African Smallholder Farmers Research Articles

When yield gaps are poverty traps: The paradigm of ecological intensification in African smallholder agriculture

Yield gaps are pervasive in African smallholder agriculture, and are large for almost all crops in all regions. There is consensus that poor soil fertility and nutrient availability are the major biophysical limitations to agricultural production in the continent. We identify two major yield gaps: (1) the gap between actual yields (YA) and the water-limited yield potential (Yw), which is the maximum yield achievable under rainfed conditions without irrigation if soil water capture and storage is optimal and nutrient constraints are released, and (2) The gap between YA, and a locally attainable yield (YL) which corresponds to the water and nutrient-limited yields that can be measured in the most productive fields of resource endowed farmers in a community. Estimates of these two yield gaps are given for major crops, together with a framework for how yield gaps can be estimated in a pragmatic way for different farming systems. The paradigm of ecological intensification which focuses on yield potential, soil quality and precision agriculture is explored for the African context. Our analysis suggests that smallholder farmers are unable to benefit from the current yield gains offered by plant genetic improvement. In particular, continued cropping without sufficient inputs of nutrients and organic matter leads to localised but extensive soil degradation and renders many soils in a non-responsive state. The lack of immediate response to increased inputs of fertiliser and labour in such soils constitutes a chronic poverty trap for many smallholder farmers in Africa. This necessitates a rethink for development policy aimed to improve productivity and address problems of food insecurity.

Participatory action research (PAR) as an entry point for supporting climate change adaptation by smallholder farmers in Africa

Emerging trends of a changing and increasingly variable climate have introduced new livelihood challenges in rain-fed smallholder agricultural systems that predominate in Sub-Saharan Africa (SSA). The capacity of local farming communities and their institutions to respond to the new and emerging impacts of climate change is often constrained by lack of access to information and improved technologies, as well as poor support mechanisms to promote assimilation of new knowledge. This threatens to heighten vulnerability of the majority of SSA's rural communities who are already facing severe problems of food insecurity and a declining soil resource base. In this paper we use two case studies from Wenchi district in Ghana and Makoni in Zimbabwe to communicate how participatory action research (PAR) methodology, characterised by iterative planning–action–reflection cycles, was coupled with a new concept of field-based farmer learning centres to build adaptive capacity of smallholder farmers to climate change. The study was part of a University of Zimbabwe—led project supported under the Climate Change Adaptation in Africa (CCAA) programme to explore the state of resilience in African smallholder farming. The PAR and learning centre processes enabled communities, local leaders, and extension agents and researchers to establish the, hither to, imperceptible link between poor soil fertility and rising institutional challenges within communities. Institutional conflicts related to land tenure and sharecropping arrangements between migrant farmers and native landowners were addressed in Ghana, while local institutions supporting traditional social safety net mechanisms were revitalized in Zimbabwe. In both cases, it was apparent that farmers faced multiple stresses, at the core of which were poor and declining soil fertility and weakening local institutions. The worsening rainfall distribution and increasing cases of drought are broadening the scope for vulnerability, often driving competing claims and conflicts. PAR was successfully used as an entry point, empowering communities to self-mobilize and self-organize to co-learn and experiment with integrated soil fertility management (ISFM) technologies and other improved farming practices. They realised opportunities for achieving high crop yields and generate surpluses in good years. Strengthening local institutional capacity to revitalise community safety nets proved an essential ingredient for enhancing adaptive capacity of smallholders to climatic shocks. The PAR process was a major driver of effective partnerships among community members, extension, policy makers and researchers, but ensuing success generated a new set of social challenges that could not be addressed within the short timescale of the project. We conclude that PAR was a suitable mechanism for supporting self-organization and co-learning processes among smallholder farmers and their service providers, enabling them to use ISFM technologies and strengthen their local institutions around natural resource management. This revealed the scope for building adaptive capacity of these communities against climate change and variability.

Soils, science and the politics of knowledge: How African smallholder farmers are framed and situated in the global debates on integrated soil fertility management

The paper addresses an important and often overlooked cultural aspect of smallholder agriculture in sub-Saharan Africa (SSA). This relates to how different policy organisations conceptualise soil management problem, its causes and solutions and how these framings intersect with, and incorporate smallholders’ indigenous knowledge. The article provides a brief review of the positionality of modernists and post-modernists on knowledge production and the politics which the process entails. Considering the ideology of some continental and global initiatives on integrated soil fertility management (ISFM), the paper identifies and addresses institutional framings of soil fertility problem in SSA. It also analyses the political economy [and ecology] of soil management in SSA; and investigates how farmers’ knowledge are incorporated into ISFM in the sub-continent. Drawing from some empirical evidences, the paper suggests that there is need for an economically viable and socio-culturally acceptable framework for the integration of both western and local knowledge in ISFM.

Failing to Yield? Ploughs, Conservation Agriculture and the Problem of Agricultural Intensification: An Example from the Zambezi Valley, Zimbabwe

Agricultural intensification, or increasing yield, has been a persistent theme in policy interventions in African smallholder agriculture. This article focuses on two hegemonic policy models of such intensification: (1) the ‘Alvord model’ of plough-based, integrated crop-livestock farming promoted in colonial Zimbabwe; and (2) minimum-tillage mulch-based, Conservation Agriculture, as currently preached by a wide range of international agricultural research and development agencies. An analysis of smallholder farming practices in Zimbabwe's Zambezi Valley, reveals the limited inherent understanding of farmer practices in these models. It shows why many smallholder farmers in southern Africa are predisposed towards extensification rather than intensification, and suggests that widespread Conservation Agriculture adoption is unlikely.

Oviposition Induced Volatile Emissions from African Smallholder Farmers’ Maize Varieties

Maize (corn), Zea mays, is a genetically diverse crop, and we have recently shown that certain open pollinated varieties (OPVs) of Latin American origin possess a trait not present in mainstream commercial varieties: they produce volatiles in response to stemborer oviposition that are attractive to stemborer parasitoids. Here, we tested whether a similar tritrophic effect occurs in the African OPVs 'Nyamula' and 'Jowi'. Herbivore induced plant volatiles (HIPVs) were collected from plants exposed to egg deposition by the stemborer Chilo partellus. In a four-arm olfactometer bioassay, the parasitic wasp Cotesia sesamiae preferred samples containing HIPVs from plants with eggs to samples collected from plants without eggs. EAG-active compounds, including (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), were released in higher amounts from the egg induced headspace samples. Our results suggest that this oviposition trait is not limited to S. American Z. mays germplasm, and that it could be used to increase indirect defense against attack by stemborers.

A research agenda to explore the role of conservation agriculture in African smallholder farming systems

Controversy surrounds the promotion of conservation agriculture (CA) in smallholder farming systems in sub-Saharan Africa. The introduction of CA is a profound change in farm management. Benefits in reduced erosion and stabilized crop production may be obtained, but technical performance at field level is but one of the determinants of adoption. For various reasons, all of the CA principles are not always fully implemented by farmers and results not as favourable as expected. As with other approaches to increasing agricultural productivity, the production constraints, farmers’ objectives, and the expected benefits and costs of implementing CA are important aspects that influence adoption. At farm and village levels, trade-offs in the allocation of resources become important in determining how CA may fit into a given farming system. At a regional level, factors such as the market conditions, interactions among stakeholders and other institutional and political dimensions become important. At each level, opportunities or difficulties emerge that enhance or impede development, adaptation and adoption of CA. The ex-ante identification of situations for where CA (and which form of CA) is appropriate demands research from a multi-stakeholder, multi-level, and interdisciplinary perspective. Recommendations are made where research is required to address key knowledge gaps.

RNA interference as a resistance mechanism against crop parasites in Africa: a ‘Trojan horse’ approach

Biological crop pests cause serious economic losses. In Africa, the most prevalent parasites are insect pests, plant pathogenic root-knot nematodes, viruses and parasitic plants. African smallholder farmers struggle to overcome these parasitic constraints to agricultural production. Crop losses and the host range of these parasites have continued to increase in spite of the use of widely advocated control methods. A sustainable method to overcome biological pests in Africa would be to develop crop germplasm resistant to parasites. This is achievable using either genetic modification (GM) or a non-GM approach. However, there is a paucity of resistant genes available for introduction. Additionally, the biological processes underpinning host parasite resistance are not sufficiently well understood. The authors review a technology platform for using RNA-mediated interference (RNAi) as bioengineered resistance to important crop parasites in Africa. To achieve acquired resistance, a host crop is stably transformed with a transgene that encodes a hairpin RNA targeting essential parasitic genes. The RNAi sequence is chosen in such a way that it shares no homology with the host's genes, so it remains 'inactive' until parasitism. Upon parasitism, the RNAi sequence enters the parasite and post-transcriptional gene silencing (PTGS) mechanisms are activated, leading to the death of the parasite.

Isoschaftoside, a C-glycosylflavonoid from Desmodium uncinatum root exudate, is an allelochemical against the development of Striga

In East African small-holder farming of maize, the cattle forage legume, Desmodium uncinatum is used as an intercrop due to its allelopathic inhibition of parasitism by Striga hermonthica, an obligate parasitic weed that can devastate the maize crop. Bioassay-guided fractionation of the root extract of D. uncinatum revealed isoschaftoside to be the main compound in the most potent fraction inhibiting growth of germinated S. hermonthica radicles. Bioassays repeated with isoschaftoside isolated from a different plant source, Passiflora incarnata, proved it to be a biologically active component. Analysis of the root exudates produced by hydroponically grown D. uncinatum showed isoschaftoside to be present in the hydroponic media at biologically active concentrations of 10–100 nM.

Videos Bridging Asia and Africa: Overcoming Cultural and Institutional Barriers in Technology-Mediated Rural Learning

Will African farmers watch and learn from videos featuring farmers in Bangladesh? Learning videos on rice seed management were made with rural women in Bangladesh. By using a new approach, called zooming-in, zooming-out, the videos were of regional relevance and locally appropriate. When the Africa Rice Center (AfricaRice) introduced them to Africa in 2005, many scientists and civil servants thought that the videos would be culturally inappropriate and hence irrelevant to African smallholder farmers. However, African farmers who watched the videos did not experience cultural barriers. In fact, they enjoyed seeing another part of the world where farmers faced similar problems as theirs, and were able to solve these problems by themselves. To promote wider uptake of the videos a key challenge was to overcome the institutional barriers. Public sector agencies especially had to be convinced that African smallholders appreciated Asian videos. By watching the videos with farmers and listening to their feedback, the skeptical scientists and service providers changed their minds. By 2009, the Bangladeshi rice seed videos had been translated into 20 African languages and so became national products. Videos made according to the zooming-in zooming-out approach can bring farmer-to-farmer extension to a higher level of social aggregation.

Linking smallholder agriculture and water to household food security and nutrition

Promoting household food security and reducing malnutrition rates of a growing population with the same amount of water is a challenge facing South African nutritionists and agriculturalists alike. Apart from non-food related effects of agriculture in general, the crop and livestock production practices of the South African smallholder farmer may have nutritional implications, primarily when practised on residential land and resulting in home consumption. Yet, few studies have systematically investigated the impact thereof. It appears that crop diversification, gender issues and nutrition education are among the important factors that strengthen the link between agriculture and nutrition. Since food production is the most water-intensive activity in society, nutritional water productivity (i.e. nutrition per volume water) of foods and the nutritional water footprint of diets should be investigated in order to achieve a sustainable solution. This implies that both the demand for a diet consisting predominantly of water-productive plant products, as well as the supply thereof, be addressed.

Groundnuts as ‘economic crop’ or ‘wife of the home’ in Northern Nyasaland

This paper examines points during the 1930s in which the colonial state in Nyasaland attempted and failed to bring groundnuts more into the colonial export economy. Nyasaland colonial officials, the Department of Agriculture, European export companies and the British Colonial Office attempted to establish the groundnut as an ‘economic crop’ for African smallholder farmers in the Northern Province of Nyasaland in the 1930s. Their failure was in part due to competing and conflicting interests: payment of hut taxes, reduction of millet production, improvement of food security, payment of railway costs, and reduction of migration. Farmers actively resisted colonial efforts to sell groundnuts to European buyers. The paper addresses the question: how can we understand the nature of colonial state power in relation to Nyasaland peasant agricultural practices in the 1930s? I argue that conflicting interests within the colonial state, as well as external constraints led to efforts to both stabilize and exploit the Nyasaland farmer in the Northern Province. These competing agendas helped lead to a failed effort at groundnut promotion. Colonial officials' actions were linked to ideas about gender, ethnicity and migration. Lack of colonial scientific knowledge about groundnuts, including their gendered role in the local food system contributed to the failure. The focus on groundnuts is a lens through which to understand the nature of colonial power in Nyasaland and the role of agricultural science in the colonial state. The paper contributes to broader discussions about multiple historical geographies of colonialism, the nature of African colonial states, and the relationship of African farmers to colonial states.

How do small-holder farmers understand insurance, and how much do they want it? Evidence from Africa

Innovative micro-insurance schemes can be one tool to help African small-holder farmers cope with climate variability and change. A critical challenge for implementing such risk transfer programs, however, is helping participants understand how insurance operates, and there is evidence that farmers with a poor understanding of insurance are less likely to use it. One of the proposed tools to help farmers understand insurance is a simulation game, through which farmers can gain first-hand experience with a functioning insurance market. This paper reports on the results of experiments in Ethiopia and Malawi, investigating farmers’ understanding and the effectiveness of a role-playing game at improving that understanding. Our results suggest a generally poor understanding of basic insurance concepts, and are consistent with past results in suggesting that better understanding correlates with greater willingness to purchase it. Our results also suggest that role-playing games may be an important tool for improving understanding, but that they do not necessarily out-perform more conventional training practices.

Identifying key entry-points for strategic management of smallholder farming systems in sub-Saharan Africa using the dynamic farm-scale simulation model NUANCES-FARMSIM

African smallholder farming systems are complex, dynamic systems with many interacting biophysical subcomponents. In these systems the major inputs and outputs are managed by human agency – the farmers. To analyse potential developmental pathways of smallholder farming systems in sub-Saharan Africa (SSA), we recognised the need for a tool that can capture the effects and consequences of decision-making on the use of resources. Here we describe and apply such a new modelling tool, developed within the NUANCES framework (Nutrient Use in ANimal and Cropping systems: Efficiencies and Scales), called NUANCES-FARMSIM (FARM SIMulator), an integrated crop – livestock model developed to analyse African smallholder farm systems. NUANCES-FARMSIM was used to analyse a representative case study farm in the highlands of Western Kenya, a site for which each of the components of FARMSIM has been thoroughly tested. We present the results of a sensitivity analysis which showed the model to be sufficiently robust to identify key management options that explain most of the variability in farm productivity, and the long-term consequences of these options for the case study farm. The analyses showed clearly that the most important decisions are those related to the interactions between the different components of the farm and therefore justify the need of integrating crop and livestock components within one modelling tool. The allocation of limited resources across the farm, and the way organic matter is recycled or redistributed within the farm determines the long-term production capacity of the system. The results of the sensitivity analyses further showed that for the case study farm in Western Kenya a strong focus on improving the reliability of the subsystem level or process descriptions will only result in minor improvement in simulating productivity at farm level.

An integrated evaluation of strategies for enhancing productivity and profitability of resource-constrained smallholder farms in Zimbabwe

In African smallholder agriculture, improved farm-scale understanding of the interaction between the household, crops, soils and livestock is required to develop appropriate strategies for improving productivity. A combination of models was used to analyse land use and labour allocation strategies for optimizing income for wealthy (2.5 ha with eight cattle) and poor (0.9 ha without cattle) farms in Murewa, Zimbabwe. Trade-offs between profitability, labour use and partial nutrient balances were also evaluated for alternative resource management strategies. Farm data were captured using the Integrated Modelling Platform for Mixed Animal-Crop Systems (IMPACT), which was directly linked to the Household Resource use Optimization Model (HROM). HROM was applied to optimize net cash income within the constraints specific to the households. Effects of alternative nutrient resource management strategies in crop and milk production were simulated using the Agricultural Production Systems Simulator (APSIM) and RUMINANT models, respectively, and the output evaluated using HROM. The poor farm had a net income of US$ 1 yr −1 and the farmer relied on selling unskilled labour to supplement her income. The poor farm’s income was marginally increased by US$18 yr −1 and the soil nitrogen (N) balance was increased from 6 to 9 kg ha −1 yr −1 by expanding groundnut production from the previous 5–25% of the land area. Further increases in area allocated to groundnut production were constrained by lack of labour. On the poor farm, maize production was most profitable when cultivated on a reduced land area with optimal weeding. The wealthy farm had a maize-dominated cropping system that yielded a net cash balance of US$290 yr −1, mainly from the sale of crop produce. Net income could be increased to US$1175 yr −1, by re-allocating the 240 hired labour-days more efficiently, although this reallocation substantially reduced partial soil N and phosphorus (P) balances by 74 kg N ha −1 and 11 kg P ha −1, respectively, resulting in negative nutrient balances. Few opportunities existed to increase productivity and income of the smallholder farms without inducing negative nutrient balances. On the wealthy farm, groundnut was the least profitable crop; shifting its production to the most fertile field did not improve income unless the groundnut residues were fed to lactating cows. The analysis carried out in this paper highlights the need to develop practical technological recommendations and development interventions that consider farm resource endowment (land, fertilizers, manure and labour), variability in soil fertility within farms and competing resource use options.

Structure and Dynamics of Livestock Marketing in Rural Uganda: Constraints and Prospects for Research and Development

ABSTRACT The projected global expansion in consumption of meat and other livestock products potentially offers sub-Saharan African small-holder farmers opportunities to escape from the poverty trap. A necessary condition for exploiting this potential is the establishment of marketing systems that provide farmers with reasonable incentives to participate in the market. In this study, the functioning of livestock markets in rural Uganda is analyzed based on a survey of 401 livestock keepers, complimented with focus group discussions with livestock traders and policy planners. The first part is dedicated to the empirical analysis of key organizational and institutional arrangements. Constraints along the marketing chain as well as potential institutional solutions are illuminated. The second part is dedicated to propositions of new areas where more work and new results are needed to improve the functioning of the livestock marketing chain. Empirical findings indicated the following institutional constraints: a poor market information system, lack of grades and standards, lack of trade finance, poor contract enforcement and dispute settlement, disorganized actors, high transaction risks, and poorly developed marketing infrastructure. The study recommends a structured approach to livestock marketing, market infrastructure development, and emphasis on arbitration systems as specific ways of improving the efficiency of livestock marketing in Uganda and other developing countries.

Yield gaps, nutrient use efficiencies and response to fertilisers by maize across heterogeneous smallholder farms of western Kenya

The need to promote fertiliser use by African smallholder farmers to counteract the current decline in per capita food production is widely recognised. But soil heterogeneity results in variable responses of crops to fertilisers within single farms. We used existing databases on maize production under farmer (F-M) and researcher management (R-M) to analyse the effect of soil heterogeneity on the different components of nutrient use efficiency by maize growing on smallholder farms in western Kenya: nutrient availability, capture and conversion efficiencies and crop biomass partitioning. Subsequently, we used the simple model QUEFTS to calculate nutrient recovery efficiencies from the R-M plots and to calculate attainable yields with and without fertilisers based on measured soil properties across heterogeneous farms. The yield gap of maize between F-M and R-M varied from 0.5 to 3 t grain ha−1 season−1 across field types and localities. Poor fields under R-M yielded better than F-M, even without fertilisers. Such differences, of up to 1.1 t ha−1 greater yields under R-M conditions are attributable to improved agronomic management and germplasm. The relative response of maize to N–P–K fertilisers tended to decrease with increasing soil quality (soil C and extractable P), from a maximum of 4.4-fold to −0.5-fold relative to the control. Soil heterogeneity affected resource use efficiencies mainly through effects on the efficiency of resource capture. Apparent recovery efficiencies varied between 0 and 70% for N, 0 and 15% for P, and 0 to 52% for K. Resource conversion efficiencies were less variable across fields and localities, with average values of 97 kg DM kg−1 N, 558 kg DM kg−1 P and 111 kg DM kg−1 K taken up. Using measured soil chemical properties QUEFTS over-estimated observed yields under F-M, indicating that variable crop performance within and across farms cannot be ascribed solely to soil nutrient availability. For the R-M plots QUEFTS predicted positive crop responses to application of 30 kg P ha−1 and 30 kg P ha−1 + 90 kg N ha−1 for a wide range of soil qualities, indicating that there is room to improve current crop productivity through fertiliser use. To ensure their efficient use in sub-Saharan Africa mineral fertilisers should be: (1) targeted to specific niches of soil fertility within heterogeneous farms; and (2) go hand-in-hand with the implementation of agronomic measures to improve their capture and utilisation.

Multiple benefits of manure: The key to maintenance of soil fertility and restoration of depleted sandy soils on African smallholder farms

Manure is a key nutrient resource on smallholder farms in the tropics, especially on poorly buffered sandy soils, due to its multiple benefits for soil fertility. Farmers preferentially apply manure to fields closest to homesteads (homefields), which are more fertile than fields further away (outfields). A three-year experiment was established on homefields and outfields on sandy and clayey soils to assess the effects of mineral nitrogen (N) fertilizer application in combination with manure or mineral phosphorus (P) on maize yields and soil chemical properties. Significant maize responses to application of N and manure were observed on all fields except the depleted sandy outfield. Large amounts of manure (17 t ha−1 year−1) were required to significantly increase soil organic carbon (SOC), pH, available P, and base saturation, and restore productivity of the depleted sandy outfield. Sole N as ammonium nitrate (100 kg N ha−1) or in combination with single superphosphate led to acidification of the sandy soils, with a decrease of up to 0.8 pH units after three seasons. In a greenhouse experiment, N and calcium (Ca) were identified as deficient in the sandy homefield, while N, P, Ca, and zinc (Zn) were deficient or low on the sandy outfield. The deficiencies of Ca and Zn were alleviated by the addition of manure. This study highlights the essential role of manure in sustaining and replenishing soil fertility on smallholder farms through its multiple effects, although it should be used in combination with N mineral fertilizers due to its low capacity to supply N.

Variable grain legume yields, responses to phosphorus and rotational effects on maize across soil fertility gradients on African smallholder farms

Promiscuous soyabean varieties have potential to contribute significantly to income generation, food security and soil N budgets on smallholder farms. One of the major factors limiting this potential is farmers’ preference to allocate nutrient resources to food security cereal crops on the most fertile fields, leaving grain legumes to grow on residual fertility on infertile fields. Two experiments were conducted to: (i) compare the current farmer practice with targeting manure and single super phosphate (SSP) to soyabean in a three-year rotation cycle on two fields with different soil fertility: an infertile sandy soil and a more fertile clay soil; and (ii) assess the effects of variability of soil fertility within and across farms on productivity of soyabean and groundnut. In the first experiment, soyabean (<0.2 t ha−1) and maize yields (<0.7 t ha−1) without fertilizer were poor on a degraded sandy soil. Both crops responded poorly to SSP due to deficiency of other nutrients. Manure application significantly increased soyabean and maize yields, led to yield stabilization over three seasons and also significantly increased the proportion of N2 fixed by soyabean (measured using 15N natural abundance) from 60% to 83%. On the sandy soil, P was used more efficiently and gross margins were greater when SSP and manure were applied to maize in a maize–soyabean rotation. Soyabean and maize yields without fertilizer inputs were larger on clay soil with moderate fertility (0.4–0.7 t ha−1 and 2.0–2.3 t ha−1 respectively) and were significantly increased by application of SSP and manure. Within rotations, P recovery was higher when manure and SSP were applied to maize (43 and 25%) than when applied to soyabean (20 and 19%). However, application of manure to soyabean on the clay was more profitable than application to maize for individual crops and within rotations. In the second experiment, soyabean and groundnut yields were largest (∼1 and ∼0.8 t ha−1 respectively) on plots closest to homesteads on wealthy farms, which were more fertile due to good past management. Yields were poor (< 0.5 t ha−1) on other fields which previously had received little nutrient inputs. Soyabean and groundnut yields correlated well with available P (R 2 = 0.5–0.7) and soil organic C (SOC) contents (R 2 = 0.4–0.6). For smallholder farmers to maximise benefits from legume production they need to focus attention on the more fertile plots, although production should be optimized in relation to maize. Targeting nutrients to maize as currently practiced by farmers was more efficient and economic under poor soil fertility conditions, whilst potential exists to increase income by targeting manure to soyabean on the more fertile soils.

Niche-based assessment of contributions of legumes to the nitrogen economy of Western Kenya smallholder farms

Nitrogen (N) deficiency is a major constraint to the productivity of the African smallholder farming systems. Grain, green manure and forage legumes have the potential to improve the soil N fertility of smallholder farming systems through biological N2-fixation. The N2-fixation of bean (Phaseolus vulgaris), soyabean (Glycine max), groundnut (Arachis hypogaea), Lima bean (Phaseolus lunatus), lablab (Lablab purpureus), velvet bean (Mucuna pruriens), crotalaria (Crotalaria ochroleuca), jackbean (Canavalia ensiformis), desmodium (Desmodium uncinatum), stylo (Stylosanthes guianensis) and siratro (Macroptilium atropurpureum) was assessed using the 15N natural abundance method. The experiments were conducted at three sites in western Kenya, selected on an agro-ecological zone (AEZ) gradient defined by rainfall. On a relative scale, Museno represents high potential AEZ 1, Majengo medium potential AEZ 2 and Ndori low potential AEZ 3. Rainfall in the year of experimentation was highest in AEZ 2, followed by AEZ 1 and AEZ 3. Experimental fields were classified into high, medium and low fertility classes, to assess the influence of soil fertility on N2-fixation performance. The legumes were planted with triple super phosphate (TSP) at 30 kg P ha−1, with an extra soyabean plot planted without TSP (soyabean-P), to assess response to P, and no artificial inoculation was done. Legume grain yield, shoot N accumulation, %N derived from N2-fixation, N2-fixation and net N inputs differed significantly (P<0.01) with rainfall and soil fertility. Mean grain yield ranged from 0.86 Mg ha−1, in AEZ 2, to 0.30 Mg ha−1, in AEZ 3, and from 0.78 Mg ha−1, in the high fertility field, to 0.48 Mg ha−1, in the low fertility field. Shoot N accumulation ranged from a maximum of 486 kg N ha−1 in AEZ 2, to a minimum of 10 kg N ha−1 in AEZ 3. Based on shoot biomass estimates, the species fixed 25–90% of their N requirements in AEZ 2, 23–90% in AEZ 1, and 7–77% in AEZ 3. Mean N2-fixation by green manure legumes ranged from 319 kg ha−1 (velvet bean) in AEZ 2 to 29 kg ha−1 (jackbean) in AEZ 3. For the forage legumes, mean N2-fixation ranged from 97 kg N ha−1 for desmodium in AEZ 2 to 39 kg N ha−1 for siratro in AEZ 3, while for the grain legumes, the range was from 172 kg N ha−1 for lablab in AEZ 1 to 3 kg N ha−1 for soyabean-P in AEZ 3. Lablab and groundnut showed consistently greater N2-fixation and net N inputs across agro-ecological and soil fertility gradients. The use of maize as reference crop resulted in lower N2-fixation values than when broad-leaved weed plants were used. The results demonstrate differential contributions of the green manure, forage and grain legume species to soil fertility improvement in different biophysical niches in smallholder farming systems and suggest that appropriate selection is needed to match species with the niches and farmers’ needs.

Soil type, management history and current resource allocation: Three dimensions regulating variability in crop productivity on African smallholder farms

Soil fertility varies markedly within and between African smallholder farms, both as a consequence of inherent factors and differential management. Fields closest to homesteads (homefields) typically receive most nutrients and are more fertile than outlying fields (outfields), with implications for crop production and nutrient use efficiencies. Maize yields following application of 100 kg N ha −1 and different rates and sources of P were assessed on homefields and outfields of smallholder farms in Zimbabwe. Soil organic carbon, available P and exchangeable bases were greater on the homefields than outfields. In each of three experimental seasons, maize yields in homefield control plots were greater than in the outfields of farms on a granitic sandy and a red-clay soil. Application of mineral N significantly increased maize yields on homefields in the first season (2.1–3.0 t ha −1 on the clay soil and 1.0–1.5 t ha −1 on the sandy soil) but the effects of N alone were not significant on the outfields due to other yield-limiting factors. Greatest yields of about 6 t ha −1 were achieved on the clayey homefield with 100 kg N ha −1 and 30 kg P ha −1 applied as single super phosphate (SSP). Manure application gave greater yields (3–4 t ha −1) than SSP (2–3 t ha −1) in the sandy homefield and in the clayey outfield. Maize did not respond significantly to N, dolomitic lime, manure and P on the sandy outfield in the first and second seasons. In the third season, manure application (∼17 t manure ha −1 year −1) on the sandy outfield did result in a significant response in grain yields. Apparent P recovery in the first season was 55–65% when P was applied at 10 kg ha −1 on the clayey homefield (SSP), clayey outfield (SSP and manure) and sandy homefield (manure) with apparent P recovery less than 40% when P was applied at 30 kg ha −1. On the sandy outfield, P recovery was initially poor (<20%), but increased in the successive seasons with manure application. In a second experiment, less than 60 kg N ha −1 was required to attain at least 90% of the maximum yields of 2–3 t ha −1 on the sandy homefield and clayey outfield. N use efficiency varied from >50 kg grain kg −1 N on the infields, to less than 5 kg grain kg −1 N on the sandy outfields. Apparent N recovery efficiency by maize was greatest at small N application rates with P applied. We conclude that blanket fertilizer recommendations are of limited relevance for heterogeneous smallholder farms. Targeted application of mineral fertilizers and manure according to soil type and past management of fields is imperative for improving crop yields and nutrient use efficiencies.