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Food security in Sub-Saharan Africa, particularly in semi-arid tropics (41% of the region; 6 months of dry season) is threatened by droughts, dry spells and infertile soils. In Zimbabwe, 74% of smallholder farming areas are located in semi-arid areas mostly in areas with soils of low fertility and water holding capacity. The dominant crop in these areas, maize (Zea mays L.), is susceptible to drought. Under smallholder farming in Zimbabwe, conventional tillage entails cutting and turning the soil with a mouldboard plough thereby burying weeds and crop residues. Seed is planted by hand into a furrow made by the plough, ensuring that crops germinate in relatively weed free seedbeds. Inter-row weed control is performed using the plough or ox-drawn cultivators and hand hoes. Conventional tillage has been criticised for failure to alleviate negative effects of long dry spells on crops and to combat soil loss caused by water erosion estimated at 50 to 80 t ha-1 yr-1. Therefore, conservation tillage has been explored for improving soil and water conservation and crop yields. Our objective was to determine the maize yield advantage of the introduced technology (conservation tillage) over conventional tillage (farmers’ practice) based on a review of experiments in semi-arid Zimbabwe. We use a broad definition of conservation tillage instead of the common definition of =30% cover after planting. Eight tillage experiments conducted between 1984 and 2008 were evaluated. Conventional tillage included ploughing using the mouldboard plough and digging using a hand hoe. Conservation tillage included tied ridging (furrow diking), mulch ripping, clean ripping and planting pits. Field-edge methods included bench terraces (fanya juus) and infiltration pits. Results showed small yield advantages of conservation tillage methods below 500 mm rainfall. For grain yields =2.5 t ha-1 and rainfall =500 mm, 1.0 m tied ridging produced 144 kg ha-1 and mulch ripping 344 kg ha-1 more than conventional tillage. Above 2.5 t ha-1 and for rainfall >500 mm, conventional tillage had =640 kg ha-1 yield advantage. Planting pits had similar performance to ripping and conventional tillage but faced digging labour constraints. Experiments and modelling are required to test conservation tillage seasonal rainfall thresholds. Constraints to adoption of conservation tillage by smallholder farmers necessitate best agronomic practices under conventional tillage while work on adoption of alternative tillage methods continues.

Preventing the off-site effects of soil erosion is an essential part of good catchment management. Most efforts are in the form of on-site soil and water conservation measures. However, sediment trapping can be an alternative (additional) measure to prevent the negative off-site effects of soil erosion. Therefore, not all efforts should focus solely on on-site soil conservation but also on the safe routing of sediment-laden flows and on creating sites and conditions where sediment can be trapped. Sediment trapping can be applied on-site and off-site and involves both vegetative and structural measures. This paper provides an extensive review of scientific journal articles, case studies and other reports that have assessed soil conservation efforts and the sediment trapping efficacy (STE) of vegetative and structural measures. The review is further illustrated through participatory field observation and stakeholders’ interview. Vegetation type and integration of two or more measures are important factors influencing STE. In this review, the STE of most measures was evaluated either individually or in such combinations. In real landscape situations, it is not only important to select the most efficient erosion control measures but also to determine their optimum location in the catchment. Hence, there is a need for research that shows a more integrated determination of STE at catchment scale. If integrated measures are implemented at the most appropriate spatial locations within a catchment where they can disconnect landscape units from each other, they will decrease runoff velocity and sediment transport and, subsequently, reduce downstream flooding and sedimentation problems.

Since the 2007 food crisis, the Government of Benin has created enabling conditions that urge small scale farmers to intensify rice production in inland valleys. Unsuitable irrigation infrastructures and lack of willingness to act collectively has led farmers to complain about water shortage. Adoption of mitigating technologies offered by government (extension) and scientists (e.g. Benin Agricultural Research Institute, AfricaRice) has been low due to the institutional context within which farmers are working. A socio-technical approach which combines technical and institutional dimensions was used to identify and test mulching as a potential method for improving irrigation water efficiency in growing rice in upland parts of Benin's inland valleys. Rice farmers (from three production areas), an extension agent and a researcher formed a multi-stakeholder platform and collaborated to test the application of mulch (three doses) and the use of a lowland rice variety in replacement of an upland rice variety during two growing seasons. Multiple methods derived from researchers and farmers’ perspectives were used to evaluate trial results: quantitative scientific evidence was combined with qualitative evaluation using indicators agreed upon by the collaborative group. Results show that the lowland rice variety IR-841 with 10 Mg ha−1 ‘rice-straw’ mulch allows farmers to better use available water in the upland areas and increase yields. Although the preference for IR-841 over the special bred upland variety Nerica-4 is risky because of its high water demand and the uncertainty in rainfall, farmers use IR-841 for profit maximisation. Beyond its technical output, the joint experimentation facilitated the interaction of knowledge, experiences and practices among the involved actors.

Man-made and natural sediment sinks provide a practical means for reducing downstream reservoir sedimentation by decreasing soil erosion and enhancing the rate of sedimentation within a catchment. The Minizr catchment (20 km2) in the northwest Ethiopian highlands contains numerous man-made soil and water conservation (SWC) structures such as soil bunds (Erken), fanya juu ridge (Cab) and micro-trenches and natural sediment sinks such as wetlands, floodplains and grassed waterways. These sediment sinks reduce downstream sedimentation into the Koga reservoir, located at the catchment outlet, however, a large quantity of sediment is still reaching the reservoir. This study evaluates the function and effectiveness of both man-made SWC structures and natural sediment sinks in reducing sediment export from the Minizr catchment. SWC structures and natural sediment sinks were digitized using Google Earth Imagery. Sediment pins and vertical sampling through the deposit were used to quantify the amount of deposited sediment. In addition, inflow and outflow of suspended sediment data were used to calculate the sediment-trapping efficacies (STE) of man-made SWC structures (soil bunds and fanya juu ridges) and natural sediment sinks. Results reveal that 144 km soil bunds and fanya juu ridges trapped 7,920 Mg y−1 (55 kg m−1 y−1) and micro-trenches trapped 13·26 Mg y−1, each micro-trench on average trapped 23 kg y−1. The 17 ha floodplain located in the centre of the catchment trapped 9,970 Mg y−1 (59 kg m−2 y−1), while a wetland with a surface area of 24 ha, located near the outlet of the catchment, trapped 8,715 Mg y−1 (36 kg m−2 y−1). The STEs of soil bunds and fanya juu ridges, wetlands and floodplains were 54%, 85% and 77%, respectively. Substantial differences were observed between the STE of grassed and un-grassed waterways at 75% and 21%, respectively. Existing man-made and natural sediment sinks played an important role in trapping sediment, with 38% (26,600 Mg y−1) of transported sediment being trapped, while 62% (43,000 Mg y−1) is exported from the catchment and thus enters the Koga reservoir. Therefore, additional catchment treatment measures are required as an integrated catchment scale sediment trapping approach to help reduce sediment loads entering Koga reservoir. Moreover, to maximize the effectiveness of sediment trapping measures, avoid structural failure and ensure their sustainability, regular maintenance is needed.

Maize ( Zea mays L.), the dominant and staple food crop in Southern and Eastern Africa, is preferred to the drought-tolerant sorghum and pearl millet even in semi-arid areas. In semi-arid areas production of maize is constrained by droughts and poor rainfall distribution. The best way to grow crops in these areas is through irrigation, but limited areal extent, increasing water scarcity, and prohibitive development costs limit the feasibility of irrigation. Therefore, there is need for a policy shift towards other viable options. This paper presents daily rainfall analysis from Rushinga district, a semi-arid location in Northern Zimbabwe. The purpose of the rainfall analysis was to assess opportunities and limitations for rainfed maize production using 25 years of data. Data was analysed using a variety of statistical methods that include trend analysis, t -test for independent samples, rank-based frequency analysis, Spearman's correlation coefficient and Mann–Whitney's U test. The results showed no evidence of change in rainfall pattern. The mean seasonal rainfall was 631 mm with a standard deviation (SD) of 175 mm. December, January and February consistently remained the major rainfall months. The results depicted high inter-annual variability for both annual and seasonal rainfall totals, a high incidence of droughts ≥3 out of every 10 years and ≥1 wet year in 10 years. Using the planting criteria recommended in Zimbabwe, most of the plantings would occur from the third decade of November with the mode being the first decade of December. This predisposes the rainfall to high evaporation and runoff losses especially in December when the crop is still in its initial stage of growth. On average 5 to more than 20 days dry spells occupy 56% of the rainy season. Seasonal rainfall exhibited negative correlation ( P R = −0.746) with cumulative dry spell length, and wet years were free from dry spells exceeding 20 days. The most common dry spells (6–10 days), are in the range in which irrigated crops survive on available soil water. Therefore, they can be mitigated by in situ rainwater harvesting (RWH) and water conservation. The potential evapotranspiration of a 140-day maize crop was estimated to be 540 mm. Consequently, short season maize cultivars that mature in less than 140 days could be grown successfully in this area in all but drought years. However, sustainable maize production can only be achieved with careful management of the soil as a medium for storing water, which is essential for buffering against dry spells. To this end soil restorative farming systems are recommended such as conservation farming, in situ RWH techniques for dry spell mitigation and a cropping system that includes drought-tolerant cereal crops as for example sorghum and pearl millet, and perennial carbohydrate sources as for example cassava to provide stable crop yields.

This study tests a rapid, user-friendly method for assessing changes in erosion risk, which yields information to aid policy development and decision-making for sustainable natural resources management. There is currently a lack of timely, up-to-date and current information to support policy development on sustainable natural resources management in Uganda. The study was carried out in the Ngenge watershed, a typical catchment in the Ugandan Highlands, characterised by deforestation in favour of subsistence agriculture without adequate soil and water conservation measures. The watershed is experiencing soil erosion, sedimentation and flooding problems which are threatening agricultural productivity and food security. Sustainable management of environmental resources is needed to ensure a livelihood for the rural population which is dependent on the land. Historical erosion risk was evaluated in three steps using multi-temporal satellite data. First, current erosion risk was assessed by combining slope and vegetation cover during periods of high intensity rainfall. The data used for the assessment was obtained from public (free) satellite images. Erosion risk was then linked to land use and finally to the change in vegetation cover over the years 1980–2000. The analysis of erosion risk using rainfall, slope and NDVI (Normalised Difference Vegetative Index) as a proxy for vegetation cover gives an indication of the current erosion risk in the area. The results of historical vegetation cover change analysis indicate an overall increase in areas under erosion risk in the study area from 1980 to 2000. This method of erosion risk mapping provides a quick and straightforward means for identifying priority areas for interventions for soil and water resource management. Considering that resources are limited, the interventions to be appropriate have to be focused mainly on areas affected by degradation.

This paper introduces a simple two-layer soil water balance model developed to Bridge Event And Continuous Hydrological (BEACH) modelling. BEACH is a spatially distributed daily basis hydrological model formulated to predict the initial condition of soil moisture for event-based soil erosion and rainfall-runoff models. The latter models usually require the spatially distributed values of antecedent soil moisture content and other input parameters at the onset of an event. BEACH uses daily meteorological records, soil physical properties, basic crop characteristics and topographical data. The basic processes incorporated in the model are precipitation, infiltration, transpiration, evaporation, lateral flow, vertical flow and plant growth. The principal advantage of this model lies in its ability to provide timely information on the spatially distributed soil moisture content over a given area without the need for repeated field visits. The application of this model to the CATSOP experimental catchment showed that it has the capability to estimate soil moisture content with acceptable accuracy. The root mean squared error of the predicted soil moisture content for 6 monitored locations within the catchment ranged from 0.011 to 0.065cm^3cm^-^3. The predicted daily discharge at the outlet of the study area agreed well with the observed data. The coefficient of determination and Nash-Sutcliffe efficiency of the predicted discharge were 0.824 and 0.786, respectively. BEACH has been developed within freely available GIS and programming language, PCRaster. It is a useful teaching tool for learning about distributed water balance modelling and land use scenario analysis.

In semi-arid Sahel, soil erosion by water is one major factor accounting for negative nutrient balances in agricultural systems. A field experiment was conducted on a Ferric Lixisol in Burkina Faso to assess the effects of soil and water conservation barriers (stone rows or grass strips of Andropogon gayanus), the sole use of organic (compost) or mineral (urea) source of nitrogen (N), and the combined use of barriers and compost or urea nitrogen on soil nutrient and sediment erosion. The run-off barriers were put along contours. Organic carbon (OC) and nutrients losses through run-off and sediments were monitored during 2001 and 2002. Carbon and nutrient concentrations of generated sediments were very high, of 14-29 g kg-1 OC, 1.0-3.7 g kg-1 N and 0.32-0.71 g kg-1 total P. concentration in run-off water reached 0.023 g L-1 in stone rows treatments compared to only 0.002 g L-1 with treatments without barriers. Annual losses of organic C, N, P, and K were high and greatly dependant on soil loss magnitude. Compared to the control plots, soil losses were reduced on average by 84% during the 2 years with stone rows treatments, while by 71% with grass strips treatments. Compost application reduced soil loss by 75% in plots without barriers and by 89% in plots with stone rows as compared to control plots. With urea application, soil loss was on average 13% lower than in plots with compost. Integrated water and nutrient management can effectively alleviate soil, carbon, and nutrients losses by water erosion, therefore may contribute to intensifying crop production in Sahelian smallholder farming.

The synergistic effect of soil and water conservation (SWC) measures (stone rows or grass strips) and nutrient inputs (organic or mineral nutrient sources) was studied at Saria station, Burkina Faso. The reduction in runoff was 59% in plots with barriers alone, but reached 67% in plots with barriers + mineral N and 84% in plots with barriers + organic N, as compared with the control plots. Plots with no SWC measure lost huge amounts of soil (3 t ha-1) and nutrients. Annual losses from eroded sediments and runoff reached 84 kg OC ha-1, 16.5 kg N ha-1, 2 kg P ha-1, and 1.5 kg K ha-1 in the control plots. The application of compost led to the reduction of total soil loss by 52% in plots without barriers and 79% in plots with stone rows as compared to the losses in control plots. SWC measures without N input did not significantly increase sorghum yield. Application of compost or manure in combination with SWC measures increased sorghum grain yield by about 142% compared to a 65% increase due to mineral fertilizers. Yields increase did not cover annual costs of single SWC measures while application of single compost or urea was cost effective. The combination of SWC measures with application of compost resulted in financial gains of 145,000 to 180,000 FCFA ha-1 year-1 under adequate rainfall condition. Without nutrient inputs, SWC measures hardly affected sorghum yields, and without SWC, fertilizer inputs also had little effect. However, combining SWC and nutrient management caused an increase in sorghum yield.

Whether it is traditional, modern or ‘‘sustainable’’ agriculture, soil organic matter plays a key role in sustaining crop production and in preventing land degradation. A field experiment was conducted on a Ferric Lixisol at Gampela (Burkina Faso) in 2000 and 2001 to carried out the effects of tillage, fertilisation and their interaction on soil organic carbon (SOC) (0–10 cm), crop performance and microbial activities. Maize straw or sheep dung were applied separately or combined with urea in a till or no-till systems and compared with urea only and a control treatment. Sampling was done each year at 2 months after sowing and at harvest. SOC was increased in the tillage treatments in 2000 by 35% but only with 18% in 2001 suggesting reduced carbon accumulation in the absence of organic and mineral restitution. Ploughing in maize straw under conditions of N deficiency led to a drastic decrease in SOC due microbial priming effect that, was not observed when ploughing in sheep dung. In no-till system, losses, organic amendment N concentration and the soil N status determined the impact on SOC and crop productivity. The negative effect on SOC in the tillage treatment with maize straw (4.1 g kg � 1 ) was less when maize straw was combined with urea (6.2 g kg � 1 ). It is concluded that in semi-arid West Africa, without both organic resource and N inputs, soil organic matter ‘‘pays’’ for crop N nutrition. Increasing SOC accumulation while improving crop yield may be conflicting under low-input agricultural systems in semi-arid West Africa. Therefore, optimum soil organic carbon and crop performance results from a judicious combination of organic resources and inorganic N mediated by microbial activity. # 2006 Elsevier B.V. All rights reserved.

Despite several approaches that aimed at mobilising East African farmers to embrace soil and water conservation (SWC) activities, farmers hardly responded since they were seldom involved in the planning of SWC activities. Two tools that employ farmers' participation were developed and applied at Gikuuri catchment in Kenya. The first tool involved farmers to map soil erosion using their own indicators and determine the soil erosion status at catchment scale. This formed the basis upon which they undertook to plan for SWC measures at catchment scale. Farmers also predicted crop yield losses based on the soil erosion status. Farmers widely approved the soil erosion status map since their own indicators and perceptions were used. The second tool provided cash flow trends for a variety of SWC activities and farmer situations. Farmers can use land with a high, moderate or low erosion status and often have rather different socio-economic settings. The net benefits over 5 yr for bench terraces, fanya juu terraces and grass strips were illustrated to assist farmers in making informed decisions on SWC adoption. The two tools increased awareness on the need for collective actions among farmers and showed fields that cause run-on on downslope fields. The improved awareness of erosion problems and the related financial consequences increased farmers' willingness to share the investment costs for cut-off drains.

Soils in many agro-ecological zones in Eritrea, a country in sub-Saharan Africa, are low in productivity due to erosion, low organic matter and poor soil management. Recently, mechanised farming has been intensively practiced to increase agricultural food production. However, the use of heavy machinery and vehicles can cause soil compaction that reduces soil productivity and crop yield. A study was conducted to determine the in-situ soil compaction and the probability of exceeding a root restricting threshold penetration resistance (PR) value of 2 MPa in three agro-ecological zones that varied in soil type, topography and climate. Soil cone penetration resistance measurements were taken up to a depth of 600 mm in 30 m by 30 m field plots with three replicates located in the three agro-ecological zones. Simultaneously, core samples were collected to determine soil moisture and bulk density from surface and subsurface soil layers. In all the soils, moisture contents during soil penetration resistance measurements were nearly the same as the field capacity soil moisture content (¿10 kPa soil moisture suction). Penetration resistance values measured from 0 to 600 mm were averaged in depth increments of 100 mm. For clay loam (Adiguadad) and loam (Sheeb) soils, the penetration resistance values up to a soil depth class of 0¿300 mm were statistically insignificant and categorised as minimum mechanical impedance to root growth (

A methodology is presented for assessing the seriousness and impact of land degradation, from a historical and a farmer perspective, in regions where data are not available. Farmers have been directly involved in the assessment of soil, productivity and vegetation cover loss over the past decades, both on cropland and on rangeland. The results for the Bolivian mountain valleys in north Chuquisaca show that the intensive use of small cropland fields has increased soil degradation and consequently led to less water and nutrient availability. Due to the subsequent productivity loss many cropland fields have been abandoned. Rangeland is also severely degraded as a result of the excessive removal of vegetation and subsequent soil loss. Farmers are very concerned, but consider the impact of changed rainfall and less available cropland beyond their control. Consequently, no serious initiatives are taken to solve the land degradation problem. This research adduces evidence of increased land degradation in the Bolivian mountain valleys over recent decades, and stresses the need for urgent (integrated) action with regard to soil and water conservation

Tillage, organic resources and fertiliser effects on soil carbon (C) dynamics were investigated in 2000 and 2001 in Burkina Faso (West Africa). A split plot design with four replications was laid-out on a loamy-sand Ferric Lixisol with till and no-till as main treatments and fertiliser types as sub-treatments. Soil was fractionated physically into coarse (0.250–2 mm), medium (0.053– 0.250 mm) and fine fractions (< 0.053 mm). Particulate organic carbon (POC) accounted for 47–53% of total soil organic carbon (SOC) concentration and particulate organic nitrogen (PON) for 30–37% of total soil nitrogen concentration. The POC decreased from 53% of total SOC in 2000 to 47% of total SOC in 2001. Tillage increased the contribution of POC to SOC. No-till led to the lowest loss in SOC in the fine fraction compared to tilled plots. Well-decomposed compost and single urea application in tilled as well as in no-till plots induced loss in POC. Crop N uptake was enhanced in tilled plots and may be up to 226 kg N ha � 1 against a maximum of 146 kg N ha � 1 in no-till plots. Combining crop residues and urea enhanced incorporation of new organic matter in the coarse fraction and the reduction of soil carbon mineralisation from the fine fraction. The PON and crop N uptake are strongly correlated in both till and no-till plots. Mineral-associated N is more correlated to N uptake by crop in tilled than in no-till plots. Combining recalcitrant organic resources and nitrogen fertiliser is the best option for sustaining crop production and reducing soil carbon decline in the more stabilised soil fraction in the semi-arid West Africa. # 2005 Published by Elsevier B.V.

A field experiment was laid out in Burkina Faso (West Africa) on an Eutric Cambisol to investigate the interaction of organic resource quality and phosphate rock on crop yield and to assess the contribution of earthworms (Millsonia inermis Michaelsen) to P availability after phosphate rock application. Organic resources of different quality were applied at a dose equivalent to 40 kg N ha−1 with or without phosphate rock from Kodjari (Burkina Faso) at a dose equivalent to 25 kg P ha−1, and were compared with control and single phosphate rock treatments in a factorial complete block design with four replicates. Sorghum (Sorghum bicolor L. Moench) variety SARIASSO 14 was grown. Sheep dung had the highest impact on earthworm casting intensity followed by maize straw. Combining organic resources with phosphate rock reduced earthworm casting activities compared to a single application of organic resources or phosphate rock. Addition of phosphate rock to maize straw reduced P availability in earthworm casts whereas combining sheep dung or compost with phosphate rock increased P availability. The contribution of earthworms to Kodjari phosphate rock solubilisation mainly occurred through their casts, as the available P content of casts was 4 times higher than that of the surrounding soil.

Soil organic matter (SOM) controls the physical, chemical and biological properties of soil and is a key factor in soil productivity. Data on SOM quantity and quality are therefore important for agricultural sustainability. In 1990, an experiment was set up at Saria, Burkina Faso on a sandy loam Lixisol to evaluate long-term effects of tillage (hand hoeing or oxen ploughing) with or without 10 t ha-1 yr-1 of manure and fallowing on SOM and N concentrations and their distribution in particle size fractions. The field was sown annually to sorghum (Sorghum bicolor [L.] Moench). Ten years later, total organic C and total N, SOM fractions and their N concentrations, and sorghum yield were determined. Continuous sorghum cultivation without organic inputs caused significant losses of C and N in the hoed and ploughed plots. However, addition of manure to hoed plots was effective in maintaining similar levels of C and N to fallow plots. Without manure, SOM was mainly stored in the size-fraction

This paper describes the results of a survey on farmers' perceptions of the effect of woody natural vegetation on wind erosion. Sixty farmers were interviewed in three villages in northern Burkina Faso. The farmers mentioned that the presence of woody vegetation between the crops could benefit yield, but feared competition between the natural vegetation and the crop. Vegetation in a field was considered to increase deposition and decrease erosion on that field. The most important vegetative characteristics that affect wind erosion were, according to the farmers, vegetation's shape, porosity, flexibility and arrangement of the vegetation in the field. At present, most farmers do not apply this knowledge to the management of the natural woody vegetation on their fields

In the semiarid regions of sub-Saharan Africa, fertilizer recovery and nutrient release from organic sources are often moisture limited. Moreover, in these regions runoff brings about large nutrient losses from fertilizer or organic inputs. This study was conducted in the north sudanian climate zone of Burkina Faso (annual rainfall 800 mm, PET 2000 mm yr-1). We assessed the combined and interactive effects of two types of permeable barriers (stone rows and grass strips of Andropogon gayanus Kunth cv. Bisquamulatus (Hochst.) Hack.) and organic or mineral sources of nitrogen on erosion control and sorghum yield. The field experiment (Ferric Lixisol, 1.5% slope) was carried out during three rainy seasons and consisted of 2 replications of 9 treatments, in which the barriers were put along contours and combined with compost, manure and fertilizer nitrogen (N). Compared with the control plots, the average reduction in runoff was 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 rows reduced soil erosion more than grass strips (66% versus 51%). Stone rows or grass strips without N input did not induce a significant increase of sorghum production. Supplying compost or manure in combination with stone rows or grass strips increased sorghum grain yield by about 142%, compared with a 65% increase due to mineral fertilizers. The sorghum grain yields at 1m upslope from the grass strips were less than those 17m from the grass strips. As stones do not compete with plants, the opposite trend was observed with stone rows. We conclude that for these nutrient depleted soils, permeable barriers improve nutrient use efficiency and therefore crop production. However, grass strips must be managed to alleviate shade and other negative effects of the bunds on adjacent crops.

We studied fine-scale spatial plant distribution in relation to the spatial distribution of erodible soil particles, organic matter, nutrients and soil water on a sandy to sandy loam soil in the Sahel. We hypothesized that the distribution of annual plants would be highly spatially autocorrelated and that this would be positively linked with the spatial distribution of erodible soil particles, soil organic matter and nutrients. Further, we hypothesized that larger vegetated patches (a relatively high vegetation cover at coarser spatial scale) will be stronger linked with erodible soil particles, organic matter and nutrients than smaller vegetated patches (a relatively low vegetation cover at coarser spatial scale). Generally, for `low cover' and `high cover' plots, spatially confined micro-elevations with a relatively high volume percentage of erodible very fine – medium sand (50–400 μm), were linked with a higher cover of annual plants. The robustness of vegetation patchiness (expressed as the proportion of the total variance accounted for by spatial dependent variance) was significantly higher for the `high cover' plots. For the `low cover' plots, higher vegetation cover was associated with higher elevations, soil moisture, and volume percentage of very fine to medium sand, but lower organic matter, total N and P. For the `high cover' plots, micro-elevations also consisted of a relatively high volume percentage of very fine to medium sand, and this was associated with dryer conditions and higher total N. Additionally, dryer conditions were weakly correlated with higher organic matter. So, micro-elevations were indirectly associated with dryer and more fertile conditions, which was opposite to what we found for the `low cover' plots.

Eritrea’s coastal zone has been identified as an area of substantial development potential. About 14,000 ha of this 5 million ha area (i.e.

Heterogeneity in space and time makes it difficult to evaluate the hydrologic effects of hedgerows in between crops. In this study a robust modelling system for designing hedgerow barrier systems is introduced. The system is demonstrated with experimental data over three years from a site in central Burkina Faso. Soils at the site are prone to crusting and consist of a sandy loam in the top layer and clay in deeper layers. Measurements comprise runoff from differently sized plots (1, 5 and 10 square meters), water storage into the soil, and rainfall. It appears that the relevant hydrologic processes can be represented with the available measurements by two compact dynamic models, each comprising two states (runoff and soil water; water use and soil water), one input (rain; potential evapotranspiration) and two spatial units (hedgerow and cropped area). The analysis of the phase planes of the models reveals which combinations of alley length and hedgerow type lead to desirable combinations of both runoff and evapotranspiration. This information can be interpreted directly in a qualitative sense or alternatively used for a Monte-Carlo analysis of the system.

Under low and poorly distributed rainfall higher food production can be achieved by increasing crop water use efficiency (WUE) through optimum soil fertility management and selection of deep-rooting cultivars, appropriate plant density and planting dates. We explored AquaCrop's applicability in selecting adaptive practices for improving maize yield and WUE under rainfed smallholder farming in semi-arid Zimbabwe. AquaCrop was first tested using field measurements without calibration. The model was subsequently applied to estimate the effect of effective rooting depth (ERD), plant density and planting date on maize yield. Simulations were done with daily rainfall data for 25 seasons. During model testing AquaCrop simulated canopy cover development well and simulated biomass accumulation showed good agreement with measured values. The model overestimated soil water, and observed final biomass and grain yield were 96 and 92% of simulated values, respectively. Model application showed that increasing ERD from 0.40 m at 32,500 plants ha−1 to 0.60 m at 44,400 plants ha−1 increased grain yield from 6.0 to 7.8 t ha−1, biomass water use efficiency by 20.5%, grain water use efficiency by 23.6% and transpiration water use efficiency by 26.8%. At 0.60 and 0.80 m ERD and 44,400 plants ha−1, biomass and grain yield, and WUE, were similar. Drainage below the rootzone was ≥40% of non-productive water losses in normal and wet seasons whilst soil evaporation contributed 47% in dry seasons at 0.80 m ERD. To improve yield and WUE, we recommend: incorporation of deep-rooting legumes, deeper-rooting cultivars (≥0.60 m effective rooting depth) and practices that improve ERD, a plant density of 44,400 plants ha−1; and practices that reduce soil evaporation e.g. mulching and addition of organic fertilisers to improve soils’ available water capacity and enhance response to mineral fertilisers. Further research should include field testing of results from this study with farmers.

Realising that rainwater harvesting (RWH) improves crop productivity, smallholder farmers in semi-arid Zimbabwe modified contour ridges traditionally used for rainwater management by digging infiltration pits inside contour ridge channels in order to retain more water in crop fields. However, scientific studies on crop yield benefits of infiltration pits have not been conclusive. Combining field-edge RWH methods such as contour ridges with infiltration pits with in-field practices may enhance crop yield benefits. Thus, the objective of the study was to assess soil moisture and maize yield improvement of combining infiltration and planting pits. Field experiments were conducted in Rushinga, Zimbabwe for three seasons at three sites using a split-plot design: main-plot factor, field-edge rainwater management method (RWMM); and split-plot factor, tillage method. Soil moisture content was measured weekly using gravimetric and Time Domain Reflectometry (TDR) methods. A household and field survey to establish farmers’ perceptions, typology and availability of field-edge RWMM was conducted. In order to share experiences and enhance stakeholders’ learning, field days were held. Lateral movement of soil water was measured up to 2 m downslope from infiltration pits, hence infiltration pits did not improve maize yield and soil moisture content in the cropping area. Maize yield (kg ha−1) was 45% higher under conventional tillage (2697) than planting pits (1852) but the yield gap decreased from 90 to 30% in the first and third year respectively. The value of infiltration pits is in reducing soil erosion by water and growing high value horticultural crops inside and close to pits, a view shared by host farmers and other stakeholders. Planting pits are an option for farmers without access to draught power and a fall-back method. Research is required to determine soil moisture, maize yield benefits and waterlogging risk in fields with underlying impermeable layers that enhance lateral flow of water.

In northern Burkina Faso (West Africa), a study was undertaken to explore the possibilities of restoring the infiltration capacity of crusted soils through the stimulation of termite activity. Treatments consisted of the application of a mulch of a mixture of wood and straw without insecticides (resulting in “termite plots”) and the application of the same mulch and an insecticide (Dursban with chloropyrifos as the active ingredient) to prevent termite activity (resulting in “non-termite plots”). Three rainfall simulations of 60 minutes duration with an intensity of 50 mm/h at an interval of 24 hours between the first and the second and 72 hours between the second and third simulation were applied, to study the effect of consecutive showers on termite-modified soil characteristics. Cumulative infiltration amounts, final infiltration rates, soil water content and porosity were larger and bulk density was smaller on termite plots as compared to non-termite plots. This suggests that termites may be an important agent in soil-crust control and in the improvement of soil physical properties in Sahelian ecosystems.

Crop productivity in Rwanda is declining as a result of intensive farming on steep slopes, which leads to soil loss and declining soil fertility particularly in the northern highlands. Slow-forming terraces have been widely adopted in the northern highlands of Rwanda to control soil erosion however not much been done to evaluate their efficiency. We hypothesized that slow-forming terraces reduce soil loss and soil fertility gradients compared with non-conserved land. A field experiment compared the soil erosion rates and fertility gradients of 20+ year old terraces where sole grass strips ( Pennisetum purpureum ) or grass strips combined with infiltration ditches were used with those of land where no soil conservation technologies were applied. The experiment was conducted in the year 2010 on three landscape positions (Upperslope, Hillslope and Footslope) along a representative toposequence using farmers’ fields where potato and maize were grown in two consecutive cropping seasons. The highest annual soil loss (41.5 t ha −1 ) was recorded with plots receiving no soil conservation practices while the lowest annual soil loss (18 t ha −1 ) was recorded with grass strips combined with infiltration ditches. The slow-forming terraces showed a marked “within” spatial difference in both soil quality and crop yield. The soil in the lower part of the terraces showed as much as 57% more organic carbon content and 31% more available phosphorous than the soil in the upper part. Organic carbon (OC) was higher (OC = 2.1%) on the upperslope than on hillslope (OC = 1.9%). Less than 2.3 t ha −1 of the mean potato yield (23 t ha −1 ) and only 0.5 t ha −1 of the mean maize yield (5 t ha −1 ) were recorded on the uppermost third of the terraces on all three landscape positions. The marked soil fertility gradients indicate that the sustainability of slow-forming terraces is threatened, unless a site-specific fertilizer strategy is developed. For the sustainability of these terraces, the current practice of “harvesting” the fertile soil from the lower edge of the grass strip and using it as fertilizer for the nutrient deficient upper parts of terraces needs to be stopped.

The Rift Valley drylands of Ethiopia are characterized by sandy loam soils that have poor fertility and unreliable rainfall conditions. The aim of this study was to examine the potential benefit of rainwater harvesting by tied-ridges and improved soil fertility on maize productivity through field experimentation and simulation with the FAO's AquaCrop model. The effect of tied-ridges with and without manure on maize yield at smallholder farms was studied during the years 2009 (very dry, 96% probability of exceedance) and 2010 (normal year, 46% probability of exceedance). During a normal rainfall, the mean yield of maize grain was 4.0 Mg ha(-1) when tied-ridges were applied in combination with manure (at 4.5 Mg ha(-1) rate); 3.4 Mg ha(-1) when tied-ridges were applied without manure; 3.5 Mg ha(-1) when traditional tillage was applied in combination with manure; and 2.7 Mg ha(-1) when traditional tillage was applied without manure. Long-term simulations with the FAO's AquaCrop showed that the root zone soil water may exceed field capacity for consecutive days during above average rainfall seasons in the shallow sandy loams. The question thus is when to tie or not to tie ridges. Therefore, upon proper calibration of the FAO's AquaCrop model, the effect of tied-ridges and improved soil fertility was simulated in response to different amounts of seasonal rainfall, number of rainfall days and sowing times. Simulations revealed that, during below average rainfall seasons (280-330 mm), tied-ridges are more effective at improving crop yields than enhancing the fertility level of the soil. But during above average rainfall seasons, the rainwater that is held in tied-ridges can be more effectively utilised when the current fertility level of the soil is improved. The simulated rainwater use efficiency of maize was 6.1-6.5 kg ha(-1) mm(-1) for traditional tillage without any fertiliser, 6.8-7.3 kg ha(-1) mm(-1) for tied-ridges without any fertiliser and 11.0-12.9 kg ha(-1) mm(-1) for tied-ridges with optimum fertiliser (96% soil fertility level). It is, therefore, concluded that combined use of tied-ridges and farmyard manure can enhance maize yield under wide range of rainfall conditions (annual rainfall ranging from 280 mm to 680 mm). The Maresha-modified ridger used in this study can be popularized among Ethiopian farmers due to its simplicity and effectiveness. (C) 2012 Elsevier B.V. All rights reserved.

Precise estimation of mean flow velocity (U-mean) is imperative for accurate prediction of hydrographs and sediment yield. For overland flow, U-mean is normally estimated by multiplying the dye or salt based velocity measurement with a correction factor (alpha). A wide range of correction factors is available in the literature, all of which were derived under different experimental conditions. The selection of a suitable a has become a main challenge for accurate mean flow calculations. This study aimed to assess the variability of a with grain size (D-50) and slope (S), and to evaluate the dependency of U-mean on flow rate (Q), D-50 and S by regression analysis. Flume experiments were performed at Q varying from 33 to 1033 x 10(-6) m(3) s(-1), S ranging from 3 degrees to 10 degrees, and D-50 ranging from 0.233 to 1.022 mm. Flow velocities were measured directly with the dye tracing technique (U-dye), and derived indirectly from flow depth measurements (U-depth). The Udepth measurements were considered as U-mean. The derived alpha (U-depth/U-dye) values did not remain constant with sediment size and increase significantly with the increase of D-50. The mean alpha values for 0.230, 0.536, 0.719 and 1.022 mm sands were 0.44, 0.77, 0.82 and 0.82, respectively. Hence, due to the substantial variation of alpha with D-50, no absolute alpha value is applicable to all hydraulic and sedimentary conditions. However, mean alpha values for 0.230, 0.536 and 0.719 mm sands were found comparable with alpha values available in the literature for similar grain sizes. The influence of Q S. and D-50 on U-mean was studied by regression analysis. Regression analysis depicted the significant influence of Q and D-50 on U-mean. while the effect of slope was found to be non-significant. Comparison of the derived regression equation with five literature datasets showed that the model can predict mean flow velocities in overland flow at a reasonable accuracy as long as the mean velocity is below 0.4 m s(-1). At higher velocities the error becomes unacceptably large.

Simulation models are tools that can be used to explore, for example, effects of cultural practices on soil erosion and irrigation on crop yield. However, often these models require many soil related input data of which the saturated hy- draulic conductivity (Ks) is one of the most important ones. These data are usually not available and experimental de- termination is both expensive and time consuming. Therefore, pedotransfer functions are often used, which make use of simple and often readily available soil information to calculate required input values for models, such as soil hydraulic values. Our objective was to test the Rosetta pedotransfer function to calculate Ks. Research was conducted in a 64-ha field near Lamesa, Texas, USA. Field measurements of soil texture and bulk density, and laboratory measurements of soil water retention at field capacity (–33 kPa) and permanent wilting point (–1500 kPa), were taken to implement Rosetta. Calculated values of Ks were then compared to measured Ks on undisturbed soil samples. Results showed that Rosetta could be used to obtain values of Ks for a field with different textures. The Root Mean Square Difference (RMSD) of Ks at 0.15 m soil depth was 7.81 ¿ 10–7 m·s–1. Further, for a given soil texture the variability, from 2.30 ¿ 10–7 to 2.66 ¿ 10–6 m·s-1, of measured Ks was larger than the corresponding RMSD. We conclude that Rosetta is a tool that can be used to calculate Ks in the absence of measured values, for this particular soil. Level H5 of Rosetta yielded the best results when using the measured input data and thus calculated values of Ks can be used as input in simulation models

For thousands of years, smallholder-based crop farming in Ethiopia has been practiced with oxen ploughing using the traditional Maresha ard plough where consecutive tillage operations are undertaken perpendicular to each other. Despite its wide acceptance by smallholder farmers, long-term use of the Maresha is believed to deteriorate the soil's physical properties. This study examines the surface and subsurface infiltration, soil evaporation and penetration resistance of sandy loam soils that have been exposed to varying durations (0, 2, 7, 22 and 35 years) of cultivation after being converted from acacia-based grassland dominated by Acacia tortilis and Acacia senegal in the Central Rift Valley (CRV) of Ethiopia. The infiltration rate of the surface layer increased significantly (p = 0.05) immediately after conversion from acacia-based grassland to cultivated land. Thereafter, there was a weak decreasing trend (p > 0.05, R2 = 0.24) in infiltration rate with years of cultivation. Unlike the surface soil layer, there was no significant difference in the subsurface (below 15 cm) infiltration between the acacia-based grassland and lands cultivated for varying numbers of years. Following a rain event satisfying field capacity of the soils, the daily soil evaporation increased significantly (p = 0.05) with increased duration of cultivation. The cumulative evaporation, observed over 5 consecutive days following the last rainfall, increased by 2.4 times in the 35 years old cultivated land from the acacia-based grassland. There was also a strong correlation (R2 = 0.86) between α (the slope of the cumulative evaporation versus the square root of time) and an increase in the years of cultivation. It is, therefore, concluded that long-term Maresha cultivation along with the present soil management makes the maize crop susceptible to drought and dry-spells. Improved soil management and development of appropriate tillage are needed to maximize rainwater use efficiency and achieve a more sustained agricultural production in the drought-prone CRV of Ethiopia.

In the semi-arid region of Tigray, Northen Ethiopia a two season experiment was conducted to measure evapotranspiration, estimate yield response to water stress and derive the crop coefficient of teff using the single crop coefficient approach with simple, locally made lysimeters and field plots. During the experiment we also estimated the water productivity of teff taking into account long-term rainfall probability scenarios and different levels of farmers’ skills. During the experimental seasons (2008 and 2009), the average potential evapotranspiration of teff ranged from 260 to 317 mm. The total seasonal water requirement of teff was found to lower in contrast to the assumptions of regional agronomists that teff water requirement is comparable to that of wheat and barley (375 mm). The average single crop coefficient values ( k c ) for the initial, mid and late season stages of teff were 0.8–1, 0.95–1.1 and 0.4–0.5, respectively. The seasonal yield response to water stress was 1.04, which indicates that teff exhibits a moderately sensitive and linear response to water stress. The results suggest that teff is likely to give significantly higher grain yield when a nearly optimal water supply is provided. The study showed that, in locations where standard equipment is not affordably available, indicative (rough) crop evapotranspiration values can be obtained by using field plots and employing locally made lysimeters. The difference in economic water productivity (EWP) and the crop water productivity (CWP) for teff were assessed under very wet, wet, normal, dry and very dry scenarios. In addition two groups of farmers were evaluated, a moderately (I) and a highly skilled (II) group. The results showed that higher EWP and CWP were obtained under very wet scenario than very dry scenario. There was also a 22% increase in EWP and CWP under group II compared to group I farmers. The increase was due to a 22% reduction in unwanted water losses achieved through use of improved technology and better irrigation skills. Both EWP and CWP can be used to evaluate the pond irrigation water productivity (IWP) for a given climate, crop and soil type, and skill and technology level of the farmer. For special crops like teff extra criteria may be needed in order to properly evaluate the pond irrigation water productivity. During the experimental seasons, a high IWP for teff was attained when about 90% of the optimal water need of the crop was met. IWP can be used as an indicator as how much supplementary irrigation has to be applied in relation to the rainfall and other sources of water supply in order to assure greatest yield from a total area. However, the supplemental irrigation requirement of the crops may vary with season due to seasonal rainfall variability.

Unsustainable crop and soil management practices are major causes of soil degradation and declining soil biodiversity in West Africa. Identifying soil management practices that favor macrofauna abundance is highly desirable for long-term soil health. This study investigates the effects of long-term conservation soil management on termite and earthworm abundance and taxonomic richness in the central plateau of Burkina Faso. Trials included rotations with 5 Mg ha −1 yr −2 of organic matter added (established in 1960), application of 10 Mg ha −1 yr −1 applied with additional organic (manure or straw) and mineral inputs (established in 1980) and different tillage systems (established in 1990) where 10 Mg ha −1 yr −1 of organic matter was also applied. Soil macrofauna was surveyed at the soil surface and in the upper 30 cm using transect and monolith sampling methods, eight weeks after sorghum crop planting. A total of five termite taxa: Trinervitermes sp., Microtermes sp., Odontotermes magdalenae , Macrotermes sp. and Amitermes stephensoni ; belonging to the family of Termitidae, and two earthworm taxa: Dichogaster affinis , Millsonia inermis ; from the family of Acanthodrilidae were found. Termite taxonomic richness per treatment ranged between 1 and 4, while earthworm taxa ranged from 0 to 2. Under rotation, one termite taxa and no earthworm taxa were identified. In the organic amendment plots, three termite and two earthworm taxa were found. And light tillage (animal or hand) resulted in four termite taxa and one earthworm taxa. The two types of fauna clearly responded differently to the different conservation soil management practices. Under rotation lower recorded macrofauna population was attributed to the lower rate of applied organic matter compared to levels applied in the organic amendment and tillage trials and where more macrofauna were found. Location of food stock (rooting depth of different crops in the rotation) also had a significant effect on termite presence. Effect of rooting depth on earthworms was not observable due to the absence of earthworms in the rotation trials (possibly due to insecticide application. Manure treatments favored earthworms, while sorghum straw treatments favored termites likely due to respective preference for easy versus difficult to digest organic sources. Animal plowing and hand hoeing had similar and significantly positive effects and both termite and earthworm biological components compared to tractor tillage. We conclude that termite and earthworm abundance and taxonomic richness are most significantly affected by the type and amount of organic matter applied and tillage regimes, with rooting depth of rotations crops also playing a significant role. To promote macrofauna abundance and taxonomic richness in soils, integrated conservation soil management practices with attention to the particular needs and preferences of termites and earthworms is needed.

A method to calculate the statistical distribution of a scaling factor from soil-water retention curves consists of measuring the particle-size distribution by laser scattering, computing soil water retention curves using the Arya and Paris model, and calculating a scale factor for each function. The method was tested by comparing measured and predicted soil-water retention curves. The advantage of this method is in its simplicity and ease of use. The applicability of the method to a given soil texture depends on the verification of the calculated soil-water retention functions.

In a semi-arid environment, the main challenge for crop production is water deficit. FAO's AquaCrop model, which simulates yield response to water, has been calibrated to explore alternative water management strategies in teff crop. To calibrate and evaluate this model, we used independent data sets of the cropping season 2008 and 2009 in the northern Ethiopia. The teff crop cover, biomass and soil water simulated by the model agreed well with the observed data. The model revealed that the grain-water use efficiency of teff increased when supplementary irrigation after start of flowering was increased from 0 to 95 mm. The biomass-water use efficiency showed the opposite trend: it decreased. The implication of these results is that if land is not a constraint, and if the intention is to grow teff for its biomass, the best option is to use the available water to grow teff with deficit irrigation. But when the major intention is to produce grain, the best option is an optimum irrigation application. It is concluded that the AquaCrop model can be used to explore management options to improve teff water productivity in the area.

Two alternative in situ area rainwater conservation practices (tied ridging and mulching) were evaluated for four seasons (2004, 2007, 2008 and 2009) at an experimental station in Mekelle, Ethiopia. The objectives were to evaluate the performance of barley as influenced by mulch and tied ridge and to understand the relationships of rainfall and runoff on barley fields. About 16–30% of the seasonal rainfall resulted in runoff when barley was grown without water conservation, whereas the in situ conservation practices resulted in significantly low runoff. Tied ridging and mulching increased the soil water in the root zone by more than 13% when compared with the control. Consequently, grain yield and rainwater use efficiency increased significantly with tied ridging but not with mulching. Tied ridging increased the grain yield over the control at least by 44% during below average rainfall years. Neither mulching nor tied ridging were significantly different from the control when the seasonal rainfall was above average. Since rainfall is often unreliable, we recommend tied ridging as a water conservation technique for loams in the study area in order to mitigate the effect of drought stress in barley. However, tied ridges could be carefully opened when excess water is expected to cause waterlogging.

A debate on response strategies to rural poverty and land degradation has relevance for the core business of this journal. Based on extensive fieldwork in the mountain valleys of Chuquisaca, Bolivia, this paper discusses farming and migration in the region, and elaborates on the required components of a strategy to tackle land degradation. We argue that collective action at the micro–meso level is essential to lay the foundation for development activities and to reduce survival-driven migration rates. In order to actually change farmers into adaptive co-managers of their natural resources it is also necessary to create enabling conditions at meso-macro level. However, given the current context in Bolivia, the prospects for reducing land degradation are not good

In semi-arid Kenya, the most dominant soil types are of limited agricultural productivity due to crusting and compaction. The occurrence of soil crusting and compaction is attributed to seasonal rainfall characteristics, physical soil properties and bad tillage practices. Soil crusting and compaction decrease rainwater infiltration and increase surface runoff. Seasonal rainwater losses through increased runoff volumes reduce soil moisture and hence result in agricultural drought. The objective of this study was to examine the hydrological effects of two tillage practices with and without farmyard manure on surface runoff and soil loss of crusting and compacting soils under field conditions at Katumani in semi-arid Kenya. Field investigations on rainfall characteristics, surface runoff, soil loss, soil bulk density and soil shear strength covered two rainy seasons (short and long rains) and were done on a Chromic Luvisol. The field treatments were zero tillage and conventional tillage, and two farmyard manure applications (5 and 10 Mg ha-1). The results obtained showed significant effects of conventional and zero tillage and farmyard manure on infiltration and soil moisture, surface runoff and soil loss. Soil crusting and compaction significantly influenced the hydrological response of all treatments. These responses were attributed to seasonal rainfall events of varying amounts, intensities and duration, and treatment differences in soil surface conditions and aggregation. Farmyard manure (FYM) application enhanced infiltration and reduced soil crusting, compaction, and surface runoff during the initial stages of the rainy season. But in the mid-stages of the rainy season, the effects of FYM on soil aggregation diminished. Conventional tillage without farmyard manure led to high surface runoff and soil loss in this structurally unstable soil. Zero tillage performed poorly under these soil conditions because of high soil crusting and compaction, low rainwater infiltration and subsequent increase in surface runoff generation.

Water scarcity and drought in Africa are often in the news. The widespread tendency to relate farmers’ notion of drought to changes in the occurrence of dry spells is misguided: several recent studies have yielded little evidence of an increase in the length and/or frequency of such spells. The farmers’ concept of drought is contextual and an indirect result of land degradation. Plant production suffers because water is not available due to deteriorated physical properties of soil. Farmers’ perception of drought refers to the Green Water Use Efficiency (GWUE), i.e. the fraction of rain that is used for plant transpiration. GWUE in Africa is remarkably low: in sub-Saharan Africa, only 15% of the terrestrial rainwater is used by plants for the production of food, fodder and fibre. Although a millet crop grown under traditional circumstances uses only 50 mm in transpiration, the crop frequently suffers from agricultural drought due to excessive losses of rainwater. A range of land management practices is available to help improve GWUE. They can be classified according to their function. Hedgerow barriers and terraces can mitigate runoff; infiltration rates below hedgerows in Kenya were found to be three to eight times higher than where the crop was grown. Mulch that triggers soil fauna can improve water availability; mulching with straw from a local perennial grass in Burkina Faso doubled the water use efficiency from 1 kg mm−1 without fauna to 2 kg mm−1 with fauna. Water harvesting and water–nutrient synergy can improve water use; in case permeable barriers combined with the use of compost Sorghum yield in Burkina Faso was 2.3 times higher than in the control plots and the plots with the barriers only. Green water deserves more attention from policy makers, planners, land users, water engineers and managers.

A field experiment was conducted in Gampela (Burkina Faso) in 2000 and 2001 to assess the impact of organic and mineral sources of nutrients and combinations thereof in optimising crop production in till and no-till systems and to assess the economic benefit of these options. The study showed that under conditions of rainfall deficiency, the use of a single organic resource at an equivalent dose of 40 kg N ha−1 better secured crop yield than the application of an equivalent amount as urea-N, while a combination of organic resources and fertiliser was better in increasing crop yield than the application of the same N amount in the form of urea. In a year of rainfall deficiency, a mix of organic resources and fertiliser in both till and no-till systems increased crop water use efficiency, with the result that the farmer was able to purchase only half of the normal quantity of N fertiliser to obtain a higher yield that he would have done when all of the N was supplied in the form of urea. Under conditions where soil N is deficient, an economic benefit was achieved when urea was combined with easily decomposable organic material (e.g. sheep dung); mixing the urea at a dose of 40 kg N ha−1 with maize straw was not sufficient in alleviating the negative interaction due to the enhanced N immobilisation. The results demonstrate that the use of N fertiliser alone was risky and that a higher yield, with the accompanying economic benefit, was scarcely achieved under the prevailing rainfall conditions. The application of soil and water conservation measures can contribute greatly to increasing the economic benefit of mineral, organic or combined organic and mineral-derived nutrient application under semi-arid conditions.

A field experiment was conducted at Kaibo in southern Burkina Faso on an Eutric Cambisol during the 2000 rainy season to assess the interaction of organic amendment quality and soil fauna, affecting soil organic carbon and sorghum ( Sorghum bicolor L. Moench) performance. Plots were treated with the pesticides Dursban and Endosulfan to exclude soil fauna or left untreated. Sub-treatments consisted of surface-placed maize straw ( C/N ratio= 58), Andropogon straw ( C/N ratio= 153), cattle dung ( C/N ratio= 40), sheep dung ( C/N ratio= 17) or compost ( C/N ratio= 10) and the control. Organic amendments were applied at a dose equivalent to the application of 40 kg N ha(-1). The presence of soil fauna increased soil total carbon by 32% and grain yield production by 50%. The interaction between high C/N ratio organic amendment, Andropogon straw ( SA), and soil fauna reduced soil carbon build-up. We suggest that this is due to a priming effect of SA on soil organic matter in the presence of soil fauna. We also suggest that the interaction between soil fauna and easily decomposable organic amendment led to the smallest decrease in soil carbon build-up. It is concluded that in semi-arid West Africa, in the presence of soil fauna, soil carbon build-up is more affected by the quality of organic amendments than the quantity of carbon inputs. Sorghum grain yield production was significantly reduced in the absence of soil fauna. High C/N ratio organic amendment interacted negatively with soil fauna in its effects on crop performance. We propose that the effect of soil fauna on soil carbon build-up and crop performance can be optimised by using high quality organic matter of supplementing low-quality organic matter with inorganic nitrogen in semi-arid West Africa.

In the Sahelian zone of West-Africa, erosion by both wind and water causes a serious decline in fertility of the already low fertile soils. Despite the fact that the flow of nutrients has been intensively investigated by the use of nutrient balances, little attention has been paid to the contribution of the soil erosion to the nutrient balance. Two physically based models (WEPS and EUROSEM, both written in PCRaster) were extended with nutrient modules to investigate the role of wind and water erosion in tire loss and gain of nutrients at the scale of a Sabetian field. The models are applied at three geomorphic units in the Katchari catchment in northern Burkina Faso. WEPS can predict spatial patterns of erosion and deposition due to wind-blown particle transport. Depending on wind direction, crusting and vegetation cover net erosion or deposition can occur. When erosion occurs considerable amounts of nutrients are lost, but when deposition occurs, most of these nutrients may be regained. Soil loss by water erosion is closely related to the crust type present, which regulates infiltration and thus runoff. Nutrient losses by water erosion are small compared with those by wind erosion, but are forever lost for the area. Sediment transport by wind in saltation mode results in the largest soil and nutrient loss at the time scale of an event. (c) 2005 Elsevier B.V. All rights reserved.

Knowledge of changes in soil organic matter (SOM) fractions resulting from agricultural practice is important for decision-making at farm level because of the contrasting effects of different SOM fractions on soils. A long-term trial sited under Sudano-Sahelian conditions was used to assess the effect of organic and inorganic fertilization on SOM fractions and sorghum performance. Sorghum straw and kraal manure were applied annually at 10 t ha(-1), with and without urea at 60 kg N ha(-1). The other treatments included fallowing, a control (no fertilization), and inorganic fertilization only (urea, 60 kg N ha(-1)). Fallowing gave significantly larger soil organic carbon and nitrogen (N) levels than any other treatment. Total soil SOM and N concentrations increased in the following order: urea only 0.053 mm (particulate organic matter, POM). The POM concentrations in the control, straw and urea-only treatments were about one-half of the POM concentrations in the fallow treatment. POM concentrations increased in the following order: urea only

Human-induced degradation of natural resources in general and of soil in particular, is a major problem in many regions, including the Sudano-Sahelian zone. The combined effects of tillage and manure application on Lixisol properties and on crop performance were investigated at Saria, Burkina Faso, to find efficient soil management practices to improve soil fertility. A randomized block design with four treatments (hand hoeing only, hand hoeing+manure, ploughing only, oxen ploughing+manure) in three replications was started in 1990. Ten years later, total soil organic (SOC), particulate organic matter and C mineralization were measured. Initial SOC concentration was 4 mg/g and dropped to 2.1 mg/g soil in ploughed plots without manure and to 2.5 mg/g soil in hoed plots without manure. Manure addition mitigated the decrease of SOC in ploughed plots and even built up SOC in hoed plots, where it increased to 5.8 mg/g soil. Manure had a large effect on the fractions in which SOC was stored. In ploughed plots, a large amount of SOC was stored in physical particles >0.25 mm, while in hand hoed plots the maximum SOC was stored in finer fractions. In the topsoil, hoeing and manure resulted in a higher SOC than ploughing with no manure. However, in the 15–25 cm layer, particularly in September, particulate organic matter was greater in ploughed plots with manure than in hoed plots with manure. Crop yields were highest on ploughed+manure plots and lowest on ploughed plots with no manure. We conclude that applying manure annually mitigates the negative effect of ploughing and hand hoeing on SOC and related properties and therefore can contribute to the sustainability of the agricultural system in the Sudano-Sahelian zone.

Degraded soils in the sub-Saharan zone are often unproductive because of nutrient imbalance and an inadequate water supply. We conducted an experiment in the northern sudanian climate zone of Burkina to study the effect of integrated local water and nutrient management practices on soil water balance, sorghum performance and sorghum’s water use efficiency. The trial (Ferric Lixisol, 1.5% slope) consisted of two replications of nine treatments in which soil and water conservation (SWC) measures (stone rows, grass strips) and organic or mineral N-inputs (compost, manure, urea-N) were applied alone or in combination and compared to a control treatment with no N-input and no SWC measure. Application of compost improved soil water storage in the sorghum-rooting zone (0–80 cm) most when combined with stone rows or grass strips and when the year had well-distributed rainfall. However, during an erratic rainy season there was less soil water storage in the organic treatments than in the mineral treatment. Supplying compost increased evapotranspiration and soil drainage more than nutrient inputs did. Furthermore, stone rows allowed greater evapotranspiration and drainage than grass strips, and the two permeable barriers alone had a significant effect on soil water storage compared with treatments without barriers. In the rain-fed cropping system studied, we found that in an erratic rainy season with frequent periods of water stress, the stone rows or grass strips combined with compost reduced runoff and increased soil water storage and sorghum biomass production. These combined practices created sound soil water conditions and were able to satisfy the sorghum’s water demand for growth. We conclude that the synergistic effect of water-harvesting practices and the supply of organic or mineral resources increased water use efficiency. It seems that an optimum combination of organic resources and fertilisers could improve the water use efficiency (i.e. reduce runoff and drainage losses) and the productivity of Sahelian rain-fed agriculture.

Efficient use of external inputs and water conservation are a prerequisite of sustainable agricultural productivity in semiarid West Africa. A field experiment was carried out during 3 years (2000-2002) at Saria in semiarid Burkina Faso (800 mm of annual rainfall, PET of 2000 mm per year) to assess the effects of stone rows or grass strips of Andropogon gayanus Kunth cv. Bisquamulatus (Hochst. Hack.) as soil and water conservation (SWC) measures, the sole application of an organic (compost-N) or mineral (urea-N) nitrogen and the combined use of SWC and compost-N or urea-N on N flows and balances. The trial was conducted on a Ferric Lixisol with 1.5% slope and comprised nine treatments in two replications. The SWC measures were put along contours lines. During the three consecutive years, all treatments induced negative annual N balances (-75 to -24 kg N ha-1). The main factors explaining these negative balances were N exports by sorghum biomass and soil erosion-induced N losses. Large amounts of N (7 kg N ha-1 per year in 2000 and 44 kg N ha-1 per year in 2002) were lost in the control treatment through runoff and eroded sediments, which corresponds respectively to about 10 and 43% of the total outflow of N. Sole stone rows and grass strips reduced erosion N losses to 8 and 12%, respectively, of the total annual loss. The combined application of SWC measures and nutrients inputs reduced erosion N losses to only 2-7% of the annual N loss. The application of urea-N or compost-N led to the lowest soil N mining over the 3 years, whereas the highest N mining was observed in plots without added N. We conclude that N mining in poor fertile soils of West Africa can be mitigated through an integration of local water and nutrient management practices. (Resume d'auteur)

Growth and yield of pearl millet (Pennisetum glaucum) in the West African Sahel are characterized by significant spatial variability at short distances (5-20 m). Several studies have suggested that this variability is caused by aeolian redistribution of relatively fertile topsoil material. The objectives of this study were (i) to quantify storm-based erosion/deposition patterns within a Sahelian millet field and (ii) to determine the effects of topography, erosion and deposition on millet growth and yield. An experiment was conducted at a research station in southwest Niger, on a sandy, siliceous, isohyperthermic Psammentic Paleustalf. Twenty-one sediment catchers were installed in a 40 m x 60 m plot within a pearl millet field. Four wind erosion events occurred during the 1993 growing season. Maps of wind-blown mass transport were created for each storm by applying a geostatistical space-time procedure. Maps showing the spatial distribution in erosion and deposition were derived by differentiation of mass transport in the direction of the mean wind. For 21 sub-plots of 4 m x 5 m the elevation, wind-blown mass balance, millet growth and yield were quantified. A correlation analysis showed neither a significant relationship between millet yield and topography, nor between millet yield and erosion/deposition. It is concluded that only wind-blown sediment transport occurring within one growing season cannot explain the spatial variability in millet growth and yield. A new hypothesis is defined that suggests aeolian erosion/deposition patterns as observed in previous studies were not the cause but more likely the result of spatial variability in millet growth. (C) 2003 Elsevier B.V. All rights reserved.

Nutrient limitation is the main cause of per capita decline in crop production in the Sahel, where water shortage also limits an efficient use of available nutrients. Combining soil and water conservation measures with locally available nutrient inputs may optimize crop production and economic benefit in cereal-based farming systems. A study conducted in 2001 and 2002 at Saria, Burkina Faso (annual rainfall 800 mm, PET of 2000 mm yr–1) assessed the combined effects of two types of semi-permeable barriers (stone rows and grass strips of Andropogon gayanus Kunth cv. Bisquamulatus (Hochst.) Hack.) and the application of compost or urea on sorghum performance and economic benefits. The field experiment was carried out on a Ferric Lixisol, 1.5% slope and comprised 9 treatments in which the barriers were put along contours and combined with compost-N or urea-N. Installation of stone rows or grass strips without addition of nutrient inputs was not cost effective, although it induced sorghum yield increase (12–58%) particularly under poor rainfall conditions. Combining compost with stone rows or grass strips significantly increased sorghum yield that induced positive interaction effects (mean added effects of 185 kg ha–1 for stone rows combined with compost-N and 300 kg ha–1 for grass strips combined with compost-N). Economic benefits were substantial (109 480 to 138 180 FCFA ha–1) when compost-N was added to both stone rows and grass strips, whereas limited economic benefits were observed with the application of urea-N (1120 to 22 120 FCFA ha–1). This may provide farmers with capital to invest in soil management and may also contribute to poverty alleviation in sub-Saharan Africa.

This study appraises the effects of land use on soil properties in a typical watershed in the northwestern highland of Ethiopia. Soil samples were collected from major land use types in the watershed: natural forests, cultivated lands, grazing lands and Eucalyptus plantations. The natural forests served as a control against which to assess changes in soil properties resulting from the establishment of the other land use types. Samples were taken at two depths (0–15 and 15–30 cm) in the upstream and downstream areas of the watershed and analyzed for a range of soil properties. The soils in the cultivated fields, grazing lands and Eucalyptus plantations showed significantly higher sand content, but lower Ca2+ and Mg2+ contents and cation exchange capacity (CEC) compared to soils under natural forests. Eucalyptus soils had a statistically significant higher bulk density (BD) than soils under the other three land use types. The forest and Eucalyptus soils also differed significantly from each other in their soil organic matter (SOM) and total N contents. A significant difference in available P among soils of the four land use types was caused by the difference between cultivated and Eucalyptus soils. In contrast, the distribution of soil silt fraction Na+, K+ and pH values did not differ among the four land use types. Significant differences in many of the soil properties were also observed between soils in the two sampled villages. The study underscores the need for policies and strategies for sustainable land use that will attune objectives of economic development to environmental management at the regional and local levels.

In the semiarid Sahel, farmers commonly lay stone lines in fields to disperse runoff. This study was conducted in northern Burkina Faso to assess the chemical fertility of soil under a permanent, non-fertilised sorghum crop, which is the main production system in this area, 5 years after laying stone lines. The experimental design consisted of four plots in which stone lines had been laid. The spacing between the lines was 100 m in the first plot, 50 m in the second, 33 m in the third, and 25 m in the fourth. To determine soil chemical characteristics in relation to the stone line spacing patterns studied, soil samples were taken from subplots at regular and fixed distances from the lines at the start of the trial and then 5 years later. Under the continuous non-fertilised sorghum cropping system, the beneficial effect of stone lines on soil fertility was limited. Five years after installing stone lines, soil organic C, total N, available P and Na concentrations and soil pH had decreased. Within the plots, these same variables were higher upslope than downslope of stone lines, probably because of water storage and sediment accumulation in front of the stone lines. In plots where stone lines were relatively close together (