Issue 9, October 2012

Increasing productivity with ISFM
Climate-smart systems
Soil: nature’s Pandora’s box
Bridging yield gap
Best practices in maize
ISFM for bananas
NRM in cassava and yam
Boosting productivity of cassava
Cocoa and REDD
CRPs and NRM
Commercial products for farmers

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“The soil nutrient losses in sub-Saharan Africa are an environmental, social, and political time bomb. Unless we wake up soon and reverse these disastrous trends, the future viability of African food systems will indeed be imperiled.”
– Dr Norman Borlaug, 14 March 2003, Muscle Shoals, Alabama, USA

IITA was the first major African link in the integrated network of international agricultural research centers. It was also one of the first centers that engaged in  farming systems research. In the 1980s and 1990s, the Institute had a very strong program on natural resource management (NRM), covering aspects of soil fertility management, cropping system diversification, and improved agronomy. This, along with the emphasis on the genetic improvement of the major food crops in the humid tropics, provided an integrated program of research on sustainable agricultural development.

Over the past fifteen years, the focus of research-for-development activities at IITA shifted away from NRM, party driven by changes in the investment portfolios of important donors. With the area of soils and natural resources back on top of the development agenda and recognizing that the potential of improved germplasm can only be realized in the presence of appropriate crop and nutrient management practices, IITA has recently decided to increase its investments on NRM research for development with a particular focus on soils.

The March 2012 issue of R4D Review commemorated IITA’s 45 years. It focused on the successes, challenges, and prospects of the genetic improvement programs; these are key to the Institute’s success in improving food crop production in sub-Saharan Africa. Innovations in genetic improvement have shown how enhanced crop productivity, along with other ingredients, such as capacity building and policies, has helped to lift millions out of poverty.

This second issue for the year highlights our important work undertaken in partnership with national and international institutions in the area of sustainable NRM in sub-Saharan Africa. It also signals IITA’s renewed focus on this area of research. The articles cover the three main pillars of the NRM research-for-development agenda: (1) Integrated Soil Fertility Management, aiming at enhancing crop productivity following agroecological principles, with a livelihood focus, (2) Sustainable Land Management, aiming at rehabilitating soils for the provision of other essential ecosystem services, with a landscape focus, and (3) Climate Change, aiming at enhancing the resilience of farming systems to climate variability.

Increasing productivity the ISFM way

Farm productivity has been cited as a major entry point to achieving success in overcoming rural poverty. Photo by IITA
Farm productivity has been cited as a major entry point to achieving success in overcoming rural poverty. Photo by IITA

Bernard Vanlauwe,

The need to grow more food without depleting important natural resources makes the intensification of African agriculture essential. The Green Revolution in South Asia and Latin America raised crop productivity through the deployment of improved varieties, water, and fertilizer. However, efforts to achieve similar results in sub-Saharan Africa (SSA) have largely failed. The sustainable intensification of agriculture in SSA has gained support in recent years, especially in densely populated areas where natural fallows are no longer an option.

There is growing recognition that farm productivity is a major entry point to achieving success in overcoming rural poverty. A recent landmark event was the launching of the Alliance for a Green Revolution in Africa (AGRA). AGRA has adopted integrated soil fertility management (ISFM) as a framework for raising crop productivity through a reliance on soil fertility management technologies, with an emphasis on the increased availability and use of mineral fertilizer ( Within the refreshed IITA Strategy 2012–2020, ISFM is one of the main pillars of the natural resource management (NRM) research area.

Figure 1. Conceptual relationship between agronomic efficiency of fertilizers and organic resource and implementation of various ISFM components.
Figure 1. Conceptual relationship between agronomic efficiency of fertilizers and organic resource and implementation of various ISFM components.

Whats is ISFM?
We defined ISFM as “A set of soil fertility management practices that necessarily include the use of fertilizer, organic inputs, and improved germplasm combined with the knowledge on how to adapt these practices to local conditions, aiming at maximizing agronomic use efficiency of the applied nutrients and improving crop productivity. All inputs need to be managed following sound agronomic principles” (Vanlauwe et al. 2011a). The definition focuses on maximizing the efficiency with which fertilizer and organic inputs are used since these are both scarce resources in the areas where agricultural intensification is needed. Agronomic efficiency (AE) is defined as the extra crop yield obtained per unit of nutrient applied and is expressed in kg crop produced per kg nutrient input.

Fertilizer and improved germplasm
In terms of response to management, two general classes of soils are distinguished: responsive soils, i.e., soils that show acceptable responses to fertilizer (Path A, Fig. 1), and poor, less-responsive soils that show little or no response to fertilizer due to constraints apart from the nutrients contained in the fertilizer (Path B, Fig. 1). Sometimes, where land is newly cleared or where fields are close to homesteads and receive large amounts of organic inputs each year, a third class exists where crops show little response to fertilizer since the soils are fertile.

The ISFM definition proposes that the application of fertilizer to improved germplasm on responsive soils will raise crop yield and improve AE relative to the current farmers’ practice. This is characterized by traditional varieties receiving poorly managed nutrient inputs and/or too little of them (Path A, Fig. 1). Major requirements for achieving production gains on responsive fields within Path A (Fig. 1) include the following: the use of disease- resistant and improved germplasm, crop and water management practices, and the application of the “4R” Nutrient Stewardship Framework—a science-based framework that focuses on applying the right fertilizer source at the right rate, at the right time during the growing season, and in the right place (Fig. 2). Poor, less-responsive soils should be avoided when deploying improved germplasm and fertilizer.

Figure 2. The 4R Nutrient Stewardship model, International Plant Nutrition Institute.
Figure 2. The 4R Nutrient Stewardship model, International Plant Nutrition Institute.

Combined application of fertilizer and organic inputs
Organic inputs contain nutrients that are released at a rate determined in part by their chemical characteristics or organic resource quality. However, organic inputs applied at realistic rates seldom release sufficient nutrients for optimum crop yield. Combining organic and mineral inputs has been advocated for smallholder farming in the tropics because neither input is usually available in sufficient quantities to maximize yields and because both are needed in the long term to sustain soil fertility and crop production. Substantial enhancements in fertilizer AE have been observed in an analysis related to N fertilizer applied to maize in Africa, but these were strongly influenced by the quality and application rate of the organic resources (Fig. 3).

An important question arises within the context of ISFM: Can organic resources be used to rehabilitate less-responsive soils and make these responsive to fertilizer (Path C in Fig. 1)? In southwestern Nigeria, the integration of residues from Siamese senna (Senna siamea), a leguminous tree, reduced topsoil acidification resulting from repeated applications of urea fertilizer (Vanlauwe et al. 2005).

Figure 3. Agronomic efficiency of fertilizer N as affected by combination with different classes of organic inputs.
Figure 3. Agronomic efficiency of fertilizer N as affected by combination with different classes of organic inputs.

Adaptation to local conditions
As previously stated, soil fertility status can vary considerably between fields within a single farm and between farms with substantial impacts on fertilizer-use efficiency (see photo on next page). In addition to adjustments to fertilizer and organic input management, measures with adaptation to local conditions are needed, such as the application of lime on acid soils, water harvesting techniques on soils susceptible to crusting under semi-arid conditions, or soil erosion control on hillsides, to address other constraints. Lastly, adaptation also includes considering the farming resources available to a specific farming household, often referred to as the farmer’s resource endowment, the status of which is related to a specific set of farm typologies. ISFM options available to a specific household will depend on the resource endowment of that household.

Towards complete ISFM
Complete ISFM comprises the use of improved germplasm, fertilizer, appropriate organic resource management, and local adaptation. Several intermediate phases have been identified that assist farmers in moving towards complete ISFM, starting from the current average practice of applying 8 kg/ha of fertilizer nutrients to local varieties. Each step is expected to provide the management skills that result in improvements in yield and in AE, with technological complexity increasing towards the right (Fig. 1). Figure 1 is not intended to prioritize interventions but rather suggests a stepwise adoption of the elements of complete ISFM. It does, however, depict key components that lead to better soil fertility management. In areas, for instance, where farmyard manure is targeted towards specific fields within a farm, local adaptation is already taking place, even if no fertilizer is used. This is the situation in much of Central Africa.

A 3-week-old maize crop in two different plots within the same farm, Western Kenya
A 3-week-old maize crop in two different plots within the same farm, Western Kenya

Successful uptake of ISFM practices
Several examples can be identified where ISFM has made a difference to smallholder farmers, including (1) dual-purpose grain legume–maize rotations with targeted fertilizer applications pioneered by IITA for the moist savannas (Sanginga et al. 2003) and (2) micro-dose fertilizer applications in legume–sorghum or legume–millet rotations with the retention of crop residues and combined with water harvesting techniques in the semi-arid agroecozone (Tabo et al. 2007).

As for the grain legume–maize rotations, the application of appropriate amounts mainly of P to the legume phase ensures good grain and biomass production. The latter in turn benefits a subsequent maize crop and thus reduces the need for external N fertilizer. Choosing an appropriate legume germplasm with a low harvest index will favor the accumulation of organic matter and N in the plant parts not harvested and choosing adapted maize germplasm will favor a matching demand for nutrients by the maize. Selection of fertilizer application rates based on local knowledge of the initial soil fertility status within these systems would qualify the soil management practices as complete ISFM.

In view of the many ongoing investments related to the dissemination of ISFM practices, it is expected that the examples of successful uptake will be amplified over large areas across various farming systems.

The principles underlying ISFM have also been observed to be applicable to cassava-based systems (see other articles in this publication). Notwithstanding the good prospects for impact generated through improved soil management, several technical issues remain to be resolved. These include (1) how farmers can diagnose the soil fertility status of their plots, including non-responsiveness, (2) how ISFM recommendations vary along such within-farm soil fertility gradients, (3) how non-responsive soils can be rehabilitated (or does this not make sense under certain circumstances?), (4) what minimal level of resource endowment is required to engage in ISFM, (5) how ISFM principles can be condensed to a set of easy-to-implement rules of thumb, adapted to a specific cropping environment, (6) whether efficient fertilizer use is a valid entry point towards sustainable intensification, (7) whether ISFM produces sufficient in-situ crop residues to ensure that soil carbon values remain about a minimal threshold, (8) what minimal conditions are needed (e.g., population density, policy) to allow large-scale uptake of ISFM, and (9) how ISFM relates to conservation agriculture.

Sanginga, N., K. Dashiell, J. Diels, B. Vanlauwe, O. Lyasse, R.J. Carsky, S. Tarawali, B. Asafo-Adjei, A. Menkir, S. Schulz, B.B. Singh, D. Chikoye, D. Keatinge, and R. Ortiz. 2003. Sustainable resource management coupled to resilient germplasm to provide new intensive cereal–grain legume–livestock systems in the dry savanna. Agriculture, Ecosystems and Environment, 100: 305–314.
Tabo, R., A. Bationo, B. Gerard, J. Ndjeunga, D. Marchal, B. Amadou, G. Annou, D. Sogodogo, J.B.S. Taonda, O. Hassane, Maimouna K. Diallo, and S. Koala. 2007. Improving cereal productivity and farmers’ income using a strategic application of fertilizers in West Africa. Pages 201–208 in: Advances in integrated soil fertility management in sub-Saharan Africa: Challenges and opportunities, edited by A. Bationo, B. Waswa, J. Kihara, and J. Kimetu, J. Kluwer Publishers, The Netherlands.
Vanlauwe, B, J. Diels, N. Sanginga, and R. Merckx. 2005. Long-term integrated soil fertility management in south-western Nigeria: crop performance and impact on the soil fertility status. Plant and Soil 273: 337–354.
Vanlauwe, B, A. Bationo,  J. Chianu, K.E. Giller, R. Merckx U. Mokwunye, O. Ohiokpehai, P. Pypers, R. Tabo, K. Shepherd, E. Smaling, P.L. Woomer, and N. Sanginga. 2011a. Integrated soil fertility management: operational definition and consequences for implementation and dissemination. Outlook on Agriculture 39: 17–24.
Vanlauwe, B, J. Kihara, P. Chivenge, P. Pypers, R. Coe, and J. Six. 2011b. Agronomic use efficiency of N fertilizer in maize-based systems in sub-Saharan Africa within the context of Integrated Soil Fertility Management. Plant and Soil 339: 35–50.
Zingore, S. and A. Johnston. 2013. The 4R Nutrient Stewardship in the context of smallholder Agriculture in Africa, in: Agroecological Intensification of Farming Systems in the East and Central African Highlands, edited by B. Vanlauwe, G. Blomme, and P. Van Asten. Earthscan, UK, in press.

Climate-smart perennial systems

Banana-coffee systems in East Africa. Photo by P. van Asten
Banana-coffee systems in East Africa. Photo by P. van Asten

Laurence Jassogne,, Piet van Asten, Peter Laderach, Alessandro Craparo, Ibrahim Wanyama, Anaclet Nibasumba, and Charles Bielders

Coffee is a major cash crop in the East African highland farming systems. It represents a high proportion of export values at the national level (for example >20% for Uganda). It is also crucial for the sustainability of the livelihoods of smallholder farmers.

During a survey in Uganda, smallholder farmers explained that the income generated by coffee had sent their children to school and helped to build permanent houses. Prices of coffee have also been increasing in the past decennia, motivating them to continue growing the crop.

Although coffee is a promising cash crop, smallholder farmers that grow coffee are still vulnerable. Soil fertility is declining, pest and disease pressure is increasing, populations are rising, and land is continuously fragmented. Above all, climate change is starting to take its toll and puts further pressure on the coffee-based farming systems—directly, because temperature and rainfall have an impact on the physiology of Arabica coffee, and indirectly because the incidence and severity of certain pests and diseases such as the coffee berry borer and coffee leaf rust will increase.

Figure 1. Change of suitability for Arabica-growing areas in Uganda using MAXENT approach and based on a ‘business as usual’ climate change scenario.
Figure 1. Change of suitability for Arabica-growing areas in Uganda using MAXENT approach and based on a ‘business as usual’ climate change scenario.

Current and future suitability of coffee growing areas
In collaboration with Dr Peter Laderach (CIAT), the direct effect of climate change on the suitability of coffee-growing areas in Uganda was mapped (Fig. 1).

If the current coffee crop systems do not change (i.e., same coffee varieties and management practices), these areas will move up the slope and the suitable surface area will decrease. In this light, climate-smart coffee- based systems need to be developed to sustain the existing coffee- based systems.

Adaptation strategies in coffee systems
IITA-led field surveys in the region, combined with a literature review, revealed that there is a multitude of coffee systems that exist. This diversity reflects the variability among farmers in terms of their resource availability, objectives, political history, and opportunities (Fig. 2).

Highest yields can be obtained in systems without shade or with low shade levels (Fig. 2). However, these same systems represent higher production risks and a higher use of external inputs. In polyculture systems and forest systems, on the other hand, highest yield quality can be obtained with the minimum use of external inputs. Furthermore, they allow, among others, a better adaptation to climate change, higher carbon stocks, and more ecological services. Quantifying these trade-offs and raising awareness among farmers and other stakeholders along the coffee value chain will help informed and sustainable decisions to be made about the coffee systems.

Figure 2. Trade-offs at a farmer-plot level in coffee systems.
Figure 2. Trade-offs at a farmer-plot level in coffee systems.

The more coffee is shaded, the more it is protected from rising temperatures and extreme weather events. Shade in coffee systems can reduce the average temperature in the lower coffee canopy by a few degrees. Although shade is an interesting technology to make coffee systems “climate smart” and hence, adapted to climate change, it is not the primary reason why farmers add shade to their coffee. Shade plants often produce fruit and/or timber. This diversifies the income of the farmer.

The same happens when farmers intercrop coffee with banana. Adding banana to the system increases food security, diversifies income, and adds shading to coffee. A country-wide survey in Uganda showed that coffee/banana intercropping was a common cropping system except in North and North-West Uganda. The incidence of coffee leaf rust was 50% when coffee was intercropped with banana.

Most farmers have some shade trees in their coffee; many practice intercropping with common beans. The combination of short- and long-term benefits of such shade systems makes them ideal climate-smart candidates. Shade trees also sequester carbon, contributing to the mitigation of the effects of climate change. In the end, few farmers (<5%) have pure full-sun monocropped coffee.

Figure 3. Pie charts depict major nutrient deficiencies based on soil and foliar samples of 10 coffee farms per site (black dots).
Figure 3. Pie charts depict major nutrient deficiencies based on soil and foliar samples of 10 coffee farms per site (black dots).

Constraints in diversified systems
However, shade trees also compete with coffee for light, nutrients, and water. If this competition is not managed well, then the shaded coffee system risks collapse, especially in conditions of poor soil fertility. Due to increasing population pressure and land fragmentation, integrated soil fertility management (ISFM) will help to manage nutrient competition. In a shaded system, the turn-over of biomass contributes to nutrient recycling. Organic matter from shade trees or banana will act as in-situ mulch. However, soils in Uganda are poor and have some major nutrient deficiencies (Fig. 3).

Replenishing soil fertility by adding external inputs is necessary if farming systems need to be sustained. Adding small amounts of fertilizers adapted to site-specific deficiencies increases fertilizer use efficiency and forms part of the ISFM approach. Coffee can be a major driver for the adoption of fertilizers by smallholders since farmers are generally organized for access to output markets. The same organizational lines can then be used to provide access to input markets.

Understanding the farmers’ objectives, perceptions, and constraints is critical in identifying the adoption pathways of production-increasing technologies. To continue this research, IITA will start a case study in Rakai (Uganda) at a site of the CGIAR Research Program on Climate Change, Agriculture, and Food Security (CCAFS). Here, climate-smart coffee scenarios will be developed in a participatory manner with smallholder farmers, based on data from previous projects, interviews with individual farmers and groups, and farm measurements. Greenhouse gas emissions will be quantified to measure the mitigation potential of the existing coffee systems. Furthermore, fertilizer trials throughout Uganda will be set up to test the site-specific recommendations. IITA also plans to further advance its collaborative research efforts on modeling trade-offs and synergies in coffee smallholder systems in East Africa.

Bridging the grain legume yield gap through agronomy

Robert Abaidoo,, Steve Boahen, Anne Turner, and Mahamadi Dianda

Researcher inspecting cowpea pods. Photo by IITA

Researcher inspecting cowpea pods. Photo by IITA

IITA and its partners have made significant progress in breeding grain legumes that are high yielding and drought tolerant, and have better disease and pest resistance as well as consumer-preferred traits, such as seed size, texture, and color. The use of these new improved varieties has contributed to increases in productivity on farmers’ fields across sub-Saharan Africa.

While crop genetics is very important, the key to bridging the yield gap is to capitalize on the yield potential of a particular genotype and know how to manage it to maximize productivity in challenging environments. This is where the role of an agronomist becomes apparent: to design an integrated management system that reduces the effect of the biotic and abiotic stress factors limiting the productivity of a selected genotype in a given agroecology.Streaming and download Doctor Strange (2016)

Several collaborative projects, including N2Africa funded by the Bill & Melinda Gates Foundation through Wageningen University, are developing improved management options to enhance system productivity. The N2Africa project is being implemented in eight countries: DR Congo, Ghana, Kenya, Malawi, Mozambique, Nigeria, Rwanda, and Zimbabwe. It is a research-and-development partnership program that aims to develop, disseminate, and promote appropriate N2-fixation technologies for smallholder farmers, focusing on the major grain legumes. Although atmospheric air contains 78% N2, nitrogen (N) remains the most limiting nutrient for plant growth and also the most limited nutrient in degraded soils.

The good news is that legumes have the unique ability to fix atmospheric N through symbiotic association with root nodule bacteria. The opportunity exists through biological nitrogen fixation (BNF) to improve the yields of legumes in sub-Saharan Africa since current yields are only a small fraction of their potential. The integration of legumes in cropping systems can benefit associated cereal crops through N-sparing effects, N transfer, and non-N rotation effects. However, the process of BNF can be limited by several biotic and abiotic factors.

Enhancing biological N through bradyrhizobium inoculation and phosphorus application. Ino = inoculum, TSP = total super phosphate

Enhancing biological N through bradyrhizobium inoculation and phosphorus application. Ino = inoculum, TSP = total super phosphate

Evidence abounds that successful BNF depends on the interaction of environment (climate, rainfall day length, etc.), soil factors (acidity, aluminum toxicity, limiting nutrients), management (use of mineral fertilizers, planting dates and density, weed competition), legume species and variety, and rhizobium species and effectiveness. The current low crop productivity reported in legume-based systems can be attributed in part to the prevalence of these factors that limit BNF. In applying the study to legume-based systems, the N2Africa project expects that the identification of a combination of factors (see photos below), when appropriately managed, will optimize BNF and nutrient cycling in maize-based systems. This ability makes legumes a vital component of smallholder farming systems where the input of N fertilizer is almost negligible. Successful increases in legume productivity will lead to (1) increased availability of major sources of protein for direct consumption by rural households; (2) improved soil health through BNF and a reduced need for inorganic N fertilizers; (3) the breaking of pest and disease cycles of other crops when in rotation with legumes; and (4) improved income and health for the rural poor.

Preliminary results
In collaboration with the national agricultural research and extension systems (NARES) in the eight countries, the project has isolated several indigenous rhizobia strains, notably in Kenya, Nigeria, Rwanda, and DR Congo, from local farmlands to identify and characterize superior strains for enhanced BNF. The goal is to develop inoculum production capacity using superior native rhizobial strains through collaboration with private sector partners. In addition, several commercial inoculant strains are being evaluated to identify improved varieties with enhanced BNF for integration into specific farming systems. Results of the project have shown that the inoculation of improved soybean varieties resulted in higher yields in several project sites.

However, grain yields may be constrained in P-deficient soils, hence the combined use of P fertilizers and inoculum consistently produced higher yields (Fig. 1). Note from the same figure that responses to inoculants and P fertilizer are highly variable with yield in amended plots ranging from 0 to over 3 t/ha under on-farm conditions. This further stresses the need for local adaptation (see Vanlauwe6) and the need to observe the main factors determining such variability.

Figure 1. Range of responses to bradyrhizobium inoculation and phosphorus application.

Figure 1. Range of responses to bradyrhizobium inoculation and phosphorus application.

Within the N2Africa project, having detailed monitoring and evaluation (M&E) tools within large-scale adaptation and dissemination field campaigns is an important component of the ‘Research in Development’ concept, at the core of its learning objectives. Where soil pH and levels of P are not too low, an application of 20 kg P/ha is adequate for the proper growth of soybean, cowpea, and groundnut but in soils deficient in P or with low pH,40 kg/ha is optimum.

Related interventions
The project is also identifying high-yielding legume varieties with varying maturity durations for specific environments to provide farmers with options that will enable them to match varieties to the length of the growing season. For example, when the rain is delayed in a particular year or for some reason farmers delay planting, they can select short-maturing varieties that can fit into the remaining growing period.
A major emphasis is being placed on determining the best time to plant various legumes in several agroecologies in combination with appropriate row spacing and plant population. Planting at the right time enhances yield in many ways: (1) the growing period coincides with good rainfall despite its variability in some years; (2) the crop is exposed to optimum temperature regimes; (3) growth coincides with the optimum solar radiation and daylength that regulate vegetative and reproductive growth phases in legumes due to their photosensitivity; and (4) plants escape the major pests and diseases that limit yield.

With project partners which include the national agricultural research and extension systems, nongovernmental organizations, community-based organizations, and farmers’ associations, these technologies have been developed into recommended packages and are being demonstrated on-farm. The demonstration plots are established with the direct participation of farmers who are responsible for the day-to-day maintenance to encourage hands-on learning. Field days are also organized during the growing season for individuals and farmers’ groups to create awareness about the technologies. The project encourages women’s participation as well. Other dissemination activities involve the distribution of inputs to project participants including improved seeds, inoculants, and P fertilizer and lime at agreed prices. The project has developed training programs to improve the skills of extension agents, farmers, and other stakeholders to ensure sustainability of the results after the project ends.

It is expected that these agronomic interventions should lead to increased diversification of N2-fixing legume species in smallholder farming systems in sub-Saharan Africa, expansion in the cultivation of grain, greater productivity in legume-based farming systems, and enhanced family incomes and nutrition. In collaboration with microbiologists, plant breeders, and the private sector, the selection and dissemination of efficient rhizobial inoculant strains and improved varieties of grain legumes with enhanced BNF capacities adapted to various environmental stresses will improve the prospects of increasing legume components in cropping systems as well as enhancing the production of expanded ecosystem services.

ISFM for banana systems

Piet van Asten,, Severine Delstanche, Lydia Wairegi, Tony Muliele, Syldie Bizimana, Godfrey Taulia, Ken Giller, Peter Leffelaar, Laurence Jassogne, Philippe Baret, and Charles Bielders

Banana systems in Rwanda. Photo by Piet van Asten
Banana systems in Rwanda. Photo by Piet van Asten

Banana is the primary food crop in the Great Lakes Region, providing food and income for over 85% of the population. Unfortunately, current banana yields of 5–30 t/ha/year are low compared to potential yields of over 70 t/ha/year. Although between 25% and 70% of this yield gap can be explained by low soil fertility (Fig. 1), the use of external inputs such as fertilizers is virtually nonexistent and soil fertility is mostly managed by recycling local organic residues.

A study done by Severine Delstanche at the Catholic University of Louvain-la-Neuve (UCL) showed that very little nutrients were released from the soil through weathering of the soil minerals (Fig. 2). Hence, soil fertility depended almost entirely on the soil’s organic matter content. In banana-based farming systems, nutrient recycling is very important, as the harvest index is relatively small (<30%). This helps maintain relatively high organic matter content in the soil.

Figure 1. Primary crop constraints identified by use of boundary line approach.
Figure 1. Primary crop constraints identified by use of boundary line approach.

Furthermore, the large and perennial canopy and root system of banana help protect the soil from erosion. Banana therefore plays an important role in protecting the environment in this hilly landscape.

In addition to being an important component in sustaining soil fertility, banana plays an important socioeconomic buffer role for the smallholders. The crop provides food security as bunches are harvested throughout the year and any surplus can be sold to generate a continuous cash flow.

Banana systems particularly occur in areas with high population pressure and small (<2 ha) farm sizes. A study by Lydia Wairegi at Makerere University (Uganda), showed that fertilizer use was very profitable in the peri-urban area close to Kampala with marginal rates of return sometimes exceeding 500%. However, in areas far from the market (>150 km), the intensification process seemed less promising, but banana continues to play an important buffer role to maintain food security and protect the environment. In these remote locations, it seems wiser to invest in improved use of local (nutrient) resources, than to purchase mineral fertilizers.

Figure 2. The proportion of annual K fluxes in banana systems originating from soil mineral weathering (2%), crop<br /> residues (77%), and the crop harvest (21%).
Figure 2. The proportion of annual K fluxes in banana systems originating from soil mineral weathering.

To maintain its buffer role, banana can be integrated with other crops such as coffee and beans. The Ph.D. studies of Tony Muliele and Syldie Bizimana (both UCL) showed that intercropping beans with banana could be improved. Traditionally, farmers in Rwanda, Burundi, and DR Congo would till the banana field at the beginning of the wet season to suppress weeds and prepare the land for bean intercropping. Unfortunately, this practice damages a large proportion of the superficial root system of banana plants. Based on practices observed in Southwest Uganda, a technology of zero-tillage and mulching was tested. Beans are planted in the mulch. The planting holes for the beans were made using a stick. The use of external mulch greatly improved banana performance in eight trials across the region. However, beans did suffer some setback in some instances when the improved banana growth would lead to a more dense canopy, outshading the understory beans. In collaboration with Bioversity International and the Tropical Soil Biology and Fertility Institute of CIAT (TSBF-CIAT), a series of trials was conducted to reduce the banana canopy through leaf pruning. The results are almost ready, but they provide farmers with advice on how to best manage the trade-offs between banana and the understory legumes.

To improve fertilizer use efficiency and profitability, it will be important for farmers to apply the right nutrients at appropriate rates. To enable the identification of the most deficient nutrients (see photo) that need primary attention when applying fertilizers, compositional nutrient diagnosis (CND norms) were developed by Lydia Wairegi in Uganda and by Severine Delstanche in Rwanda. The CND norms are based on foliar analysis and allow a quick assessment of nutrient deficiencies observed within the plant. Contrary to critical norms for single nutrients, the CND allows for an integrated assessment of nutrient imbalances within the plant.

Farmers can use pictures to diagnose nutrient deficiencies for nitrogen, phosphorus, potassium, and magnesium.
Farmers can use pictures to diagnose nutrient deficiencies for nitrogen, phosphorus, potassium, and magnesium.

Besides developing fertilizer recommendations based on foliar analysis, IITA conducted a series of large nutrient omission trials in central and southwest Uganda. Based on the quantification of nutrient uptake, soil nutrient supply, and crop response, a QUEFTS model was developed to predict fertilizer requirements in collaboration with Wageningen University (WUR). This work was led by Ph.D. student Kenneth Nyombi and is currently being carried forward by Ph.D. student Godfrey Taulya. He observed that potassium nutrition was particularly important for banana to alleviate drought stress. The result from the ongoing research effort clearly shows that strong synergies can be achieved when integrating soil fertility, agronomic, and economic research approaches at the plot, farm, and regional levels.

NRM in cassava and yam production systems

Stefan Hauser,

Cassava has the potential to produce roots even under poor soil conditions. Photo by IITA
Cassava has the potential to produce roots even under poor soil conditions. Photo by IITA

Why are yields of cassava in Thailand and India three times higher than in Africa and production costs in Brazil only one-third of those here? Although Africa suffered from the Cassava Mosaic Disease pandemic and currently faces the threat of Cassava Brown Streak Disease, breeding tolerant and resistant germplasm has contributed to yield gains over the last three decades. Thailand, India, and Brazil have been successful in commercial cassava production with yields between 25 and 40 t/ha. The question arises: how can African farmers realize more of the >80 t/ha yield potential of cassava?

Natural resource management (NRM), agronomy, and crop husbandry have hardly ever been credited with “breakthrough” solutions to hunger and poverty. However, when more than 50-75% of the cassava yield potential is not being realized, major improvements are clearly possible through NRM, agronomy, and appropriate crop husbandry.

Agronomy and crop husbandry
For West Africa there is still a dearth of agronomic information on cassava. Currently a density of 10,000 plants/ha is the standard, while further increases are being recommended without concrete data on the yield responses to increased density by different growth types. Cassava varieties vary widely in their branching height and level of ramification, leading to different levels of ground cover by single plants and of the start and intensity of intra-specific competition. Cassava yield distribution within the same variety is highly biased (Fig. 1), raising questions on the optimum plant density and issues such as genetic uniformity and crop responses to edaphic (soil) factors.

Figure 1. Relative contribution to yield of individual plants of cassava variety Zizila in DRC.
Figure 1. Relative contribution to yield of individual plants of cassava variety Zizila in DRC.

One future effort will be to determine optimum plant densities for monocrops by major cassava growth types.

Intercropping cassava with maize or grain legumes is still widely practiced and needs to be improved. The short-term intercrops are cleared from the field, leaving space unused that can be invaded by weeds, thus there needs to be a follow-up either with weed control or a second crop to occupy the open space. Various crops will be tested for their capacity to perform between developed cassava plants and their contribution to total system productivity.

Weed control remains a problem as there are no postemergence herbicides that cassava would tolerate. Combinations of preemergence herbicides with appropriate planting techniques have the potential to reduce weed competition and labor. For instance, a preemergence herbicide can be applied before planting cassava. The herbicide kills germinating seeds as they break through the herbicide-sealed soil surface. The cassava stakes need to be planted in a vertical position and the orientation needs to be correct so that no emerging cassava leaves touch the soil surface. Such technologies combined with the follow-up use of postemergence herbicides with shields can drastically reduce labor and increase productivity as weeding can be done at the most efficient time.

Fertilizer use is low in Africa yet it appears certain that fertilizer or other forms of soil nutrient replenishment can contribute to yield increases, higher farm incomes, possibly to lower consumer prices, and thus to better livelihoods. Using average nutrient uptake into all cassava plant parts (dry matter basis) of 6.2 kg N/t, 1 kg P/t, and 5.3 kg K/t, a total supply of 165–25–145 kg N-P-K/ha is required to attain 50% of the current potential yield (45 t/ha fresh roots). Such amounts are unlikely to be supplied by the soil and thus nutrient supply is a crucial factor in achieving higher cassava yields.

There are no recent fertilizer response curves for cassava and yam in West Africa, hence, farmers do not know the composition and amounts of fertilizer to apply. The nutrient(s) most limiting to cassava production have not been quantitatively determined. The replenishment of any most limiting nutrient would lead to substantial yield increases. Depending on the limiting nutrient, productivity and profitability increases may be possible at a very low cost and risk. IITA uses a stepwise approach, first determining the most limiting nutrient(s) followed by elaborating the optimum quantity required and the construction of recommendations for optimal nutrient composition and quantities. IITA is currently working with the International Fertilizer Development Center on testing special fertilizer blends for cassava, addressing the augmentation of neglected nutrients such as sulfur, magnesium, zinc, and boron.

Use of other nutrient sources
Compost, manure, mulch, and rock phosphate have all been proposed as means to improve soil nutrient status and crop production. However, none of these sources has had a major impact as farmers need land to produce biomass or else infrastructure is required to mine, process, and distribute rock phosphate. Although the biological sources are important, constraints in biomass production need to be overcome first.

Figure 2. Crop yield response to planted herbaceous fallow in West and Central Africa.
Figure 2. Crop yield response to planted herbaceous fallow in West and Central Africa.

Mineral fertilizers alone cannot sustain crop production on degraded land. Soil organic matter and  soil micro-, meso-, and macro-fauna are important in maintaining soil quality and health. Traditionally, fallow phases between crops were replenishing the soils’ production capacity. With increased population densities, fallow phases have been shortened or no longer exist. Thus, soils do not recover but continue to lose their production potential. Farmers do not seem to invest in soil fertility but look for ways of coping with ever less fertile soils, thereby degrading them to a stage where cropping becomes unprofitable.

Such situations have been encountered in southern Bénin. Soil fertility and quality management techniques, such as cover crops, manure application, or any other form of organic matter and nutrient recycling have not been adopted at larger scales. In retrospect, there have been constraints to the adoption that were not considered in the process of technological development. Today, with more options available and a stronger and earlier involvement of farmers in research for development, such approaches are worth reconsidering. One such technology, using leguminous cover crops, had little if any success in cassava (Fig. 2).

Controlling the cover crop was a major problem. Consequently IITA works today on efficient and effective control methods. Pueraria phaseoloides was introduced to smallholders in southern Cameroon but it was not readily accepted as farmers immediately identified it as an aggressive weed, able to destroy crops. However, two years into the use of Pueraria, fallow farmers noticed that the weeds most difficult to control had disappeared and that it was easier to clear Pueraria than the natural fallow. Some farmers burned the Pueraria only to find the land ready to crop without major labor input. Yields of cassava, maize, and groundnut were generally higher after Pueraria, whereby the labor-saving burning produced the highest yields (Fig. 3).

Figure 3. Cassava fresh root yield in burned and mulched maize-cassava and burned maize-cassava-groundnut intercrop.
Figure 3. Cassava fresh root yield in burned and mulched maize-cassava and burned maize-cassava-groundnut intercrop.

Considering farmers’ needs
Pueraria was introduced for soil fertility replenishment but was adopted for its labor-saving effects. Soil fertility was not perceived as a problem and thus positive effects on the soil could be compromised (by burning) without compromising yields. Effects such as weed suppression and the reduction of soil-borne pests and diseases may contribute to the yield increases after Pueraria.

Livestock integration and the search for synergies
Few farmers adopted the use of green manures for soil fertility improvement because they have no direct benefits from it. Herbaceous legumes have rarely been used to feed livestock, although there is (anecdotal) evidence that livestock feed on them and that they are beneficial to growth and reproduction. In the IITA-led CRP on Humidtropics, livestock integration will be a major aspect. It will add value to green manure species when these are used to feed livestock that will also benefit from the canopies of root and tuber crops (cassava leaves) remaining at root harvest. Thus, there will be an increase in returns of animal manure to fields, and to crop yields through the combined use of green and animal manures for improved food security and farm incomes.

Efficient combinations of agronomic practices, nutrient supply, and soil management practices will be developed to increase the productivity of cassava and yam while improving the status of the natural resource base. Synergistic effects between these measures and the integration of livestock or fish farming will increase resource use efficiency and income generation as well as the quality of the farm food supplies. Due consideration of social and gender aspects in farm household operations will identify the entry point best suited for IITA’s interventions. Farmers’ feedback and innovations will be integrated into approaches on sustainable intensification to increase food production and improve rural livelihoods while enhancing the capacity of the agroecosystems to deliver essential services.

Hauser, S. and C. Nolte. 2002. Biomass production and N fixation of five Mucuna pruriens varieties and their effect on maize yields in the forest zone of Cameroon. Journal of Plant Nutrition and Soil Science 165: 101–109.
Hauser, S., C. Nolte, and R.J. Carsky. 2006. What role can planted fallows play in humid and sub-humid West Africa? Nutrient Cycling in Agroecosystems 76: 297–318.

Boosting the productivity of cassava-based systems in DR Congo

Bernard Vanlauwe,, Patrick Mutuo, Nzola Mahungu, and Pieter Pypers

The intensification of African agriculture is an essential response to the increasing demands for more food without endangering important natural resources, including the forests in the Congo basin.

Because of its potential to produce some roots, cassava is often considered a crop that “likes” poor soils but, as all other crops, it responds to and requires a sufficient amount of nutrients to produce attainable yields. The transformation of cassava from a food security to a cash crop in many countries in Africa further stresses the need for nutrient replenishment strategies in cassava-based systems since the transport of cassava roots to processing plants can accelerate the amount of nutrients exported from the farm.

In recent years, integrated soil fertility management (ISFM) (see article by Vanlauwe et al. this publication) has been accepted by many organizations as the underlying technical framework for the sustainable intensification of smallholder systems in Africa. This article highlights the progress of work on the application of ISFM principles in the context of cassava-based systems in DR Congo.

In DR Congo application of 2 bags of NPK fertilizer per hectare results in root yield increases from 30% to 160%.
In DR Congo application of 2 bags of NPK fertilizer per hectare results in root yield increases from 30% to 160%.

Current situation
In the highlands of Sud-Kivu province, cassava and common beans are among the main food crops traditionally cultivated in mixed cropping systems. Cassava monocropping is done only in marginal fields where other crops fail to yield. Farmers generally allocate about 0.2–0.3 ha (30–45% of their farm area) to cassava–legume intercropping and obtain average yields of 400–800 kg/ha of legume grains and 10–15 t/ha of cassava fresh roots. Pressure on land is very heavy due to high population densities and justifies agricultural intensification and investment in soil productivity.

In Bas-Congo province, on the other hand, farmers practice slash-and-burn agriculture. Cassava is grown for 1 or 2 years, followed by fallow periods of 2 to 4 years. In the past two decennia, the population has grown by more than 50% leading to more demands for food, especially from nearby Kinshasa. Improved and sustainable, fire-free production systems are urgently needed. In DR Congo, most farmers have no access to improved varieties and have very limited options to improve soil fertility.

Fertilizer and germplasm
First, the use of improved, pest-and disease-resistant varieties in combination with appropriate rates of NPK fertilizer was observed to result in a 30–160% increase in cassava root yields in eastern DR Congo. A visible increase was observed in yields of the stems, important for the production of planting material (see photo). In western DR Congo, cassava yields doubled from 12 to 25 t/ha with the application of moderate rates of NPK and reached over 40 t/ha with higher rates. Several initiatives are taking place to ensure that large quantities of planting materials reach smallholder farmers with specific attention being given to those varieties resistant to cassava mosaic disease and the brown streak virus.

Figure 1. Cassava storage root yields as affected by application of compound fertilizer (17 nitrogen: 17 phosphorus: 17 potassium) at rates of 0 (F0), 40 (F1), 120 (F2), and 200 (F3) kg K/ha, and green manure (TI = <em/>Tithonia sp.; CH = <i>Chromolaena </i>sp.) at 2.5 t dry matter/ha alone or together with compound fertilizer at 40 kg K/ha in two trial locations in Bas-Congo, DRC. SED: standard error of difference at P<0.05. Source: Adapted from Pypers et al. 2012
Figure 1. Cassava storage root yields as affected by application of compound fertilizer (17 nitrogen: 17 phosphorus: 17 potassium) at rates of 0 (F0), 40 (F1), 120 (F2), and 200 (F3) kg K/ha, and green manure (TI = Tithonia sp.; CH = Chromolaena sp.) at 2.5 t dry matter/ha alone or together with compound fertilizer at 40 kg K/ha in two trial locations in Bas-Congo, DRC. SED: standard error of difference at P<0.05. Source: Adapted from Pypers et al. 2012

Combined application of fertilizer and organic inputs
Fertilizer response and the effect of combining inorganic and organic nutrient resources were also evaluated on cassava systems. The most common fertilizer, NPK 17:17:17, was applied in western DR Congo with or without green manure made from Tithonia sp. or Chromolaena sp., and the effects on storage root yield were evaluated in two locations with a differing soil fertility status (Fig. 1). Both plants are commonly found in fallow vegetation in western DR Congo. Control yields were similar at the two sites (12 t/ha), but the response to fertilizer differed between the sites: storage root yields reached 40 t/ha at Kiduma but only 20 t/ha at Mbuela with the addition of 200 kg K/ha. A much larger response to Tithonia sp. green manure was also observed at Kiduma, which was likely to have been related to the higher quality and nutrient contents of the green manure grown at that site.

Combining organic and inorganic nutrient resources did not result in positive interactions. No significant differences were observed between the yields after the sole application of either fertilizer or green manure to the control and those obtained with the combined application of both nutrient sources (Pypers et al. 2012). In maize-based systems, positive interactions between organic and inorganic fertilizers often arise from better synchrony in N release and N uptake by the crop. In cassava systems, where K is more often the most limiting nutrient, such a mechanism is likely to be less relevant.

In the highlands of eastern DRC alternative spacing of cassava  (2 × 0.5 m instead of 1 × 1 m) allows integration of four lines of legumes during one season and two lines of legumes during the second season without affecting cassava densities and yields.
In the highlands of eastern DRC alternative spacing of cassava (2 × 0.5 m instead of 1 × 1 m) allows integration of four lines of legumes during one season and two lines of legumes during the second season without affecting cassava densities and yields.

In eastern DR Congo, the use of improved germplasm did not result in yield increases without the simultaneous implementation of other ISFM components. Modifying the crop arrangement by planting cassava at 2 m between rows and 0.5 m within the row, intercropped with four legume lines, increased bean yields during the first season and permitted a second bean intercrop. This can also increase the total legume production by up to 1 t/ha and resulted in additional revenues of almost $1000/ha (see photo). The crop arrangement or a second legume intercrop did not affect cassava root yields. Fertilizer application increased both legume and cassava yields, and net revenue by $400–700/ha with a marginal rate of return of 1.6–2.7. Replacing the common bean intercrop by groundnut increased net revenue by $200–400/ha, partly because of the higher market value of the grains, but mostly due to a positive effect on yield of cassava storage roots. Soybean affected cassava yields negatively because of its high biomass production and long maturity period; modifications are needed to integrate a soybean intercrop into the system.

Local adaptation
Due to the high variability in soil fertility status, the varying landscape features, and the variation in access to inputs, local adaptation is required to ensure that the investments made by cassava-producing households  result in the highest returns, in line with the resources (e.g., cash, land, labor) that these households have (see photo). Such adaptation efforts are best led by extension and development partners that have the required skills and capacity to implement such efforts at scale. In eastern DR Congo, erosion is the most urgent issue to be tackled to enhance the sustainability of cassava-based systems. Results from Southeast Asia with the use of live hedges should be explored for these environments.

Cassava land preparation varies widely in DRC partly affected by slope and water status of the land.
Cassava land preparation varies widely in DRC partly affected by slope and water status of the land.

These findings demonstrate the large potential of ISFM to increase productivity in cassava–legume systems in DR Congo. This is crucial in view of the fact that cassava is changing from being almost only a food security crop to one for which there is high demand in local and urban markets. The intensification of production is thus a prerequisite for sustaining cassava-producing households and ISFM can assist in achieving such benefits. However, these benefits were not observed in all study sites. In poor soils, productivity increases were variable or absent, and soil amendments are required. Better targeting and local adaptation of the technologies are possible with a better understanding of the conditions under which positive effects occur.

Pypers, P., J.M. Sanginga, K. Bishikwabo, M. Walangululu, and B. Vanlauwe. 2011. Increased productivity through integrated soil fertility management in cassava–legume intercropping systems in the highlands of Sud-Kivu, DR Congo. Field Crops Research 120: 76–85.
Pypers, P., W. Bimponda, J.P. Lodi-Lama, B. Lele, R. Mulumba, C. Kachaka, P. Boeckx, R. Merckx, and B. Vanlauwe. 2012. Combining mineral fertilizer and green manure for increased, profitable cassava production. Agronomy Journal, in press.