 1970b; Ko- wal and Omolokun, 1970), Burkina Faso (Christoi, 1966; IITA, 1981 ), Sierra Leone (Millington, 1 9 8 5 ) a n d Senegal (Fournier, 1967; Hulugalle and Maurya, 1991 ). Similar remarks apply to cross-tying of mounds which has been found more effective in preventing-erosion than untied mounds. The effectiveness of ridging and tied-ridges depends on the landslope, rainfall and design characteristics. For example, at Samaru (Nigeria), ridge-tying decreased the cotton yield in a wet season with 1394 mm rainfall but increased the yield in a dry season with a rainfall of 889 mm. Layenear and Hunter ( 1971 ) in Ghana reported lower maize yields from ridged plots compared with unridged plots. Hudson ( 1971 ) recommended the safety device of making the ties lower than the ridges which should be graded and should be ac- companied by a backup support system of contour terraces.

Terracing

Terracing is among other conservation methods which are less widely used in the region. Millington (1985) identified bench terracing, stone bunding,


stick bunding and contour bunding as soil conservation methods used in traditional farming in Sierra Leone (Table 7 ). Similar observations with respect to terrace farming with stone or grassed contour banks have been made else- where in parts of Nigeria in the Atacora highlands, Bauchi Plateau, Mandara Mountains (Palmer, 1958; Lawes, 1966; Kowal, 1970b) and in Niger (Allo- koto) (Birot and Galabert, 1967; Roose and Bertrand, 1971; Roose, 1990). The success of terraces in soil and water conservation depends on their design and maintenance (Lal, 1983). Kowal (1970b) reported the successful control of erosion from terraced croplands for more than 10 years without excessive soil loss or damage to the water course even during an exceptional storm of 71 mm in 1969. Similar observations from Mokwa were reported by Pal- mer ( 1958 ). Terrace farming is not widely practised apparently owing to the lack of equipment and the prohibitive costs (relative to returns from subsistence farming systems) involved in the establishment and subsequent maintenance of the terraces (Marchal, 1979, 1986). The type of terracing in relation with the climate is very important. Roose (1986, 1989) showed that diversion terracing is less adapted to semi-arid regions than the "dispersion system" of permeable microbarrage-like stone bunds because farmers need

TABLE 7

Soil loss and runoff from plots and watersheds under different management systems in Nigeria

Site Management Soil loss Runoff ( tha -1 year - t ) (%)

Samaru * Broadlands 21.0 20.4 Cropped ridges 19.6 27.9 (down slope) Cropped alternate 5.7 18.2 tied-ridges Flat cultivation 3.8 25.2 (bare) Flat cultivation 4.0 20.3 (arables)

lbadan 2

Sierra Leone 3

Bare (runoffplots) 232.6 42.1 Mulched (runoff plots ) 0.20 2.4 Terraced (watershed) 0.66 18.1 Unterraced (watershed) 2.25 18.8

Bench terracing 7.5 Stone bunding 29.5 Stock bunding 27.3 Contour bunding 18.0 No conversation 46.7-54.4

tKowal, 1970b:-'Lal, 1976, 1983; 3Millington, 1985.

l "]8 P.O. AINA ET AL.

runoff from the hilltops to complement the irrigation of their fields down slope.

Strip cropping

Strip cropping, with strips of different crops such as cover crops and grasses alternating down the slope with food crops, has been used in Senegal (Roose, 1967; Fournier, 1967 ), Burkina Faso and Niger (Roose and Bertrand, 1971 ). Strip cropping is effective (depending on the width of the strips and the land slope) in reducing erosion and runoff to as much as less than one-fifth of those under conventional system of cultivation.

Agroforestry

A variation of the strip cropping system, consisting of herbaceous vegetation as buffer strips, is the alley cropping system in which arable crops are grown in the spaces between rows of planted woody shrub or tree fallows. It is an integration of trees and shrubs with annual food crop production in an agro-forestry system (Vegara, 1982) and is known and practised by traditional farmers in many parts of the humid and sub-humid regions. The effectiveness and the beneficial effects of this system in soil and water conservation are attributed to reduced erosion and surface runoff (Table 8; Lal, 1989), low soil temperature, reduced soil moisture loss and increased soil organic matter and nutrient recycling (Mongi and Huxley, 1979; Kang et al., 1981; Wilson and Lal, 1984). The choice of the fallow species and agro-ecological conditions influences the effectiveness of the system. Promising results have been obtained in alley cropping with Gliricidia and Leucaena. The ecological compatibility of this system is yet to be established for the different ecological

TABLE 8

Effects of alley cropping and tillage methods on soil erosion under maize (first season) and cowpea (second season) from 1982 through 1987 (adapted from Lal, 1989)

Treatment Spacing (m)

Soil erosion for six growing seasons (t ha - t )

Maize Cowpea

Plowed - 25.6 3.8 No till - 0.6 0.2 Leucaena 4 3.4 0.8 Leucaena 2 0.6 0.3 Gliricidia 4 3.8 0.4 Gliricidia 2 3.4 1.1


zones of the region particularly for semi-arid regions where the association of Faidherbia albida withn various crops is well known.

Fertility management

Undoubtedly the proper use of fertilizers and liming materials will complement the effect of tillage and other soil-management systems in enhancing the productivity of the soils in the region which are characterized predominantly by low fertility status, which constrains crop reproduction. This is evident from several studies. For example, Klaij and Hoogmoed (1987) reported a strong synergistic effect from combining tillage and fertilizer use on a low fertility sandy soil in Niger, which resulted in a fourfold yield increase. The solutions to problems of plant nutrition, fertilizer and lime requirements are only practicable where availability and costs of the materials can easily and economically be met by the socio-economic conditions of the farmers.

CONCLUSIONS

The survey of literature presented in this paper indicates that the existing experimental data are rather limited for different eco-regions of West Africa. Serious gaps exist in our knowledge with respect to soil and crop factors and their responses to different tillage and soil-water conservation methods under the diverse conditions of the region. Choice of tillage and conservation systems is therefore still largely a matter of guesswork. Comparison and extra- polation of the research findings from one country to another of the same ecological conditions are also difficult because of the lack of uniform stan- dard research methods and also because socio-economic conditions are different from place to place.

A number of generalizations made from the available research information, are outlined in Table 9. The region is diverse in terms of soils, agricultural systems ecological conditions and population density. A wide range of soil-management problems, tillage systems and soil and water conservation techniques are thus expected. The traditional agricultural system of shifting cultivation and bush fallowing involving ridge-furrow and mounds as methods of seedbed preparation predominate in the region and are adaptable to different soils, crops and eco-regions. Although low in productivity, these methods are ecologically more sound and result in less environmental degradation than mechanized tillage involving primary and secondary tillage operations. A mechanized tillage system, being enforced by increasing population pressures on land and increasing food demand, is compatible with intensive row crop farming and results in differential impacts on soil in different eco-regions. In the humid and sub-humid tropics, it results in rapid soil deterioration especially on the well-drained, coarse textured Ultisols, Alfisols,


TABLE 9

Soil eco-regional guide to tillage methods for upland crops in West Africa

Moisture Texture of soil surface Constraints Tillage methods regime

Per-humid Sandy, sandy loam, Soil erosion by water, low No tillage, reduced tillage, and humid loam, sand soil fertility low mulch farming with cover

AWC ~, high soil crops, agroforestry with temperature plantation/tree crops

Per-humid Silt loam, silty clay Soil erosion, crusting and humid loam compaction, high soil

temperature

Per humid Clay loam, clay Water logging, poor traf- and humid ficability, erosion

Sub-humid Sandy loam. loamy sand, sandy clay

Soil erosion by water, crusting, compaction. drought, low soil fertility, low AWC

Semi-arid and Sandy loam, loamy Soil erosion by wind and arid sand water, drought, low regions AWC, high soil tem-

perature, sand blasting

Semi-arid Clayey, sandy clay, Soil erosion, poor traffic- regions swelling soils ability, water logging,

drought

Reduced tillage or minimum tillage, cover crops, mulch farming, agroforestry

Ridge/furrow system, surface drainage, raised beds or mounds, agroforestry

No tillage with periodic chisel plowing, mulch farming with cover crops and alley cropping

Chisel plowing, tied ridges, plowing at the end of rains, rough seed bed

Ridge/furrow system broad beds, water harvesting

Arid regions Sandy loam. loamy Wind erosion, drought, Wind breaks, reduced tillage. sand sand blasting, low water harvesting

AWC techniques

~AWC is available water capacity.

Oxisols and Inceptisols. On the other hand, in the semi-arid and arid regions which are characterized by compact, hydraulically inferior soils, deep plowing especially at the end of the rains is beneficial to soil and water conservation. Mulch farming appears to be an effective soil and water conservation measure in all eco-regions owing to enhanced rainfall acceptance, reduced erosion and runofflosses and optimized temperature in the rooting zone. The problem of inadequate mulching material especially in the arid and semi-arid regions with long dry seasons may be solved by planting cover crops. Mulch farming in combination with a no-tillage system has a wide application for grain crop production on Ultisols, Alfisols and Oxisols in the humid and sub-


humid regions as an effective soil and water conservation technique. How- ever, mulch farming is not practical in the arid and semi-arid regions. Ridging is widely practised, and is effective in soil and water conservation especially contour ridging in the more humid regions and tied-ridging on the sandy soils of semi-arid and arid regions. Agro-forestry is not yet widely integrated in food crop production in the region, and research on this aspect is presently inadequate. Adoption of soil and water conservation techniques that are compatible with large-scale mechanized farming such as terracing, which was once part of traditional hillside small-scale farming in parts of the region, is limited by the poor socio-economic conditions of most farmers in the region and the subsistence agricultural systems.

The future course for soil and water conservation research will have to ac- celerate the process of finding more desirable tillage and soil and water conservation systems as economic situations change. Research will have to be standardized and coordinated regionally to permit comparisons and application of research findings, emanating from one region to the other within similar eco-regions.

REFERENCES

Adeoye, K.B., 1982. Effect of tillage depth on physical properties of a tropical soil and on yield of maize, sorghum and cotton. Soil Tillage Res., 2:115-231.

Adeoye, K.B., 1985. Effect of some management practices on soil crusting in cultivated savanna soils. In: R.A. Sobulo and E.J. Udo (Editors). Soil Fertility, Soil Tilth and Post-clearing Land Degradation in the Humid Tropics. SSSN Proc., pp. 302-310.

Adu, S.V., 1972. Eroded savanna soils of the Navrongo-Bawku area, northern Gahana, Ghana, J. Agric. Sci., 5: 3-12.

Aina, P.O., 1979. Soil changes resulting from long-term management practices in Western Ni- geria. Soil Sci. Soc. Am. J., 43: 173-177.

Aina, P.O., 1982. Soil and crop responses to tillage and seedbed configuration in southwestern Nigeria. Proc. 9th ISTRO Conf., Osijek, Yugoslavia, pp. 72-78.

Aina, P.O., 1988. Effect of tillage rotations and cropping system combinations on soil properties and yield of corn and cowpea. Proc. I lth ISTRO Conf., pp. 543-548.

Aina, P.O., 1989. Soil erosion problems in Nigeria: Issues and perspectives of soil management and conservation. Proc. 17th Annual Conf. Soil Science Soc. Nigeria, Nsukka.

Ashrif, M.I. and Thornton, I., 1965. Effects of grass mulch on groundnuts in the Gambia. Exp. Agric., l: 145-152.

Birot, Yl. and Galabert, J., 1967. L'amelioration des rendements en agriculture par amenage- ments antierosifs et techniques culturales visant a la conservation de l'eau et du sol dans la region de l'Adder-Doutchi Maggia (Rep. de Niger), Station d'Allokoto. Premiers observations, 1966. Colloque sur la fertilite des sols tropicaux, Tananarive, 2, pp. 1316-133 I.

Boli, Z., Bep, B. and Roose, E., 199 I. Enquete ju l'erosion en zone coloumiere Nord Cameroon. Bulletin Reseau Erosion No. 11, Montpellier, pp. 127-138.

Boone, F.R., 1988. Weather and other environmental factors influencing crop responses to tillage and traffic. Soil Tillage Res., I l: 283-324.

182 P.O. AINA ET AU

Charreau, C., 1969. Influence des techniques culturales, sur le development du reuisellement et de I'erosion en Casamance. Agron. Trop. Paris, 24: 836-842.

Charreau, C. and Fauck, F., 1970. Mise an point sur l'utilization agricole des sols de la region de sefa (Casamance). Agron. Trop. Paris, 25: 151-191.

Charreau. C. and Nieou, R., 1971. k'amelioration du profil cultural dans les sols sableux et sablo-argileux de la zone tropicale seche ouest-africaine et ses incidences agronomique (d'apres les travaux des chercheurs dI'IRAT en Afrique de l'Ouest). Agron. Trop. Paris, 26: 209-255,565-631,903-978, 1183-1237.

Charreau, C. and Poulain, J., 1963. La fertilization des mils et sorghos. Agron. Trop. Paris, 18: 53-63.

Christoi, R., 1966. Assessment of erosion in Upper Volta, Oleagineux, 21:531-534. De Vleeschauwer, D. and Lal, R., 1981. Properties of worm casts under secondary tropical for-

est regrowth. Soil Sci., 132: 175-181. De Wild, J.C., 1967. Experiences with Agricultural Development in Tropical Africa. Vols. I and

II. The John Hopkins Press, Baltimore, MD. d'Hoore, J.U, 1964, Soil Map of Africa, Scale 1 to 5,000,000. Joint project no. 11, Commission

for Technical Cooperation in Africa, Lagos. Djokoto, R.K. and Stephens, D., 1961. Thirty long-term experiments under continuous crop-

ping in Ghana. I. Crop yields and responses to fertilizers and manures. Emp. J. Exp. Agric.. 29: 181-195.

Dudal. R., 1980. Soil-related Constraints to Agricultural Development in the Tropics. IRRI. Los Banos, Philippines, pp. 40-58.

Dunham, R.J., 1988. Diverse tillage systems in semi-arid West Africa. Proc. Intern. Soil Tillage Res. Organization, 1 lth Intern. Conference (Tillage and Traffic in Crop Production, 2 ), pp. 631-636.

FAO, 1976. FAO/UNESCO Soil Map of the World VI. Africa, UNESCO, Paris, pp. 307. FAO, 1987. Consultation on Irrigation in Africa. FAO, Rome, Paper no. 42, pp. 21-28. Fournier, F., 1967. Research on Soil Erosion and Soil Conservation in Africa. Aft. Soils, 12:

53-96. Gillier, P., 1964. kes coques d'arachides: leu-utilisation en agriculture. Oleagineux, 19: 473-

475. Greenland, D.J., 1981. Soil management and soil degradation. J. Soil Sci., 32: 301-322. Greenland, D.J. and Lal, R., 1977. Soil Conservation and Management in the Humid Tropics.

Wiley, Chichester, 285 pp. Guiscafre, J., 1961. Conservation des sols, et protection des cultures par bandes brise - vent.

Cantous Doukonla Tchatibali et Wina (Cameroun). Bois For. Trop., 79:17-29. Hsiao. T.C., Toole, J.C.O. and Tomar, V.S., 1980. Water Stress as a Constraint to Crop Produc-

tion in the Tropics. IRRI, Los Banos, Philippine, pp. 339-371. Hudson. N.W., 1971. Soil Conservation. Batsford, London, 320 pp. Hulugalle, N., Lal, R. and TerKuile, C.H.H., 1986. Amelioration of soil clearing of a tropical

rainforest. Soil Sci., 78: 86-91. Hulugalle, N.R. and Maurya, P.R., 1991. Tillage systems for the West Africa Semi-Arid Tropics.

Soil Tillage Res., 20:187-199. IITA, 1981. International Institute of Tropical Agriculture, Annual Report for 1980, Ibadan.

Nigeria, 45 pp. Ike, I.F., 1986. Soil and crop responses to different tillage practices in a ferruginous soil in the

Nigeria savanna. Soil Tillage Res., 6: 261-272. Ike, I.F., 1988. Soil compaction under different tillage methods and its effect on growth and

yield of cowpea. Paper presented at the 16th Annual Conference Soil Sci. Soc. of Nigeria held at Minna 27-30 November 1988, 6 pp.


Kang, B.T., Wilson, G.F. and Sipkens, L., 1981. Alley cropping maize and leucaena in Southern Nigeria. Plant Soil, 63:165-179.

Kannegieter, A., 1967. Zero-cultivation and other methods of reclaiming pueraria fallow land for food crop cultivation in the forest zone of Ghana. Trop. Agric., 128: 1-8.

Kannegieter, A., 1969. The combination of short term pueraria fallow, zero cultivation and fertilizer application: Its effects on following maize crop. Trop. Agriculturist, 125: 77-89.

Klaij, M.C. and Hoogmoed, W.B., 1987. Crop response to tillage practices in Sahelian soils. Workshop on soil, water and management systems for rain-fed agriculture in the Sudano- Sahelian zone, Niamey, Niger.

Kowal, J., 1970a. The hydrology of a small catchment basin at Samaru, Nigeria. IlI. Assessment of surface runoff under varied land management and vegetation cover. Niger. Agric. J., 7: 120-133.

Kowal, J., 1970b. Effect of an exceptional storm on soil conservation at Samaru, Nigeria. Niger. Geog. J., 13: 163-173.

Kowal, J.M. and Kassam, A.H., 1976. Energy load and instantaneous intensity of rainstorms at Samaru, Northern Nigeria. Trop. Agric. (Trinidad). 53:185-197.

Kowal, J. and Omolokun, A.O., 1970. The hydrology of a small catchment basin at Samaru, Nigeria. I. Seasonal fluctuations in the height of the ground water table. Niger Agric. J., 7: 27-40.

Kowal, J. and Stockinger, K., 1973. Usefulness of ridge cultivation in Nigerian agriculture. J. Soil Water Conserv., 28(2): 136-137.

Lal, R., 1973. Effects of seed bed preparation and time of planting on maize (Zea mays) in Western Nigeria. Exp. Agric., 9:303-313.

Lal, R., 1975. Roles of mulching techniques in tropical soil and water management. IITA, Oba- dan, Tech. Bull., No. 1, 38 pp.

Lal, R., 1976a. No-tillage effects on soil properties under different crops in Western Nigeria. Soil Sci. Soc. Am. J., 40: 762-768.

Lal, R., 1976b. Soil erosion problems of an Alfisol in Western Nigeria and their control. IITA Monogr., no. 1, 30 pp.

Lal, R., 1979. Importance of tillage systems in soil and water management in the tropics. In: R. Lal (Editor), Soil Tillage and Crop Production. IITA Proc. Ser., 2: pp. 25-32.

Lal, R., 1981. Soil erosion problems on alfisols in Western Nigeria. VI. Effects of erosion on experimental plots. Geoderma, 25:215-230

Lal, R., 1983. Soil conditions and tillage in the tropics. In: I.O. Akobondu and A.E. Deutsch (Editors), No-tillage Crop production in the Tropics.

Lal, R., 1984a. Mechanized tillage systems effects on soil erosion from an Alfisol in watersheds cropped to maize. Soil Tillage Res., 4: 349-360.

Lal, R., 1984b. Soil erosion from'tropical arable lands and its control. Adv. Agron., 37:183- 248.

Lal, R., 1985. A soil suitability guide for different tillage systems in the tropics. Soil Tillage Res., 5: 179-196.

Lal, R., 1986a. Effects of eight tillage treatments on a tropical Alfisol 1. Maize growth and yield. J. Sci. Food Agric., 37: 1073-1082.

Lal, R., 1986b. No-tillage and minimum tillage systems to alleviate soil related constraints in the tropics. In: M.A. Sprague and G.B. Triplett (Editors), No-tillage and Minimum Tillage Systems. Wiley, New York, NY, pp. 261-317.

Lal, R., 1989. Agroforestry systems and soil surface management of a tropical Alfisol II. Soil erosion and nutrient loss. Agrofor. Syst., 8:97-111.

Lal, R. and Dinkins, E.L., 1979. Tillage systems and crop production on an Ultisol in Liberia. In: R. Lal (Editor), Soil Tillage and Crop Production. IITA Proc. Ser., 2:221-234.

Lal, R. and Hahn, S.K., 1973. Effects of methods of seedbed preparation, mulching and time of


planting on yam in Western Nigeria. Symp. Proc. Int. Soc. Trop. Root Crops, Int. Inst. Trop. Agric., Ibadan, Nigeria, 2-12 December 1973.

Lal, R., Wilson, G.F. and Okigbo, B.N.. 1979. No-till farming after various grasses and legumi- nous cover crops. Field Crop Res., 1: 71-84.

Lawes, D.A., 1957. Preliminary. report on soil-water-crop relationships at Samaru, Northern Nigeria. Emp. Cotton Grow. Rev., 34: 155-160.

Lawes, D.A., 1966. Rainfall conservation and the yields of sorghum and groundnut in Northern Nigeria. Exp. Agric., 2: 139-146.

Layenear, P. and Hunter, H.B., 1971. Effect of seedbed preparation on maize (Zea mays L) Ghana. J. Agric. Sci., 4: 71-78.

Le Buanec, B., 1971. Influence du labour sur les rendements de riz et de mais et sols ferralli- tiques. Afr. Soils. 17: 173-179.

Lemaitre, C., 1954. Les problemes de la conservation des sols an Niger et le "Gao". Proc. 2nd Inter-Afr. Soils Conf., 1: 569-580.

Marchal, J.Y., 1979. L'espace de technicicus et celui de paysaus. In: Maitrise de l'espace afraire et developpement. Memoire ORSTOM, Paris No. 89: 245-252.

Marchal, J.Y., 1986. Vingt annres de lutte anterosive au nord du Burkina Faso. Cahier OR- STOM Pedol, 22, 2: 173-180.

Maurya, P.R., 1986. Effect of tillage and residue management on maize and wheat yield and on physical properties of an irrigated sandy loam soil in northern Nigeria. Soil Tillage Res., 8: 161-170.

Mensa-Bonsu and Obeng, H.B., 1979. Effects of cultural practices on soil erosion and maize production in the semi-deciduous rainforest and forest-savanna transitional zones of Ghana. In: R. Lal and D.J. Greenland (Editors), Soil Physical Properties and Crop Production in the Tropics. Wiley, Chichester, pp. 509-519.

Millington, A.C., 1985. Soil management under urban fringe farming systems in Freetown, Sierra Leone. Soil Use Land Manage. Am. S.C.S.A. Special.

Mongi, H.O. and Huxley, P.A., 1979. Soil Research in Agroforestry. International Council for Research in Agro-Forestry (ICRAF). Nairobi, Kenya.

Morgan, W.B. and Pugh, I.C., 1969. West Africa. Methuen, London. Nicou, R., 1974. Contribution to the study and improvement of the porosity of sandy and sandy

clay soils in the dr)," tropical zone. Agron. Trop., 29:1100-1127. Nicou, R., 1976. Le labour, technique d'economie de l'eau pour la zone Sahelo-Soudonienne

ouest Africaine. In: Efficiency of Water and Fertilizer Use in Semi-Arid Tropics. FAO/IAEA Report no. 192, pp. 75-100.

Nicou, R. and Chopart, J.L.. 1979a. Root growth and development in sandy and sandy clay soils of Senegal. In: R. Lal and D.J. Greenland (Editors), Soil Physical Properties and Crop Production in the Tropics. Wiley, Chichester, pp. 375-384.

Nicou, R. and Chopart, J.L., 1979b. Water management in sandy soils of Senegal. In: R. Lal (Editor), Soil Tillage and Crop Production. IITA, Proc. Ser. 2: 248-257.

Nye, P.H., 1952. Studies on the fertility of Gold Coast Soils IV. The potassium and calcium status of the soils and the effects of mulch and kraal manure. Emp. J. Exp. Agric., 20: 227- 233.

Obi, M.E. and Nnabude, P.C., 1988. The effects of different management practices on physical properties of a sandy loam soil in northern Nigeria. Soil Tillage Res., 12:81-90.

Ofori, C.S., 1973. The effect of ploughing and fertilizer application on the yield of cassava (31a- nihot esculenta Crantz). Ghana J. Agric. Sci., 6:21-24.

Ofori, C.S. and Nandy, S., 1969. The effect of method of soil cultivation on yield and fertilizer response of maize grown on a forest ochrosol. Ghana J. Agric. Sci., 2: 19-24.

Ogunremi, L.T., Lal, R. and Babalola, O., 1986. Effects of tillage methods and water regimes on


soil physical properties and yields of upland rice for an Ultisol in southeast Nigeria. Soil Tillage Res., 6: 305-324.

Okigbo, B.N., 1977. Farming systems and soil erosion in West Africa. In: D.J. Greenland and R. Lal (Editor), Soil Classification and Management in Humid Tropics. Wiley, Chichester.

Okigbo, B.N., 1979. Effects of preplanting cultivation and mulching on the yield and perform- ance of cassava (Manihot esculenta). In: R. Lal (Editor), Soil Tillage and Crop Production. IITA Proc. Ser. 2: 75-92.

Okigbo, B.N. and Lal, R.. 1977. Role of cover crops in soil and water conservation. FAO Soil Bull., 33: 97-108.

Opara-Nadi, O.A. and Lal, R., 1986. Effects of tillage methods on physical and hydrological properties of a tropical Alfisol. Z. Pflanzenernaehr. Bodenkd., 149: 235-243.

Palmer, J.E.S., 1958. Soil Conservation at Mokwa, Northern Nigeria. Trop. Agric. (Trinidad), 35: 34-40.

Pereira, H.C. Wood, R.A., Brzostowski, H.W. and Hosegood, P.H., 1958. Water conservation by fallowing in semi-arid tropical East Africa. Emp. J. Exp. Agric., 24:213-228.

Rockwood, W.G. and l.al. R., 1974. Mulch tillage: a technique for soil and water conse~'ation in the tropics. SPAN: Prog. Agric., 17: 77-79.

Roose, E., 1967. Dix annres des mesure de l'erosion et du ruissellement au Senegal. Agron. Trop. Paris, 22: 123-152.

Roose, E.J., 1973. Dixsept anndes de mesures experimentales de l'erosion et du ruissellement sur un sol ferralliqiue sableux de basse Cote d'Ivoire - Contribution a l'etude de l'erosion hydrique en milieu intertropical. ORSTOM, Abidjan: Th~se Doct. Ingeneur FAC. Sci. Abid- jan No. 20, 125 pp.

Roose, E., 1975. Natural mulch or chemical conditioner for reducing soil erosion in humid tropical areas. In: Soil Conditioners. SSSA, Special Publication No. 4:131-138.

Roose, E.J., 1977a. Application of the universal soil loss equation of Wischmeier and Smith in West Africa. In: D.J. Greenland and R. Lal (Editors), Soil Conversation and Management in The Humid Tropics. Wiley, U.K., pp. 177-188.

Roose, E.J., 1977b. Erosion et ruisellement en Afrique de l'Ouest. Vingt annres de mesures en petites parcelles experimentales. Travaux et Doc. ORSTOM Paris No. 12, 108 pp.

Roose, E., 1978. Approach to the definition of rain erosivity and soil erodibility in West Africa. In: M. de Boodt and D. Gabriels (Editors), Workshop of the Assessment of Erosion in USA and Europe. Wiley, U.K., pp. 153-164.

Roose, E., 1986. Terraces de diversion au microbarrages permeable? Analyse de deux de- marches de conservation de l'eau et des sols chez les petites fermes de la zone Soudano- Sohelienne d'Afrique Occidentale. Cah. Orstom Pedol., 22 ( 2 ): 81-92.

Roose, E.J., 1976. Use of the universal soil loss equation to predict erosion in West Africa. Soil Conserv. Soc. Am. Spec. Publ., 21 : 60-74.

Roose, E.J., 1988. Soil and water conservation lessons from steep-slope farming in French- speaking countries of Africa. In: W.C. Moldenhauer and N.W. Hudson (Editors), Conser- vation Farming on Steeplands. SWCS, Ankeny, IA, pp. 129-139,

Roose, E.J., 1989. Diversity of traditional and modern strategy for soil and water conservation. Ecological and ethnical impact in Sudano-Sahelian areas of Western Africa. ORSTOM Montpellier, 20 pp. Paper presented at the Sixth ISCO Conference, Addis Abeba, Ethiopia, in press.

Roose, E., 1990. Methodes traditionalles de gestion de l'eau et de sols en Afrique occidentale Soudano-sohelienne: difinition, fouctionnement limits, et ameliorations possible. Bulletin Reseau Erosion, No. 10, Montpellier, pp. 98-107.

Roose, E.J. and Bertrand, R., 1971. Contribution a l'etude de la methode des bandes d'arret pour lurer contre l'erosion hydrique en Afrique de l'Ouest. Resultats experimentaux et observations sur le terrain. Agron. Trop. Paris, 26: 1270-1283.


Roose. E. and Plot. J., 1984. Runoff, erosion and soil fertility restoration on the Mossi Plateau (Burkina Faso). In: Challenge in African hydrology. Harare. IASH Publ. No. 144: 485-498.

Soil Conservation Society of America, 1982. Resource Conse~'ation Glossary. Ankeny, IA. Some, L. and Ovaltara, B., 1991. Effet du ruisellement el de e'irosion sur le li lan hydrix et le

rendement en sorgho a saria (Burkina Faso). Bulletin Reseau Erosion No. 11, Montpellier. pp. 139-155.

Tourte, R., Nicou, R. and Boulieu, A., 1963. Quelques techniques de culture des mils et sorghos au Senegal. Possibilites de la culture mecanique. Agron. Trop. Paris, 18: 65-72.

Vegara, N.T., 1982. New Directions in Agroforestry: The potential of Tropical Legume Trees. Improving Agroforestry in the Asia Pacific Tropics. East-West Centre, Honolulu, Hawaii.

Verney, R. and Williams, P., 1965. Results of studies of erosion on experimental plots in Da- homey. Proc. OAU/STRC symposium on the maintenance and improvement of soil fertility, Khanoum.

Willis, W.O., 1976. Soil water management and other factors that affect crop production. In: Efficiency of Water and Fertilizer Use in Semi-arid Regions, IAEA Rep., 192: 1-19.

Wilson, G.F. and Lal, R., 1984. New approaches towards soil and water conservation in tropical Africa. In: R. Lal, P.A. Sanchiez and R.W. Cummings (Editors), Land Clearing and Devel- opment in the Tropics. A.A. Balkema, Rotterdam.

tillage methods and soil and water conservation in west africa

Documents