Search

Sustainable intensification of African farming systems has been high on the agenda of research and development programs for decades. System innovations such as integrated soil fertility management (ISFM) and conservation agriculture have been proposed to tackle the complex challenges farmers face. In this study, we assess how different factors at the plot, farm and institutional level can influence the adoption of ISFM. We employed a stratified sampling approach to randomly select 285 and 300 farmers in Tamale, northern Ghana and Kakamega County, western Kenya, respectively. These two locations were selected to understand the underlying reasons for their divergent adoption levels. Ordinal regression models were used to identify determinants of adoption. In Tamale, adoption rates of ISFM are much lower than in Kakamega. Only 3% of the farmers fully adopted the recommended practices compared with 36% in Kakamega. The low availability of improved seeds is a major reason for the lower uptake of the complete ISFM paradigm in Tamale. The econometric analysis revealed that plot level variables such as soil carbon, soil texture, slope and plot area had a significant effect on the number of adopted ISFM components at both locations. Moreover, family labor availability is also an important factor. Other farm and household characteristics, such as off-farm occupation, livestock ownership, and membership in associations, matter for Kakamega only. Key policy recommendations include promotion of locally available organic resources and improved access to improved seeds in Tamale.

Sub-Saharan Africa (SSA) is plagued by low productivity and little research is available on the attainable responses and profitability to applied nutrients under variable environments. The objective of this study was to determine the attainable maize grain response to and potential of profitability of N, P and K application in SSA using boundary line approaches. Data from experiments conducted in SSA under AfSIS project (2009–2012) and from FAO trials database (1969–1996) in 15 countries and constituting over 375 different experimental locations and 6600 data points are used. Both response to fertilizer and value cost ratio (VCR) are highly variable and no more than 61 % cases for N, 43 % for P and 25 % for K attain VCR of 2 or more. Also, based on the recent AfSIS data, VCR exceeds 1 in just 67 % (N), 57 % (P) and 40 % (K) of the cases, even when best management practices are applied on a research farm, and interest rates are zero. Chances for profitability are highest when soil organic carbon is 1–2 % and control maize grain yield is 1–3 t ha−1 but also depends on relatively static soil properties (primarily texture and mineralogy) that are not under farmer control. We conclude that return on investment of macronutrient fertilizer is highly variable and can be substantially increased by helping farmers decide where to apply the fertilizers. Consequently, farmers need access to information on factors influencing economic returns of fertilizer use in order to make the right decisions.

Understanding the patterns of land degradation indicators can help to identify areas under threat as basis for designing and implementing site-specific management options. This study sort to identify and assess the patterns of land degradation indicators in selected districts of western Kenya. The study employed the use of Land Degradation Sampling Framework (LDSF) to characterize the sites. LDSF a spatially stratified, random sampling design framework consisting of 10 km × 10 km blocks and clusters of plots. The study broadly identified and classified the indicators and attributes of land degradation into soil and site stability, hydrologic function and biotic integrity. Assessment of general vegetation structure showed that over 70% of the land was under cropland with forests accounting for 8% of the area. Sheet erosion was the major form of soil loss. High variability was observed for the soil properties and this can be due to both inherent soil characteristics as well as land management practices. There was distinct variation in the soil properties between the topsoil (0–20 cm) and the subsoil (20–30 cm) with the topsoil having higher values for most of the parameters compared to the subsoil. Using coefficient of variation (CV) as criteria for expressing variability, Ca, TON, Mg, SOC and silt were most variable soil properties for the 0–20 cm depth. Moderate variability (CV 0.15–0.35) was observed for CEC, P, K and clay while Na, Sand and pH had the least variability (CV

SUMMARYReduced tillage is said to be one of the potential ways to reverse land degradation and ultimately increase the productivity of degrading soils of Africa. We hypothesised that crop yield following a modest application of 2 t ha−1 of crop residue in a reduced tillage system is similar to the yield obtained from a conventional tillage system, and that incorporation of legumes in a cropping system leads to greater economic benefits as opposed to a cropping system involving continuous maize. Three cropping systems (continuous maize monocropping, legume/maize intercropping and rotation) under different tillage and residue management systems were tested in sub-humid western Kenya over 10 seasons. While soybean performed equally well in both tillage systems throughout, maize yield was lower in reduced than conventional tillage during the first five seasons but no significant differences were observed after season 6. Likewise, with crop residue application, yields in conventional and reduced tillage systems are comparable after season 6. Nitrogen and phosphorus increased yield by up to 100% compared with control. Gross margins were not significantly different among the cropping systems being only 6 to 39% more in the legume–cereal systems relative to similar treatments in continuous cereal monocropping system. After 10 seasons of reduced tillage production, the economic benefits for our cropping systems are still not attractive for a switch from the conventional to reduced tillage.

SUMMARYMany food production systems in sub-Saharan Africa are constrained by phosphorus (P). We hypothesized that within legume-cereal rotation systems: targeting P to the legume phase leads to higher system productivity, and that use of grain legumes leads to better economic returns than use of herbaceous legumes. Four P application regimes: (i) no P, (ii) P applied every season, (iii) P applied in season 1 only and (iv) P applied in season 2 only were tested for four seasons in three cropping systems (continuous maize, mucuna-maize rotation and soybean-maize rotation) in a split plot experiment set up in Nyabeda, western Kenya. Treatments where P was applied were better than no P treatments. While continuous cereal systems showed the need for application of P every second season, rotation systems involving mucuna and soyabean indicated that application in one out of three seasons could be sufficient. Nitrogen fertilizer equivalence was 52 to >90 kg N ha−1for soyabean and 37 to >90 kg N ha−1for mucuna, depending on P fertilization and season. Analysis of marginal rates of return (MRR) showed that soybean-maize rotation with one application of P was the most economically viable option, with an MRR of at least 147% compared to other non-dominated options.

The effects of conservation tillage, crop residue and cropping systems on the changes in soil organic matter (SOM) and overall maize–legume production were investigated in western Kenya. The experiment was a split-split plot design with three replicates with crop residue management as main plots, cropping systems as sub-plots and nutrient levels as sub-sub plots. Nitrogen was applied in each treatment at two rates (0 and 60 kg N ha−1). Phosphorus was applied at 60 kg P/ha in all plots except two intercropped plots. Inorganic fertilizer (N and P) showed significant effects on yields with plots receiving 60 kg P ha−1 + 60 kg N ha−1 giving higher yields of 5.23 t ha−1 compared to control plots whose yields were as low as 1.8 t ha−1 during the third season. Crop residues had an additive effect on crop production, soil organic carbon and soil total nitrogen. Crop rotation gave higher yields hence an attractive option to farmers. Long-term studies are needed to show the effects of crop residue, cropping systems and nutrient input on sustainability of SOM and crop productivity.

Soil fertility depletion has been described as the single most important constraint to food security in West Africa. Over half of the African population is rural and directly dependent on locally grown crops. Further, 28% of the population is chronically hungry and over half of people are living on less than US$ 1 per day as a result of soil fertility depletion. Soil organic carbon (SOC) is simultaneously a source and sink for nutrients and plays a vital role in soil fertility maintenance. In most parts of West Africa agro-ecosystems (except the forest zone), the soils are inherently low in SOC. The low SOC content is due to the low shoot and root growth of crops and natural vegetation, the rapid turnover rates of organic material as a result of high soil temperatures and fauna activity particularly termites and the low soil clay content. With kaolinite as the main clay type, the cation exchange capacity of the soils in this region, often less that 1 cmol kg 1 , depends heavily on the SOC. There is a rapid decline of SOC levels with continuous cultivation. For the sandy soils, average annual losses may be as high as 4.7% whereas with sandy loam soils, losses are lower, with an average of 2%. To maintain food production for a rapidly growing population, application of mineral fertilizers and the effective recycling of organic amendments such as crop residues and manures are essential especially in the smallholder farming systems that rely predominantly on organic residues to maintain soil fertility. There is need to increase crop biomass at farm level and future research should focus on improvement of nutrient use efficiency in order to increase crop biomass. Research should also focus on ways of alleviating socio-economic constraints in order to increase the legume component in the cropping systems. This will produce higher quality fodder for the livestock and also increase biomass at farm-level. This paper reviews various strategies and lessons learnt in improving soil organic carbon status in West Africa soils. 2006 Elsevier Ltd. All rights reserved.

In southern Africa, soil nutrient reserves are being depleted because of continued nutrient mining without adequate replenishment. The consequent downward spiral of soil fertility has led to a corresponding decline in crop yields, food insecurity, food aid and environmental degradation. The central issue for improving agricultural productivity in southern Africa is how to build up and maintain soil fertility despite the low incomes of smallholder farmers and the increasing land and labour constraints they face. Under this review five main options namely: inorganic fertilizers, grain legumes, animal manures, integrated nutrient management and agroforestry options appropriate to smallholder farmers are presented. Issues addressed in the use of inorganic fertilizers are reduction in fertilizer costs, timely availability and use efficiency. Legumes can be used to diversify farm system productivity but this requires P and lime application to support better legume growth and biological nitrogen fixation (BNF) as well as development of markets for various legume products. Manure availability and quality are central issues in increasing smallholder farm productivity and increasing its efficiency through proper handling and application methods. Integrated nutrient management of soil fertility by combined application of both inputs will increase use efficiency of inputs and reduce costs and increase profitability; but the challenge is often how to raise adequate amounts of either inorganic or organic inputs. Issues such as quality of inputs, nutrient balancing, labour to collect and transport organic inputs and their management need to be optimized. These are the challenges of adoption as are the scaling up of these options to millions of small-scale farmers.

Soil carbon plays a key role in maintaining crop productivity in the soils in sub-Saharan Africa (SSA). This is more so considering that most smallholder farmers cannot afford the use of adequate amounts of inorganic fertilizers to restore the proportion of nutrients lost through crop harvests, soil erosion and leaching. Complicating the situation is the huge proportion of land under threat of degradation in the form of soil erosion and nutrient decline. There are numerous opportunities for improving soil carbon as a basis of ensuing sustainable agriculture. This paper discusses the role of soil carbon in agricultural production, with special focus on sub-Saharan Africa. First, the paper presents a discussion on the functions of soil carbon (biological, chemical and physical). This is followed by a look at the causes of carbon variation across agroecosystems. Management of soil carbon and productivity is evaluated in the context of resource availability, quality and soil organic matter pools. Drawing from the integrated soil fertility management practices in Africa, the paper discusses various strategies for organic carbon management and the implication of the same on crop productivity and soil properties. A special focus is given to the lessons learned from long-term experiments across Africa.

Conservation agriculture (CA) offers an opportunity to reverse prevailing land degradation and consequent loss of productivity often occasioned by intensive soil tillage in cropping systems in most parts of sub-Saharan Africa (SSA). A long term experiment was established in Nyabeda Western Kenya in 2003 to evaluate the effect of tillage and crop residue application on maize and soybean yields, and on soil properties. The experiment was set up as a split-split-split plot design with four replicates and involved a factorial combination of tillage system (reduced and conventional tillage), cropping system (continuous cereal, soybean-maize rotation and intercropping), crop residue – maize stover – management (plus and minus crop residue) and nitrogen (N) application. Results showed that tillage influenced performance of maize although significant tillage effects were observed in only 5 out of the 15 seasons analyzed. Overall average maize grain yields were 2.9 ton ha−1 in reduced tillage and 3.6 ton ha−1 in conventional tillage systems. Application of crop residue increased seasonal maize grain yield in reduced tillage (340 kg ha−1) and in conventional tillage (240 kg ha−1), but the only significant crop residue (CR) effect was observed in season 10. Differences in maize yields between the two systems were attributed to phosphorus availability as it was demonstrated that application of crop residue in the reduced tillage resulted in better availability of P than without crop residue application.

The prevailing low food production in sub-Saharan Africa is an issue of great concern especially since Africa south of the Sahara is the only remaining region of the world where per capita food production has remained stagnant. This chapter reviews long-term experiments in Africa in the context of shifting paradigms related to tropical soil fertility management from first external input paradigm right through to the current Integrated Soil Fertility Management (ISFM) approach, which is a culmination of the participatory methods developed along the paradigm shift. Long term experiments (LTE) are an important source of evidence for soil fertility decline and provide crucial datasets for the development of sustainable management practices for tropical land-use systems and the amelioration of global climatic and environmental change impacts. A survey was undertaken to identify some ongoing long-term trials distributed across east, south and western Africa and in different agro-ecological zones. A long-term Experiment was defined as that extending over a period of over 5 years and more. Results from these trials are discussed in detail. Inference has been drawn from these findings and includes the following key findings: a) All long term trials showed yield decline, often with a relatively rapid fall to a low level equilibrium; b) At all sites, there were positive yield responses to one or more nutrients added as mineral fertilizers, which were consistent for the duration of the experiments highlighting the effectiveness of mine­ral fertilizers in increasing yield in arable farming systems in Africa; c) Soil organic matter (SOM) also declines significantly when land is cultivated; d) Prolonged treatments using only inputs of organic matter also showed yield declines, although the positive impact were sustained longer than for inorganic fertilizers alone in most cases; e) Rotational treatments, including sequences with legume crops and fallow periods had lower yield declines than monocultures; f) The best results invariably were those treatments that combined inorganic and organic inputs.

Africa is the continent with the lowest fertilizer use per hectare ­notwithstanding the fact it possesses geologically old, infertile and degraded soils. This chapter discusses the agro-ecological zones (AEZ) and main soil types in Africa followed by a section on the extent, effects and costs of land degradation including issues of soil productivity and profitability associated with fertilizer use in Africa. There are a variety of soil types in the five major agro-ecological zones of Africa. Ferralsols and the Acrisol are dominant in the humid zones. Ferralsols are dominant in the sub-humid zone and so are Lixisols while in the semi-arid zone Lixisols have the larger share. Sixty five percent of the agricultural land in Africa is degraded and soil fertility depletion, a manifestation of soil degradation, is currently a serious threat to food security among small-holder farmers. Because of this state of affairs there is a strong case for enhanced fertilizer use. Maize yield has reportedly increased over the control due to NPK fertilizer application from various AEZ and when soils are amended with lime and manure yield response has been even higher. Indeed, there is credible evidence of fertilizer response and profitability in Africa relative to other parts of the world, particularly, for maize and rice thus making fertilizer investment worthwhile. In conclusion, there is need for agricultural intensification through efficient use of soil nutrient and water resources. Technologies need to be adapted to the specific bio-physical and socio-economics circumstances of the small scale farmers in Africa. There is also need to focus more on increasing the fertilizers use efficiency and the development of the local fertilizer sector in order to make them more profitable.

The full potential of legume based technologies can only be achieved if farmers widely adopt these technologies. Widespread adoption of legume-based technologies calls for the identification of spatial and temporal niches of legumes in the farming systems. The entry point may be immediate provision of food and fodder but also contribution to soil fertility improvement and erosion control among others. Whichever the objective, there is need to demonstrate the immediate benefits to be accrued by the adopters if large scale adoption is to be achieved. There is need to address challenges hindering widescale adoption of legume technologies such as availability of improved germplasm, poor markets and policy. Whereas the national systems play a key role in promoting legumes, the private sector continues to effectively complement government efforts to increase availability and accessibility to quality seed. Flow of knowledge cannot be achieved through a narrow prescriptive approach, but requires development and testing of a battery of possible interventions for soil fertility improvements suited to the specific agroecological environment together with the farmers. Better and innovative mechanisms are required for sharing of knowledge between all those involved in trying to improve the productivity of smallholder agriculture. This should cover the use or both print and audio media as complement to the traditional approaches such as the use of field days and onfarm demonstrations. The focus of this chapter is therefore to look at how the above issues have been addressed in adapting and disseminating legume-based technologies in Africa.

Preface.- Chapter 1: Agro-ecological distribution of legumes in farming systems and identification of biophysical niches for legumes growth, Nandwa SM (The late), Obanyi SN and Mafongoya PL.- Chapter 2: Socio- economic contribution of Legumes to Livelihoods in Sub-Saharan Africa Odendo M, Bationo A and Kimani S.- Chapter 3: Inter And Intra-Specific Variation Of Legumes And Mechanisms To Access And Adapt To Less Available Soil Phosphorus And Rock Phosphate, Nandwa SM (The late), Bationo A, Obanyi SN, Rao IM, Sanginga N and Vanlauwe B.- Chapter 4: Legume based cropping systems and soil biodiversity dynamics Karanja NK, Kimenju JM, Esilaba AO, Jefwa J and Ayuke F.- Chapter 5: Strategies to adapt, disseminate and scale out legume based technologies Mugendi DN, Waswa BS, Mucheru MW and Kimetu JM.- Chapter 6: Comparative Analysis of the Current and Potential Role of Legumes in Integrated Soil Fertility Management in West and Central Africa Bationo A, Kimetu J, Vanlauwe B, Bagayoko M, Koala S and Mokwunye AU.- Chapter 7: Comparative Analysis of the Current and Potential Role of Legumes in Integrated Soil Fertility Management in East Africa Mugendi DN, Waswa BS, Mucheru M, Kimetu JM and Palm C.- Chapter 8: Comparative Analysis of the Current and Potential Role of Legumes in Integrated Soil Fertility Management in Southern Africa Mapfumo P.- Chapter 9: Options for improving plant nutrition to increase common bean productivity in Africa Jansa J, Bationo A, Frossard E and Rao IM.- Conclusion and the way forward

An on-farm experiment was conducted in Western Kenya (Busia district) in the long rain season of 2005 to investigate the effects of conservation tillage on soil properties and the crop yields. The experiment based on a split–split plot design with three replicates and six core treatments arranged in a factorial combination of nitrogen application and cropping systems was adopted. Maize variety IR (striga resistant) was planted as a test crop, soybean (SB20) variety as an intercrop and for maize–legume rotation. Soil pH, moisture content and organic carbon were analysed in soil whereas total P and N were analysed in the plant tissue. Conservation and conventional tillage systems combined with cropping systems (intercropping, rotation and continuous) at 0 and 60 kg N/ha application were tested. Residue incorporation was done to all plots. The soil was sampled before and after harvesting to compare the effects of the treatments. Weeding for conservation tillage plots was by hand pulling. Combinations of conservation tillage, continuous and with application of 60 kg N/ha, for maize gave the highest yield of 2.8 t/ha. The combination of conservation tillage, rotation and application of 60 kg N/ha gave 2.5 t/ha maize grain. Combination of conservation tillage rotational cropping system at 60 kg N/ha application gave the highest soybean yield (1.23 t/ha). Soil carbon showed that there was significant difference between the conservation tillage and conventional tillage as well as the increase of the soil carbon from initial level of 1.44% to the highest percentage soil carbon of 1.9%.

In Kenya, soil acidity is a major contributor to declining soil fertility and 20% of the soils are acidic and are considered to be of low fertility. Most farmers are unaware of the benefits of liming acid soils. A study was carried out during the 2005 and 2006 long rain seasons at Kuinet in Uasin Gishu District of the Rift Valley Province in Kenya to delineate the effects of Minjingu phosphate rock (MPR) and agricultural lime as liming materials on yields of soybeans intercropped with maize. The maize responded to application of soil amendment materials for the first season with the diammonium phosphate and lime (DAPL) treatment giving the highest maize yields of 6.19 t ha–1 compared to the control which gave 1.36 t ha–1. Soybean yields were low in the first season with the DAPL treatment and control treatment giving yields of 0.32 and 0.14 t ha–1, respectively. This, however, changed significantly after the variety was changed in the second season, with yields going up to 0.68 t ha–1 for the triple superphosphate and lime (TSPL) treatment. From the study, it was concluded that there is potential for growing soybean in Uasin Gishu District of Kenya. However, a study and/or research is recommended to screen and identify a suitable variety for increased soybean yields in this district.

Sub-Saharan African (SSA) region continues to experience perennial hunger, poverty and poor health of its people. Agricultural production has remained low over decades and is declining to extremely low staple maize yields below 0.5 t ha–1 season–1 at the smallholder farm scale, against the potential of 4–5 t ha–1 season–1 given modest levels of inputs and good crop husbandry. Constraints contributing to low productivity are numerous, but the planting of poor-quality seed, declining soil fertility, poor markets and value addition to products significantly contribute to poor productivity. Partnerships for development are weak even though there are numerous technologies to improve and sustain agricultural production arising from extensive research and extension in SSA. But, technology adoption rates have been extremely slow, and in some cases we find no adoption. In this chapter we highlight constraints which are bottlenecks for achievement of a green revolution in Africa. Success efforts are reported, but we moot a focus on efficient utilization of abundant and affordable African natural resources, such as phosphate rocks to replenish depleted phosphorus in soils. We argue that to achieve an African green revolution, partnerships with concerned global communities and national institutions, including universities, NGOs, CBOs and farming communities, need to be strengthened. Specifically, human capacity at all levels should be built through training. Without private sector’s strong participation on acquisition of inputs and marketing proven products, it will be difficult to achieve a green revolution.

Combined application of organic resources and mineral inputs is integral to sustainable soil fertility management but in-situ production of adequate organic matter is often limited by P availability. An experiment was set up at Nyabeda in Western Kenya aimed at (1) quantifying the contribution of herbaceous and grain legumes to nitrogen supply in a cereal-legume rotation system and (2) quantifying the impact of targeting phosphorus (P) to certain phases of the rotation on overall maize grain yield. In this split-split plot experiment, Mucuna pruriens was used as the herbaceous legume while soybean was used as the grain legume. Results obtained in the two seasons of the study indicated that the use of either mucuna or soybean as previous crop significantly increased maize grain yield with or without the addition of nitrogen fertilizer. More than 5 tons ha-1 of maize grain yield was realised in season two following the addition of phosphorus fertilizer at both season one and season two compared to about 3 tons ha-1 of maize grain yields obtained when no P was added. It could be concluded that in this region, the addition of P fertilizer is an integral management option to ensure optimal utilization of the nitrogen fixed by the legume crop. Using P during the legume season may be sufficient to supply P requirements to the succeeding cereal crop. Also, applying P to the mucuna or soybean legume crop was not any different from applying it both to the legume and cereal crops indicating that farmers can save labour and cash by applying P only to the legume. The good performance of maize planted after mucuna was an indication that mucuna could be used by farmers in the region as an N source (Nitrogen Fertilizer Equivalency (NFE) >100 kg N ha-1) thus reducing cost of buying N fertilizers. Although soybean showed a lower NFE of 40 kg N ha-1, it had higher economic benefits and could thus be more acceptable to the farmers. These findings could be confirmed by using more than two cereals and legume rotation cycles

Crop production in sub-Sahara Africa is constrained by low soil phosphorus (P) content. A study was conducted in western Kenya to explore alternative P inputs and ways of optimizing their effectiveness and profitability. A field experiment established in 2007 studied the effects of Minjingu phosphate rock (MPR) and triple superphosphate (TSP) on maize, common beans and soybean yield. MPR and TSP were applied seasonally at a rate of 0, 12.5, 25 and 50 kg P ha−1 either alone or in combination. Application of P, irrespective of amount, resulted in significantly higher grain yield and total biomass for maize, common beans and soybean compared with the 0 P treatment. Applying P at 12.5 kg ha−1 resulted in significantly (ρ ≤ 0.05) lower maize, common beans and soybean grain yields than all the other P rates. On the other hand, application of P at 25 kg ha−1 resulted in similar yields to the higher P application rates. Relative agronomic effectiveness of MPR was similar for both maize and soybeans in most seasons, confirming that MPR has high potential for direct application in these soils. Switching from no application to P applied at 12.5 and also 25 kg P ha−1 attracts a marginal rate of return of at least 200 %. Switching from 25 kg P ha−1 to any of the other options attracted MRR < 200 %. This implies that adoption of either MPR or TSP by farmers in western Kenya is profitable for maize and soybeans production, given that MRRs were above 100 % minimum acceptable rate of return which is a requirement for farmers to change from one technology to another.