Technologies for sustainable management of sandy Sahelian soils

Bationo, A.¹; J. Kihara¹; B. Waswa¹; B. Ouattara¹ and B. Vanlauwe¹

Keywords: Sandy soils, inorganic and organic fertilizers, microdose, fertilizer pleacement, water harvesting

Introduction

The Sahelian zone of West Africa is the home of the world’s poorest people, 90% live in villages and gain their livelihood from subsistence agriculture. Per capita food production has declined significantly over the past three decades. According to the FAO, total food production in Sahelian countries grew by an impressive 70% from 1961 to 1996, but it lagged behind the population which doubled causing food production per capita to decline by approximately 30% over the same period. High population densities have necessitated the cultivation of marginal lands that are prone to erosion. Consequently, present farming systems are not sustainable.

Sahelian countries produce 80% of their total cereal production under very difficult conditions. Rainfall is generally low, variable and unreliable (Toupet 1965) with a growing period of 60-100 days (Sivakumar, 1986). The average annual rainfall of the cultivated zones varies from 300 to 900 mm and the ratio of annual rainfall to annual potential evapo-transpiration varies from 0.20 to 0.65.

The rainfall in West Africa shows a significant north-south gradient because of the inter-seasonal movement of the inter-tropical convergence zone, north and south of the equator. The rainfall in the Sudano-Sahelian zone is low, variable, and unreliable. Time-dependent variations in rainfall are quite common in the region with coefficient of variation of annual rainfall ranging between 15-30% (Figure 1). Nicholson (1981) showed that in 1950, rainfall over West Africa was above normal, at some locations as high as 250% above normal. However, in 1970, rainfall was below normal throughout the region. As a result of rainfall variability average yields of sorghum and pearl millet have shown significant inter-annual variability (Figure 2).

In addition to the unpredictable climatic conditions, the region is experiencing declining production as a result of negative nutrient balances reported for most cropping systems. Stoorvogel and Smaling (1990) reported that in 1983 49 kg/ha or an average of 9.3 million kg of nutrients was lost in sub-Saharan Africa. Projections for the year 2000 showed that nutrient losses will increase to 60 kg per year. In Burkina Faso, with a cropped area of 6.6 million ha, the nutrient losses were estimated at 95,000 tonnes of N, 12,128 tonnes of P and 65,357 tonnes of K or the equivalent of US$159 million for the purchase of N, P and K fertilizers (Table 1).

Figure 1. Percentage deviation of annual rainfall from the mean at Niamey, Niger

Figure 2. Sorghum and millet yields in the Sahel over the period 1961 to 1996

Table 1. Nutrient losses for some West African countries

This situation has stemmed from increasing population pressure, and soil degradation in particularly drought-prone region where the soils are naturally infertile. Currently over a quarter of West Africa sub-region’s population of two hundred million inhabitants are threatened by food insecurity. It is estimated that the productivity of land currently under cultivation should increase by at least 3% per annum in order to meet the food demands of the regions population. Any program aimed at reversing the declining trend in agricultural productivity and preserving the environment for present and future generations in West Africa should begin with soil fertility restoration and maintenance.

This paper highlights various soil fertility restoration technologies tested in the sandy sahelian zones of West Africa. The paper discusses the crop production environment in West Africa, the combined use of organic and inorganic nutrient sources, fertilizer application technologies, cropping systems manage­ment, combined water harvesting and nutrient management (INM), phosphate rock utilization and plant genetic utilization for drought resistance before presenting future research challenges and conclusions.

Crop production environment

The most common cropping systems in the Sudano-Sahelian zones involve growing several crops in association as mixtures or intercrops. This practice provide the farmer with several options for returns from land and labour, often increased efficiency with which scarce resources are used, and reduce dependence upon a single crop that is susceptible to environmental and economic fluctuations. Types of crop associations differ from place to place, with ecological zone, farm size, human population, and soil fertility as well as with cultural and socio-economics factors.

In the Sudanian zone, sorghum based cropping systems are common. Millet, maize, groundnut, and cowpea are important components of this system (Steiner, 1984); whereas in the Sahelian zone, the cropping system is millet based, with millet/cowpea and millet/groundnut being the most important cropping patterns. While considerable information is available on fertilizer requirements for single crop species, little is known for intercropping.

The increase in production of pearl millet and sorghum has been due to the expansion of area cultivated and not to the increase of productivity per unit area. In Nigeria, whereas the area under cultivation doubled between 1979 and 1994, the yield per hectare declined from 1 tonne to 0.89 tonne over the same period. Groundnut and cowpea are the two predominant grain legumes in the Sudano-Sahelian zone. Groundnut occupies 2.7 million hectare of arable land and cowpea 6 million. The countries of West Africa have been traditional exporters of groundnut, but production has declined recently. Cowpea production on contrary has increased over the years, and in some countries such as Niger, it has more than doubled, largely because of an increase in area cultivated. Yields of cowpea grain are generally low, ranging between 50 and 300 kg/ha in marked contrast to yields of over 1,000 kg/ha obtainable on research station and by large scale commercial enterprises in Northern Nigeria. The potential for increased yields in the region is therefore high.

Crop livestock interaction

Crop-livestock interactions are evolving in West Africa, in relation to increased human and livestock populations and the resultant agricultural inten­sification (Smith et al., 1997). Such scenarios tend to force closer integration of crops and livestock, such that soil fertility replenishment depends on manure and urine from livestock, which in turn depend heavily on crop residues for feed. The most intense and integrated crop-livestock interactions occur in the drier parts of West Africa, in the Sudano-Sahelian zone, where annual rainfall is less than 800 mm. Over 50% of the ruminant population of West and Central Africa is found in the dry savannas and Sahel and the numbers of these livestock are predicted to increase at a rate of about 2% per annum between 1988 and 2025 (Winrock, 1992).

Interactions between crops and livestock can be beneficial or, if resources are limiting or overexploited, competitive. The transfer of nutrients from rangelands to cultivated fields by livestock provides the means for a redistribution of nutrients in time and space. Land that is unsuitable for cropping can be grazed thereby producing manure that in turn may make other land more suitable for crop production because livestock transform poor quality, bulky vegetation into products of high economic and nutritive value (Delgado et al., 1999). More specifically, direct positive contributions of livestock to soil fertility through manure and urine may include improved organic matter content, supply of nutrients (especially N and P), improved nutrient retention capacity, and a better soil physical condition as a result of improved water holding capacity.

It is also important to be aware of the potential negative effects of livestock on soils in order that these could be minimized. Livestock can contribute to vegetation removal and, through trampling, soil compaction, thereby accelerating soil runoff and erosion which may be associated with increased soil bulk density and decreased infiltration rates (Delgado et al., 1999). Through voiding ingested nutrients, especially N in urine, which is subsequently volatilized, livestock may induce nutrient loss to the system (Mohamed-Saleem and Fitzhugh, 1995).

Soil types and their fertility status

The sandy Sahelian soils are dominated by low activity clay soils consisting mainly of Entisols and Alfisols (Kang 1985). Entisols are mainly composed of quartz sand. Alfisols have a clay accumulation horizon and a high base saturation because of lower rainfall and leaching. Both soils have poor structural stability, low water retention and nutrient holding capacity, low organic matter content and low effective cation exchange capacity (ECEC) and are highly susceptible to drought (Kang 1985, Bationo and Mokwunye 1991). Phosphorus is the most limiting factor to crop production in these sandy soils of Sahelian zone followed by nitrogen. Available P in these soils is less than 2 mg P/kg while the amount of total P ranges between 25 to 340 mg/kg with a mean of 109 mg/kg (Manu et al., 1991). These soils therefore present major constraints for arable production.

Nutrient balances are negative for many cropping systems suggesting that farmers in West African countries are mining their soils (Table 2). In Burkina Faso, estimates indicate that in 1983, for a total of 6.7 million hectares of land cultivated, soil nutrient mining amounted to a total loss of 95,000 tonnes of N, 12,100 tonnes of P and 65,000 tonnes of K. The significance of these figures is alarming when it is realized that productivity of these soils in their native state is already low because of low native plant nutrient fertility levels.

Table 2. Aggregated nutrient budgets (losses) for some West African countries

There is a close relationship between the main dominant soil types in the various agro-ecological zones, while Ferrasols are dominant in the humid zones, Lixisols and Arenosols constitute the major part of in the Sudano-Sahelian zones.

The main physico-chemical features of these soils are as follows:

• Arenosols (Psammients in the American classification).
• Arenosols mainly consist of quartz with low water retention capacity and low nutrient content. These soils account for about 6%, 15%, 10%, 30%, 13% and 30% of the area of Burkina Faso, Mali, Mauritania, Niger, Nigeria and Senegal respectively.
• Lixisols (Alfisols in the American classi­fication).

They form 80%, 46%, 52%, 55%, 34% and 59% of the total area of Benin, Burkina Faso, Ghana, Guinea Bissau, Nigeria and Togo respectively, these are soils which have a horizon characterized by the accumulation of clay with a low nutrient accumulation capacity but are well saturated in cations.

A survey of the physical and chemical pro­perties of selected soils from West Africa was undertaken and multivariate analysis techniques such as discriminate analysis to compare the pearl millet producing soils with the sorghum/maize soils used (Table 3).

Most of the millet, sorghum, and maize-producing soils in West Africa are sandy and it influences the physical and chemical characteristics of these soils. A majority of the West African soils have low levels of organic matter, total nitrogen, and effective cation exchange capacity (ECEC). The main source of nitrogen is accumulated organic matter and its naturally low level of ECEC could be attributed to low organic matter and clay contents, the correlation being higher with organic matter content. The kaolinitic mineralogy of the soil systems also influenced ECEC.

Nitrogen sources and management

Bationo and Mokwunye (1991) reported a very significant relationship between soil organic matter (SOM) and total nitrogen in the Sudano-Sahelian. Soil organic matter is highly correlated with the clay content of soil and as result of the sandy nature of the soils in the Sudano-Sahelian zone, total organic matter remains very low in most of the soils in the region. In many cropping systems, little or no agricultural input is added to the soil. This leads to a decline in soil nitrogen which frequently results in lower crop yields or soil productivity. As predicted by Stoorvogel and Smaling (1990) countries like Burkina Faso, Mali, Niger and Senegal lose as much as 14, 8, 16, 12 kg N/ ha per year respectively (Table 4).

In the Sudano-Sahelian zone, there is a flush of N at the beginning of the wet season. The magnitude of this flush seems to be proportional to the duration of the preceding dry season (Semb and Robinson, 1969). This flush may range between 13 to 183 kg N/ ha, but to what degree this release of N will be beneficial to the subsequent crop depends on the intensity and the frequency of the rains early in the season. Annual crops will not have developed sufficiently to utilize a significant proportion of the N flush early in the season hence, most of it is susceptible to losses through leaching (Ssali et al. 1986).

Table 3. Univariable analysis of variance of the pearl millet and sorghum/maize soils groups

Table 4. N, P, and K losses in selected West African countries

Due to a lack of synchrony between nutrients released by soil organic matter and mineral fertilizer, nutrients especially N, are lost through leaching, volatilization, and denitrification. As pointed out by Myers et al. 1994, synchrony can be promoted by manipulating plant demand (controlling planting date, duration of crop to be grown, use of crops with different growth patterns in multiple cropping systems).

For many years several scientists in the Sudano-Sahelian zones initiated research to assess the efficiency of N fertilizers in order to increase food production (Christianson and Vlek 1991, Bationo et al. 1989, Bationo and Vlek 1998). Urea and calcium ammonium nitrate (CAN) are the most common sources of N used by farmers. Results of trials undertaken to evaluate these two sources and methods of application of nitrogen led to the conclusions that: 1) fertilizer N recovery by plants was low; 2) there is a higher loss of N at the placement point of urea (>50%) and the mechanism of N loss is believed to have been associated with ammonia volatilization; 3) losses of N from CAN were less than from urea because one-half of the N in CAN is in the non-volatile nitrate form; 4) although CAN is a lower N analysis fertilizer than urea, it is attractive as an N source because of its low potential for N loss via volatilisation and its low soil acidifying properties (Table 5) (Bationo et al. 2003). Point placement of CAN outperformed urea point placed or broadcast and ¹⁵N data from similar trials indicate that uptake by plants was almost three times higher than that of urea applied in the same manner (Figure 5).

Table 5. Recovery ¹⁵N fertilizer by pearl millet applied at Sadore, Niger, 1985

Figure 3. Effect of broadcast and point application methods for Urea and CAN on grain yield of pearl millet Christianson and Vlek (1991) found that the optimum N rate for sorghum is 50 kg/ha and 30 kg/ha for pearl millet

Nitrogen use efficiency can be increased through rotation of cereals with legumes and through the optimization of planting density. Bationo and Vlek (1998) reported a N-use efficiency of 20% in the continuous cultivation of pearl millet but the value increased to 28% when pearl millet was rotated with cowpea. In a study by Bationo et al. (1989) found a strong and positive correlation between planting density and response to N fertilizer.

Crop rotation

Rotation of cereals and legumes is a cheaper means of improving N availability. Cereal/legume rotation effects on cereal yields have been reported by several researchers (Bagayoko et al. 1996; Bationo et al. 1998; Bationo and Ntare 1999). Table 6 shows the effect of cowpea-millet rotation on millet grain and total biomass production. In a period of three years, there was an increase of about 3 t/ha of total dry matter production when millet was grown in rotation with cowpea.

Table 6. Millet grain and total dry matter yield at harvest as influenced by millet/cowpea cropping system at Sadore (Niger)

Nitrogen use efficiency increased from 20% in continuous pearl millet cultivation to 28% when pearl millet was rotated with cowpea. Nitrogen derived from the soil is better used in rotation systems than with continuous millet (Bationo and Vlek 1998). Nitrogen derived from the soil increased from 39 kg N/ha in continuous pearl millet cultivation to 62 kg N/ha when pearl millet is rotated with groundnut. Those data clearly indicate that although all the above biomass of the legume will be used to feed livestock and not returned to the soil, rotation will increase not only the yields of succeeding cereal but also its nitrogen use efficiency (Bationo and Vlek 1998).

The response of legumes to rotation was also significant and legume yields were consistently lower in monoculture than when rotated with millet (Figure 4). This suggests that factors other than N alone contributed to the yield increases in the cereal-legume rotations.

Phosphorus sources and management

Among soil fertility factors, phosphorus deficiency is a major constraint to crop production in the Sudano-Sahelian zone. For many years, research has been undertaken to assess the extent of soil phosphorus deficiency, to estimate phosphorus requirement of major crops, and to evaluate the agronomic potential of various phosphate rock (PR) from local deposits (Bationo et al., 1990). About 80% of the soils in sub-Saharan Africa are short of this critical nutrient element and without the use of phosphorus, other inputs and technologies are not effective. However, sub-Saharan Africa use 1.6 kg P/ha of cultivated land as compared to 7.9 and 14.9 respectively for Latin America and Asia. It is now accepted that the replenishment of soil capital phosphorus is not only a crop production issue, but an environmental issue and P application is essential for the conservation of the natural resource base.

Figure 4. Effects of nitrogen and rotation on legume stover yield (kg/ha), average of four years (1989-1992) at Tara and Bengou, Niger

Availability and total P levels of soil are very low in the sandy Sudano-Sahelian zones of West Africa (SSZWA) as compared to the other soils in West Africa (Manu et al. 1991). For the sandy Sahelian soils total P values can be as low as 40 mg P/kg and the value of available P less than 2 mg P/kg. A study of the fertility status of selected pearl millet producing soils of West Africa, (Manu et al. 1991) found that the amount of total P in these soils ranged from 25 to 340 mg/kg with a mean of 109 mg/kg. The low content of both total and available P parameters may be related to several factors including 1) parent materials, which are mainly composed of aeolian sands, contain low mineral reserves and lack primary minerals necessary for nutrient recharge; 2) a high proportion of total P in these soils is often in an occluded form and is not available to crop (Charreau, 1974); 3) low level of organic matter and the removal of crop residue from fields. Organic matter has a favourable effect on P dynamics of the soil; in addition to P release by mineralization, the competition of organic ligands for Fe and Al oxides surface can result in a decrease in P fixation of applied and native P.

The P sorption characteristics of different soil types have been investigated and compared to the soils of the more humid regions. The soils of the SSZWA have very low capacity to fix P (Sanchez et al., 1980). For pearl millet producing soils, Manu et al. (1991) fitted the sorption data to Langmuir equation (Langmuir 1918), values of maximum P sorbed ranged from 27 mg/kg to 253 mg/kg with a mean of 94 mg/kg.

Phosphorus deficiency is a major constraint to crop production and responses to nitrogen applications are substantial only when both moisture and phosphorus are not limiting. Field trials were established to determine the relative importance of N, P and K fertilizers. The data in Table 7 indicates that from 1982 to 1986 the average control plot yield was 190 kg grain/ha. The sole addition of 30 kg P/ha without N fertilizers increased the average yield to 714 kg/ha. The addition of only 60 kg N/ha did not increase the yield significantly over the control and the average grain yield obtained was 283 kg/ha. The data clearly indicate that P is the most limiting factor in these sandy Sahelian soils and there is no significant response to N without correcting first for P deficiency. When P is applied the response to N can be substantial and with the application of 120 kg N/ha pearl millet grain yields of 1,173 kg/ha were obtained as compared to 714 kg/ha when only P fertilizers were applied. Over all years the addition of potassium did not increase significantly the yield of both grain and total dry matter of pearl millet.

Table 7. Effect of N, P, and K on pearl millet grain and total dry matter (kg/ha) at Sadoré and Gobery (Niger)

The use of alternative locally available phosphate rock

Phosphate Rock utilization

Despite the fact that deficiency of P is acute on the soils of West Africa, local farmers, use very low amounts of P fertilizers partly because of the high cost. The use of locally available phosphate rock (PR) could be an alternative to imported P fertilizers. Bationo et al. (1987) showed that direct application of local PR could be more economical than imported water-soluble P fertilizers. Bationo et al. (1990) showed that Tahoua PR from Niger was suitable for direct application, but Parc-W from Burkina Faso had less potential for direct application. The effectiveness of local phosphate rock depends on its chemical and mineralogical composition, the most important feature being the ability of carbonate ions to substitute for phosphate in the apatite lattice which influences the solubility, and controls the amount of phosphorus available to crops (Smith and Lehr, 1966). As shown in Table 8, in medium to long-term experiments, Tilemsi PR which is a medium-reactive rock with a total P₂O₅ of 29%, was practically equivalent to TSP per unit P (Zapata and Roy, 2004). Its relative agronomic effectiveness averaged about 80% compared to TSP.

Table 8. Yield of Millet, groundnut, sorghum, cotton and maize with Tilemsi PR and TSP in Mali, 1982-87

Phosphorus (P) placement and P replenishment with Phosphate rock

Fertilizer management practices also play an important role in increasing P use efficiency. The data in Table 9 clearly shows that hill placement of small qualities of P fertilizers resulted in a higher phosphorus use efficiency (PUE) when compared to the broad­casting of 13 kg P/ha as recommended by the extension services.

Single Superphosphate (SSP), Tahoua Phos­phate Rock (TPR) and Kodjari Phosphate Rock (PRK) were broadcast (BC) and/or hill placed (HP). For pearl millet grain P use efficiency (PUE) for broadcasting SSP at 13 kg P/ha was 18 kg/kg P but hill placement of SSP at 4 kg P/ha gave a PUE of 83 kg/kg P. Whereas the PUE of TPR broadcast was 16 kg grain/kg P, the value increased to 34 kg/kg P when additional SSP was applied as hill placed at 4 kg P/ha. For cowpea fodder PUE for SSP broadcast was 96 kg/kg P but the hill placement of 4 kg P/ha gave a PUE of 461 kg/kg P. Those data clearly indicate that P placement can drastically increase P use efficiency and the placement of small quantities of water-soluble P fertilizers can also improve the effectiveness of phosphate rock (Table 9).

PUE in the sandy soils of West Africa can also be dramatically increased with the adoption of improved crop and soil management technologies. Whereas the absolute control recorded 33 kg ha^-1 of pearl millet grains, 1,829 kg ha^-1 was obtained when phosphorus, nitrogen and crop residue was applied to the ridge and fallowed leguminous cowpea in the previous season. Results indicate, for the grain yield, that PUE increases from 46 with only P application to 133 when P is applied in combination with nitrogen, crop residues and the crop is planted on ridges in a rotation system (Table 10).

Combined organic and inorganic nutrient sources

Combined application of organic resources and inorganic inputs resources in agricultural production has gained increasing popularity in the recent years. Farmyard manure and crop residues are frequently the most widely used because of their availability to farmers. Although most of these are often low in N and P, numerous research reports show large crop yield increases resulting from combination of organic resources and mineral fertilizers in the Sahelian zone of West Africa (Abdullahi and Lombin 1978; Bationo et al., 1993; Bationo et al., 1998; Pieri, 1989).

Table 9. Effect of P sources and placement on pearl millet and cowpea yield (kg/ha) and P use efficiency (PUE) (kg/kg P)

Table 10. Effect of mineral fertilizers, crop residue (CR) and crop rotation on pearl millet yield (kg/ha) and phosphorus use efficiency (PUE) Sadore, Niger, 1998 rainy season

Manure plays a substantial role in enhancing crop yields on nutrient poor West African soils (Sedogo, 1993). According to Bationo et al. (2004), studies conducted in Mali, Burkina Faso and Niger show that manure collected from stables and applied alone produced about 34-58 kg of cereal grain dry matter (DM)/t manure and 106-178 kg of DM/t manure in stover. The application of manure together with inorganic fertilizer gave yields of 80-90 kg of grain DM/t manure and 84-192 kg of stover DM/t manure. Combined use of manure and mineral fertilizer show a long-term increase of sorghum yields over years (Figure 5). Bationo and Mokwunye (1991) have also shown that manure can help supply P when they found no difference between applying 5 t/ha of FYM and 8.7 kg P/ha as Single Superphosphate. According to Palm (1995) for a modest yield of 2 t/ha of maize the application of 5 t/ha of high quality manure can meet the N requirement but this cannot meet the P requirements in areas where P is deficient.

Figure 5. Sorghum grain yield as affected by mineral and organic fertilizers over time. Source: Sedogo (1993)

A significant effect between crop residue and mineral fertilizer on crop yield has also been reported (Bakayoko et al. 2000). From a long-term experiment initiated in 1984 in Sadore, Niger, Bationo et al. (1993) found that grain yield declined to 160 kg/ha in un-mulched and unfertilized plots as compared to 770 kg/ ha with a mulch of 2 t crop residues per hectare and 1,030 kg/ha with 13 kg P plus 30 kg N/ha. Further, the combination of crop residue and mineral fertilizers resulted in grain yield of 1,940 kg/ha. Table 11 shows that, at the same level of target P, treatments with combined mineral and organic fertilizers had the highest yields demonstrating the advantage of combined nutrient application. In 2004 for example, the use of only inorganic P sources yielded respectively 1,350 kg/ha and 3,156 kg/ha of cowpea fodder whereas the same rate in the combined organic-inorganic form (6 t manure, 3 kg P and 30 kg N/ha) gave 2,088 kg/ha and 4,219 kg/ha at Banizoumbou and Karabedji respectively (Table 11).

Table 11. Optimum combination of plant nutrients for millet grain and cowpea fodder (kg/ha) at Banizoumbou and Karabedji, Niger, 2004 cropping season

In different parts of SSA, application of organic residues has been shown to increase soil P availability due to the complexation of iron and aluminium by organic acids (Sahrawat et al., 2001; Kretzschmar et al., 1991), resulting in better root growth (Hafner et al., 1993), improve potassium (K) nutrition (Rebafka et al., 1994), protect young seedlings against soil coverage during sand storms (Michels et al., 1995), increase water availability (Buerkert et al., 1996), and reduced soil surface resistance by 65% and topsoil temperature by over 4ºC (Buerkert et al., 1996), maintain soil organic carbon in the topsoil, increase the ECEC and subsequently the nutrient holding capacity of these soils (Bationo and Mokwunye 1991; De Ridder and van Keulen 1990). The application of 4 t of crop residue per hectare for example maintained soil organic carbon at the same level as that in an adjacent fallow field in the top soil but continuous cultivation without mulching resulted in drastic reduction organic carbon (Figure 6). These effects are stronger especially in the Sahelian zone, but weaker in other areas with lower temperatures, higher rainfall and heavier soils (Buerkert et al., 1996). Organic amendments have also been reported to reduce the capacity of the soil to fix P thereby increasing P availability for uptake and hence higher P use efficiency (Sahrawat et al., 2001).

Figure 6. Effect of different management practices on soil organic carbon content after 14 years of cultivation, Sadore, Rainy season 1997

Availability of organic inputs in sufficient quantities and quality is one of the main challenges facing farmers and researchers today (Bationo and Buerkert 2001). On fields of unfertilized local cultivars, grain yield averaged only 236 kg/ha and mean residue yields barely reached 1,300 kg/ha. In light of many competing uses for biomass such as fodder and fuel for cooking, it is unlikely that the recommended levels of crop residue could be available for use as mulch. At planting, although farmers require at least 2 t/ha of crop residue for mulch, only 250 kg/ha of crop residue is presently available.

Cropping systems management

Widespread use of cereal/legume rotations or intercrops has been suggested as a means to sustainably meet increasing food demands in West Africa (Alvey et al., 2001). Integration of cereals and legumes in production systems has been tested in order to utilize the N fixation ability and the P solubilization potential of legumes. Nitrogen-15 (¹⁵N) for example used to quantify the amounts of nitrogen fixed by cowpea under different soil fertilization levels has shown that nitrogen derived from the air (NDFA) varies from 65 to 88% and can reach up to 89 kg N/ha (Bationo and Vlek 1998). The legume-cereal rotations will increase both the yield of succeeding cereal crops and its nitrogen use efficiency. In addition to that, the above-ground legume biomass is high quality forage for livestock.

Rotation of cereals and legumes is a cost effective means of improving soil fertility and productivity. Field experiments at several sites in West Africa have shown cereal yield increases in cereal/ legume rotations of between 15 and 79% compared with continuous cereal systems. Rotation of cereals with legumes increases N use efficiency. For example, Bationo and Vlek (1998) have shown that nitrogen use efficiency increased from 20% in continuous pearl millet cultivation to 28% when pearl millet was rotated with cowpea. However, the degree to which N is involved as a driving force in these effects remains unclear (Bagayoko et al., 2000; Bationo et al., 1998). It has been assumed by many workers that the positive effect of rotations arises from the added N from legumes in the cropping system. Some workers, however, have attributed the positive effects of rotations to an improvement of soil biological and physical properties and the ability of some legumes to solubilize occluded P and highly insoluble calcium bounded phosphorus by legume root exudates (Arhara and Ohwaki 1989; Sahrawat et al. 2001) and to the lower amounts of plant parasitic nematodes (Pierce and Rice, 1988). Other advantages of crop rotations include soil conservation (Stoop and Staveren 1981), organic matter restoration (Spurgeon and Grisson 1965) and pest and disease control (Sunnadurai 1973; Pierce and Rice, 1988).

The data in Figure 7 illustrate the response of pearl millet grain to the crop rotation and to different inputs of organic and inorganic fertilizers. Farmer’s practices yielded 161 kg/ha with no fertilizer and 631 kg/ha with the application of 13 kg P and 45 kg N/ha However, when these mineral fertilizers were combined with 2.7 t/ha of manure or crop residue in rotation with cowpea, yields of 1,504 kg/ha were achieved. The highest yield was observed in treatments involving rotation as opposed to no rotation treatments at the same rate of manure or crop residue.

Changes in soil chemical properties from long-term cropping system management trials, monitored in different agro-ecological zones of the Sudano-Sahelian region showed that rotations resulted in significantly higher soil pH, total N and effective cation exchange capacity (ECEC) (Bationo et al. 1995). In the long-term cropping system management studies in the Sahel, rotation systems were found to have higher levels of organic carbon compared to the continuous cropping system (Figure 8). This could partially be due to the contribution made by the fallen leaves of the cowpea crop in the crop rotation.

Figure 7. Effect of different N and P rates on pearl millet grain yield, Sadore, Niger, 2004 rainy season

Figure 8. Effect of phosphorus and cropping system on soil organic carbon, Sadore, Niger, 1995

Combined water harvesting and integrated nutrient management (INM)

The intensification of crop production requires an integration of soil, water and nutrient management that is locally acceptable and beneficial for smallholder farmers (Zougmore, 2003a). Some methods such as terraces, minimum tillage, contour plots, tied ridges, stone bunds/lines, vegetative bands/diguettes, buttages, and Zaï (half moons) are used in the semi-arid lands of West Africa. According to Rockstrom et al. (2001) the keys to improved water productivity and mitigating intra-season dry spells in rainfed agriculture are maximizing the amount of plant available water and plant water uptake capacity. This implies systems that partition more incident rainfall to soil storage and less to runoff, deep percolation and evaporative loss, as well as crops that provide more soil cover and root more deeply (Rockstrom et al., 2001). In the Central Plateau of Burkina Faso stone bounds alone doubled sorghum yield compared to plots without stone bounds and therefore, can reduce risks of crop failure in erratic rainfall years (Zougmore, 2003a). Taonda et al., (2003) found that water harvesting alone with stone bunds did not improve yields but the combination of water harvesting with stone bunds or zai plus manure more than doubled sorghum yields when compared to the control (Table 12).

Table 12. Variation of the grain yields of the sorghum during four years (kg/ha)

A study by Zougmore and Zida (2000) has clearly shown the effects of the spacing between installations of stone bunds on decreasing runoff, erosion and increasing yields. A spacing of 50 m reduced runoff by 5%, a spacing of 33 m by 12% and that of 25 m by 23%. Yield losses were reduced by 21% with a spacing of 50 m and 61% with a spacing of 25 m. With lower rainfall than normal, whereas yield increases were 58% with spacing of 50 m, it increased up to 343% with a spacing of 25 m. In a normal year increases were however not as much and the trend was even negative in years when rainfall was higher than the long-term average.

Zougmore et al. (2003a) found an average reduction in runoff of up to 59% in plots with barriers alone, but reached 67% in plots with barriers + mineral N and 84% in plots with barriers + organic N. On average, stone bunds reduced soil erosion more than grass strips (66% versus 51%). Integrated water and nutrient management may help to alleviate poverty and may empower smallholder farmers to invest in soil management for better crop production (Zougmore, 2003a).

The higher yields with Zai only (1,968 kg/ha) compared to N (1,490 kg/ha) and P (1,524 kg/ha) treatments showed that water is the most limiting resource to crop production in the West African Sahel (Figure 9). Higher yields were observed when water harvesting techniques using Zai were combined with either N or P pointing to better fertilizer use efficiency.

Figure 9. Effect of Zai, N and P on sorghum ear and grain yield in Tougouri, Burkina Faso, 2004 (Badiori et al. unpublished data)

Restoring favourable soil moisture conditions by breaking up the surface crust to improve water infiltration (half-moon technique) with appropriate nutrient management could be an effective method for the rehabilitation of degraded soil and improving productivity (Zougmore et al., 2003b). Animal drawn rippers and subsoilers could increase water productivity by increasing water infiltration and storage as well as root penetration (Rockstrom et al., 2001). Considerable yield increases above “farmers’ practices” (i.e. flat cultivation and no fertilizer) could be realized by combining tied-ridged tillage with inputs of mineral N and P fertilizer, reaching maize grain yield levels of six times the prevailing yield under farmers’ practices of approximate 1 t/ha (Jensen et al., 2003).

Conclusions

The use of rotation systems, organic and inorganic nutrients combinations and water harvesting technologies are an attractive alternative to the traditional farming systems in the Sahel, not only for increased food production but also for soil fertility improvement. However, few resource poor farmers have adopted the technologies proposed by researchers because of their capacity to invest in onerous soil fertility management and other socio-economic factors. To be adopted, researchers have to test their technologies in a participatory approach with land users. Consideration of the economic benefits of the technologies may be an important prerequisite for the success of adoption of the technologies. Besides, markets are increasingly becoming part of the research process since farmers have to trade cash crops and excess food crops produced.

Future research challenges include combining rainwater and nutrient management strategies to increase crop production and prevent land degradation, increasing the legume component for better integration of crop-livestock production systems, exploiting the genetic variation for nutrient use efficiency and integration of socio-economic and policy research with the technical solutions. Another very important issue for research is how to increase crop biomass availability at the farm level to alleviate the constraint of non-availability of organic amendments in order to maintain adequate soil organic matter levels for favourable soil conditions. Providing farmers with appropriate technologies and alternatives to soil mining and integrating the above and below ground resources conservation will contribute to the maintenance of soil quality. Selection of genotypes that can efficiently associate with Vesicular-Arbuscular Mycorrhizal (VAM) for better utilization of P applied as indigenous phosphate rock will increase benefits from these resources and increase their appeal to farmers. Use of decision support systems, modeling, and GIS is important in order to extrapolate research findings to other areas in which successful technologies can be expanded/scaled out to reach several farmers.

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¹ The Tropical Soil Biology and Fertility Institute of CIAT, P.O. Box 30677 00100 Nairobi, Kenya