18th AFA Int’l Annual Fertilizer Forum & Exhibition Feb., 7-9-2012, Sharm El-Sheikh , Egypt Maritim Jolie Ville Hotel
IPNI Approach to Nutrient Management in Sub-Saharan Africa
Mr. Adrian M. Johnston Vice President, Asia& Africa, IPNI USA
2/18/2012
Nutrient Management in Sub-Saharan Africa
Adrian Johnston IPNI Vice President – Asia & Africa Shamie Zingore IPNI Africa Director
Co‐Author: Dr. Shamie Zingore, Nairobi, Kenya
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IPNI is supported by leading fertilizer manufacturers and industry associations
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IPNI Current Programs Eastern Europe and Central Asia North America Middle East
China
Mexico Africa
Northern Latin America
SE Asia
India
Brazil
Latin America Southern Cone Australia
• 30 Ph.D. scientists in 10 program areas • 140 R&D projects in 2010, 75% dealing with increasing yields
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Global Population and Food Security
• Population increases are placing greater pressure on the food security of certain regions of the world • Potential food security challenges are going to affect all world residents, regardless of their location.
Agricultures Role in Food Production • “Food security is a global challenge, played out on a local scale”. • “Challenges of this scale (food security) have been met in the past – between 1961 and 2008, agriculture output increased by 179 percent globally. In many parts of the world, these production increases were achieved by intensification”. Brian Keating and Peter Carberry, CSIRO, Australia
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World Population – Projected Changes Values shown are % Year
World
Asia
Africa
Europe
L.A.
N.A.
Oceania
2000
6,115
60.5
13.4
11.9
8.5
5.2
0.5
2005
6,512
60.5
14.1
11.2
8.6
5.1
0.5
2010
6,909
60.3
15.0
10.6
8.5
5.1
0.5
2015
7,302
60.1
15.8
10.1
8.5
5.0
0.5
2020
7,675
59.9
16.6
9.6
8.4
5.0
0.5
2025
8,012
59.6
17.5
9.1
8.4
5.0
0.5
2030
8,309
59.2
18.3
8.7
8.3
4.9
0.5
2035
8,571
58.7
19.2
8.4
8.2
4.9
0.5
2040
8,801
58.2
20.1
8.0
8.2
4.9
0.5
2045
8,996
57.7
21.0
7.8
8.1
4.9
0.6
2050
9,150
58.2
21.8
7.6
8.0
4.9
0.6
World Population database, FAO, 2008
11% 0.5%
7.6% 59% popn
5.5% 15%
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Fertilizer use
Average per hectare fertilizer use rates as kilograms of nutrients (NPK) by fertilizer markets in 2008/09
Source: IFDC; derived from FAO data
Nutrient Depletion Rates ‐ Africa
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The context of smallholder farming systems in Sub-Saharan Africa • ‘Low’ input use • Highly variable soil fertility conditions • Mixed cropping systems • Predominantly rain-fed agriculture • Socio-economic constraints
The context of smallholder farming systems Village land (600 ha) Wealthier farmers’ cropland FZ4 (25 ha)
FZ2 (46 ha)
FZ2 (43 FZ2 ha)
3 t ha-1
5 t ha-1
Wet and dry season grazing Communal grazing land
Livestock
400 ha
Grazing of crop residues Poorer farmers’ cropland
Fodder Manure
FZ4 86 ha
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Impact of Soil Quality on Nutrient Response
Malawi data, Zingore
Reality check – Sub‐Saharan Africa Farm Type
Farm Size
Cattle
Family Fertilizer Manure Annual maize Size use use requirement
1
3.0 ha
8
8
300 kg
6 t
720 kg
2
1.2ha
3
6
100 kg
2 t
540 kg
3
0.5 ha
0
4
0 kg
0 kg
360 kg
Distribution 25% (125 million people) 30% (150 million people) 45% (225 million people)
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Farmer type 1 (25% of farms – 125 million people) Area
Maize Groundnut/ beans Total
Cost of Amount Amount production harvested consumed/ (US $) reserved
Amount sold
Income ($)
2 ha
275
6 t
1 t
5 t
1000
0.5 ha
103
0.8 t
0.2 t
0.6 t
180 1180 ($3.2/day)
Farmer type 2 (30% of farms – 150 million people) Area
Maize Groundnut/ beans Total
Cost of Amount production harvested ($)
Amount consumed/ reserved
Amount sold
Income ($)
1 ha
128
2 t
0.7 t
1.3 t
260
0.2 ha
49
0.3 t
0.2 t
0.1 t
60 320 ($0.9/day)
Farmer type 3 (45% of farms – 225 million people) Area
Cost of Amount Amount production harvested consumed/ ($) reserved
Amount sold
Income ($)
Maize
0.4 ha
38
0.3 t
0.3 t
0
0
Groundnut/ beans Total
0.1 ha
5
0.1 t
0.1 t
0
0 150 ($0.4/day)
Farm type 3 cannot grow sufficient food. Typically work for other farmers and paid in maize or little cash (US$150).
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Why did fertilizer use remain low? Unfavorable demand and supply situation Demand • Poor and variable crop response • High fertilizer:output cost ratios • lack of market information • lack of resources to purchase fertilizer • lack of information to use fertilizer efficiently
Supply • unfavorable business climate for the private sector • fertilizer distribution is unprofitable due to weak/dispersed demand; small market; high transportation costs; high finance costs.
Current interventions • Access: ‘Smart’ subsidies – governments to commit >10% budgets to agriculture • Buying: Improving farmers’ ability to purchase fertilizer by improving their access to credit • Selling: Improving market information by increasing investment in market information systems • Insurance: Protecting farmers against low and volatile output prices • Organization: Empowering farmer organizations by capacity building
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Current interventions • Policy & Regulation: Reducing fertilizer sourcing costs: o lowering trade barriers o adopting common quality standards o Harmonizing regional markets • Infrastructure: Reducing fertilizer distribution costs by improving road and rail infrastructure (Trade Corridors)
Analyzing farmers’ practice: Yield gaps
(Yp) (Ya max) (Ya) (Y)
Ya max
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Consolidation of Information on Fertilizer Access and Use in Africa • Synthesis and dissemination of science‐based information on fertilizers in crop production and sustainability. • Highlight cropping systems which offer good opportunities for intensification, and show opportunities to use nutrients to increase yields and quality. • Improve our understanding of how we can make recommendations in the variable soil environment common to Africa.
Agronomic Research to Determine Potential Productivity and Validate Site‐Specific Practices • Nutrient response trials in Kenya, Mozambique, Zimbabwe, Tanzania, Malawi and Uganda. • Evaluate both macro, secondary and micronutrient responses. • Use multiple research locations within each country to assess the spatial variation in nutrient deficiencies. • Collaborate with National and International research partners in research. • Link development work with national extension and NGO partners.
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Nutrient Expert, Maize – Nutrient Decision Support • A computer based DSS tool to formulate nutrient recommendations for maize. • Incorporates results from local research, expert knowledge from the region, local nutrient sources, risk management for water, and profit analysis. • Farmers with their advisors set their own yield goal for maize. • Both a recommendation tool, as well as a learning tool.
Development of Nutrient Expert
See at: www.ipni.net/SEAP 24
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Educational and Training Material on Nutrient Management • Develop posters, brochures, training manuals, handbooks and AV materials on nutrient management. • Develop materials for farmers, advisors, dealers and scientists and their students. • Focus on not only nutrients, but the target crops – maize and legumes. • Goal is to both create awareness and build capacity to support the extension efforts.
Field diagnosis tools
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Collaboration with Research & Development Partners
AGRA – Alliance for a Green Revolution in Africa ASHC - Africa Soil Health Consortium AfSIS - Africa Soil Information Service
The internet and mobile phone provide two complementary options for acquiring needed information and transmitting a recommendation
Computer via Web connection
Mobile phone via Web connection
Mobile phone SMS compatible
Nutrient Expert Web
Nutrient Expert Mobile-web
Nutrient Expert Mobile-SMS*
* Uses Interactive Voice Response (IVR)
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Conclusions The opportunity to improve food grain production in SS Africa is great, and fertilizers are one of the tools which will play a major role in this. IPNI plans to address the nutrient management issue by: Consolidating and sharing information on fertilizer use, approaches to making recommendations, and economics of response. Conduct research and demonstration trials to establish appropriate measures to overcome the nutrient deficiency challenges. Develop and promote decision making tools that will enable farmers and their advisors make better fertilizer recommendations. Develop, publish and distribute publications, posters for dealerships, radio programs and video footage to support fertilizer use.
We look forward to integrating technologies from all IPNI global regions in our efforts in Africa, making our program a shared effort with those regions with similar climate.
Thank You
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IPNI Approach to Nutrient Management in Sub-Saharan Africa1 Adrian Johnston and Shamie Zingore International Plant Nutrition Institute Saskatoon, Canada and Nairobi, Kenya
Abstract In the face of a growing food crisis in Sub-Saharan Africa (SSA), there are renewed efforts to help farmers to increase crop productivity mainly by increasing fertilizer use and improved crop varieties. However, sustainable crop production intensification in SSA will remain elusive unless the fundamental issues of providing the crops with adequate nutrients in their correct balance under highly variable soil fertility conditions are properly addressed. The Africa program of IPNI has been developed to work with its industry member companies, and collaborate with national agricultural research and extension systems, Universities, and International Agricultural Research Systems to make a contribution to sustainable nutrient management in SSA. The goal is to synthesize information and develop research programs to encourage fertilizer use in ways that are technically efficient, economically viable, and environmentally friendly. The program will facilitate the establishment of a network for sharing information on site- and crop-specific best fertilizer management practices for various cropping systems among researchers in Africa and other regions. Emphasis will also be placed on supporting and collaborating with existing projects to develop and promote innovative mechanisms to transfer knowledge on best fertilizer management practices to input suppliers, agricultural service providers, extension services and farmers. The program will focus on sites in the subhumid zones, due to their high potential for agricultural intensification, and will cover both commercial estates and subsistence oriented agricultural systems. Introduction Most of the soils in sub-Saharan Africa (SSA) are inherently infertile, and poor agricultural management practices during the past decades have led to a severe decline in their productive capacity. Chronic food shortages and malnutrition in Africa are a result of poor crop productivity linked to multiple soil nutrient deficiencies. Fertilizer use is extremely low in much of the SSA region (Av. 13 kg/ha), despite large areas with high potential for intensification, and this is one of the main factors explaining lagging agricultural productivity growth. Cereal crop yields have stagnated at about 1 t per ha over the past 5 decades, despite an increase in food demand by 3-3.5% per year due to a rapidly growing population (Eicher and Kupfuma 1998). Presently, chronic food insecurity affects 28% of the 700 million people who live in SSA. At the current trends of population growth, cereal crop productivity must grow by 4% annually, or more than double by 2020, to make SSA self-sufficient in cereal production (Badiane and Delgado 1995). Given the low levels of fertilizer use and poor soils in sub-Saharan Africa (SSA), fertilizer use must increase if the region is to reverse the current trends of low crop productivity and land degradation. There are renewed efforts to raise fertilizer use in SSA from the current 8 kg to 50 kg/ha by improving the marketing, policy and socio-economic environment to increase fertilizer availability at prices affordable to smallholder farmers. However, to realize full agronomic, economic and environmental benefits of increased fertilizer use, the fundamental issues of providing the crops with adequate nutrients under the highly variable soil fertility conditions in SSA must be properly addressed. Farmers often are doing the best they can with available limited resources, but they often lack a good understanding of how best to manage higher rates of fertilizers. For example, the recent investment in fertilizer in Malawi under the national subsidy programme resulted in an increase in fertilizer use by 20 kg/ha. National food production was increased by 60%, but estimated agronomic N use efficiency was only14 kg grain/kg fertilizer N. Agronomic efficiencies of 30 kg grain/kg fertilizer N can be achieved with good management, and this suggests that yield could readily be doubled with the same fertilizer. Current recommendations for nutrient management are mostly blanket in nature, based on plot level performance of single technologies, but there is increasing realization that systematic refinement of the recommendations to reflect the heterogeneous conditions at farm and landscape levels is crucial to optimize crop productivity (Giller et al., 2006). Within smallholder farms, fields can be identified that exhibit different patterns of responsiveness to applied nutrients: poorly responsive fertile fields, poorly responsive infertile fields, and responsive medium-to-infertile fields (Tittonell et al., 2008; Zingore et al., 2007).
Sustainable crop production intensification in SSA will largely depend on improving the efficiency with which scarce fertilizers are used by smallholder farmers under highly variable soil fertility conditions. Research in SSA has established Integrated Soil Fertility Management (ISFM) as a framework for boosting crop productivity through combing fertilizer use with other soil fertility management technologies, based on site conditions. Vanlauwe et al. (2010) 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.’ The 4R Nutrient Stewardship developed by the fertilizer industry worldwide focusing on applying the right fertilizer source at the right rate, at the right time in the growing season, and in the right place provides an essential basis for optimizing use of nutrient within the ISFM framework, holistically evaluating the implications for sustainability. Despite concerted nutrient management research for several decades there still exist major limitations in translating science into practice in SSA. The IPNI program has identified some key gaps that will need to be addressed, including: • Information on the status of crop productivity and nutrient use is not readily available in formats easily accessible to various categories of users. • Results from past research on fertilizer management practices are fragmented and not available to guide and prioritize investment in nutrient management. • Major gaps exist in direction and leadership to consolidate information on nutrient management in the region and produce outputs required to effectively support farmers to make informed decisions on plant nutrient management. • Current efforts to promote fertilizer use are based on nutrient management to achieve small increases in productivity and there are limited efforts to assess the potential production of favourable regions in SSA. • No clear impact pathways and decision support tools to support sustainable intensification of smallholder agriculture in SSA. • There are efforts to revise fertilizer recommendations in many countries (e.g. Malawi, Tanzania, Zimbabwe, Kenya), but frameworks to systematically develop and package fertilizer recommendations are lacking. • Major gaps exist between research and extension systems with a long time lag (often more than 20 years) in applying scientific research results into products to improve nutrient management by smallholder farmers. 4R Nutrient Stewardship The main focus of IPNI Africa program is to provide leadership and direction in plant nutrient management research and development in SSA guided by the principles of the 4R Nutrient Stewardship (www.ipni.net/4R). The 4R Nutrient Stewardship Framework developed by the fertilizer industry worldwide aims to provide the context for efficient on‐farm nutrient management practices with irreducible simplicity focused on four central components: applying the right fertilizer source at the right rate, at the right time in the growing season, and in the right place. Smallholder farms in SSA exhibit substantial heterogeneity (lack of uniformity) in soil fertility within short distances, and the 4R Nutrient Stewardship should address this variability to increase nutrient use efficiencies. Although fertilizer recommendations in SSA mostly cover N and P only, analysis of nutrient deficiencies show an increase of constraints to crop production with decreasing soil fertility status. • Depleted soils that cover wide areas are associated with multiple nutrient deficiencies and addition of the ‘right source’ of fertilizer that provide base cations (K and Ca) and micronutrients (Zn and B) in addition to N and P is required to significantly increase yields. • The ‘right rate’ of fertilizer application has also been found to have profound effects on nutrient use efficiency, with on farm experiments showing that agronomic and economic returns diminish rapidly on most poor soils when nutrient applications rates exceed 60 kg N/ha and 10 kg P/ha.
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In addition to the standard timing of basal and top dressing fertilizer application, ‘right time’ of fertilizer application in SSA should be flexible and adjustable to the highly variable inter- and intraseasonal rainfall as a risk mitigation strategy. The ‘right place’ is often critical when low rates of fertilizer are used, with spot-application more effective at placing nutrients where crops can use them effectively.
Each of the four “rights” is directly related to the other three in at least one way, interconnected into a unified, effective system. When viewed holistically, 4R Nutrient Stewardship can have wider effects on the sustainability of agricultural systems in SSA beyond the immediate benefits in terms of crop productivity. Research and Development Plans The Africa Program activities are initially covering maize based cropping system in the East and Southern Africa sub-humid zone, due to its high prospects and potential for agricultural intensification. The sub-humid zone maize-based cropping system constitutes 38% of the total land area in SSA and covers large areas in East and Southern Africa (Deckers 1993). The zone has good prospects for agricultural growth due to favourable rainfall (av. 700-1200 mm/yr) and is important for its high potential for maize production, the staple food crop for the region (FAO 2001). The program will also partner with the IFDC West Africa Division to establish research and training activities to enhance profitability and sustainability cocoa and oil palm production in humid zones in West Africa. 1. Consolidation of information on fertilizer access and use in Africa Despite extensive research on the role of fertilizer in improving crop productivity in Africa, much of the information remains fragmented as there has been little effort to consolidate results from wide-ranging studies. As a consequence, evidence for the potential of impact of fertilizers on increasing crop productivity and the best management practices that contribute most to yield increases have often been incoherent. To fill this gap, IPNI is collaborating with various research and development institutions to develop a database and information system to collate and analyze data on fertilizer access, use and the potential impact of fertilizer in Africa. Products will be developed to make the information accessible to various stakeholders in agricultural research and development. Some of the key benefits of the information system will be: • Synthesis and dissemination of science-based information on the role of fertilizer in sustainable increase in crop productivity and maintenance of soil resource base. • Highlighting cropping systems that offer good opportunities for intensification and show gaps in nutrient management for achieving high yield levels. • Improved understanding of the pathways and knowledge systems necessary to sustainably increase crop productivity in heterogeneous farming systems in Africa. 2. Agronomic research to assess potential crop productivity and validate site-specific BMPs To optimize and sustain crop productivity in the region, there is need to appropriately define the potential and attainable yields for medium and high potential benchmark sites and develop target yields that suit food security and economic objectives of various categories of farmers. The coordinated research program builds on recent research results in SSA that have shown drastic soil fertility variability at the farm and landscape levels, leading to variable crop productivity and crop response to additions of fertilizer and organic nutrient resources (Zingore et al., 2007). Consequently, large yield gaps arise from soil fertility differences between fields due to a combination of inherent and management factors. To achieve this goal, the IPNI Africa program has initiated a regional coordinated experiment to determine best nutrient management practices for maize. • Standardized nutrient omission trials are being implemented in partnership with collaborators in Kenya, Mozambique and Zimbabwe, with additional sites to be established in Tanzania, Malawi and Uganda in 2012. • The nutrient omission trials, located at multiple sites, are designed as a diagnostic tool to identify which of the macro-nutrients N, P and K are limiting maize growth, and to determine the possible other constraints related to soil secondary and micro-nutrients and soil acidity. • In nutrient omission trials, one of the nutrients is omitted while the others are applied at rates considered as non-limiting in all treatments.
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At each site, trials are established at multiple locations to determine spatial variation, which will serves to establish the link between the crop response and soil characteristics and provides the basis for predicting and mapping of nutrient deficiencies. The coordinated research project will provide a standard approach for use by the national and international agricultural research systems, development partners and extensions services to develop, validate and promote best nutrient management practices. A GIS database is under development to collect information on maize production potential and nutrient deficiencies in SSA, and will be used to integrate information from current and future IPNI and collaborators’ trials. The database will be used to highlight areas with the greatest potential for intensification and where R&D investments can be prioritized.
3. The maize nutrient management decision support tool – Nutrient Expert for Maize The process to develop site-specific fertilizer BMPs takes into account complex factors that not only affect soil-water-crop relations, but also socio-economic factors. The IPNI South-East Asia region developed Nutrient Expert for Maize, a robust computer-based decision support tool that integrates complex factors in a simple and enables local experts to strategically formulate nutrient management guidelines for maize and other crops. The tool has been adapted for African cropping systems and will be validated from 2012. • Nutrient Expert for Hybrid Maize supports farmers to increase yields and profits by suggesting a meaningful yield goal for specific locations and by providing the best nutrient management strategy required to attain the yield goal. • Nutrient Expert for Maize allows determination of yield goal taking into account the potential yield for the specific area, the attainable yield with optimal nutrient management and farmer objectives (food security or income). • The tool also presents a simple profit analysis comparing costs and benefits between the farmer‘s current practice and the recommended alternative improved practice. • Nutrient Expert for Maize was designed in a way that it can be used as a learning tool, providing field staff with information of how to determine flexible nutrient application rates that are suitable for the variable soil fertility conditions within smallholder systems, fertilizer composition, quick helps, instant summary tables and graphs, plus allowing a great amount of flexibility in use of the tool. This provides added value in moving from blanket recommendation to developing nutrient management recommendation that match conditions in specific sites. • The guidelines provided by this software are consistent with the scientific principles of Site-Specific Nutrient Management (SSNM) that include (Doberman and Cassman, 2002): • Utilize indigenous nutrient sources available on-farm • Apply adequate amounts of fertilizer N, P, K, and other nutrients to minimize nutrient-related constraints and achieve high yield • Achieve high profitability in the short and medium term • Avoid the luxury uptake of nutrients by the crop • Minimize depletion of soil fertility • Use of proper agronomic practices, such as plant spacing, time, rate, place and type of fertilizer, crop reside management etc, to increase fertilizer use efficiency. Promotion of results from Nutrient Expert for Maize is enhanced by use of other field-based tools, such as soil test kits, leaf colour charts and ‘Crop Doctor’ that allow farmers to diagnose nutrient requirements for crops. Nutrient Expert for Maize is easily adaptable to different environments, and work is already underway to validate and test the decision tool under various conditions in Africa. 4. Development of material and training programs to promote BMPs The IPNI Africa program will also develop products for innovative and effective transfer of knowledge on best fertilizer management practices to input suppliers, public and private agricultural service providers, extension services and farmers. Various IPNI materials (including posters, training manuals, handbooks, audio-visual productions etc) will be adapted and new materials specific to Africa developed for different categories of end users. The ‘Maize Doctor’ nutrient management guide is available from the Africa Program.
Over the next 5 years, the IPNI Africa Program will contribute to the Soil Health Consortium established the Gates Foundation to raise awareness of good fertilizer management and ISFM amongst stakeholders and providing information to improve decision-making by policy makers, extension workers, input suppliers. This project will also provide IPNI with a good opportunity to contribute to the development of educational curricula to improve effectiveness of University teaching programs, and the development of knowledge products for various categories of users.
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5. Collaboration Central to the success of IPNI to have impact in Africa will be development of effective partnerships with relevant partners. The program works very closely partners from national agricultural research and extension systems, Universities, International Agricultural Research Systems and other stakeholders, in implementation on activities. The Africa program has also been positioned to influence major research and development programs on nutrient management in Africa. Some of the Key programs IPNI is collaborating with include: Alliance for a Green Revolution in Africa (AGRA): Linkages have been established with the Soil Health Program of AGRA to backstop demonstration to promote best nutrient management practices. Activities include: providing technical support and strengthen the capacity of the projects to conduct adaptive research, and data; building the capacity of project to use decision support tools for improving dissemination of nutrient management recommendations; developing a database for regional integration and synthesis of results from SHP projects to facilitate innovative regional learning on agricultural intensification in SSA. Africa Soil Health Consortium: The IPNI Africa program is a key partner in the Africa Soil Health Consortium project coordinated by CABI: ‘Supporting knowledge sharing on integrated Soil Health.’ The Africa Soil Health Consortium project is developing customized materials for use in initiatives to improve crop productivity in SSA, with a focus to raise awareness of best nutrient management practices amongst stakeholders and providing information needed to improve decision-making by policy makers, extension workers and input suppliers. Africa soil Information Services (AFSIS): IPNI has joined efforts with the AfSIS project, whose main focus is to develop the digital soil map for Africa. The AfSIS project is implementation diagnostic nutrient omission trials that will provide standard approaches and tools, norms and principles for improving crop productivity and nutrient use efficiencies in SSA. Diagnostic trials have been installed in major farming systems in five countries in SSA, namely Mali, Malawi, Kenya, Tanzania and Nigeria and will provide consistent, large-scale mechanisms for testing the efficacy of fertilizer and soil fertility ameliorants use. The results from the diagnostic trials will provide information for understanding nutrient deficiencies in key pilot sites, which will provide important input to the datasets from the coordinated maize trial. Concluding Remarks • There is growing interest to help farmers in Africa to increase productivity, with a major focus on investments increase fertilizer access. • Insufficient attention has been given to develop, manage and and disseminate knowledge on fertilizer BMPs. • IPNI efforts in Africa, supported by IPNI’s global experience, will make a major contribution to fill this gap. References Badiane O, Delgado C (1995) "A 2020 vision for food, agriculture, and the environment in Sub-Saharan Africa:" 2020 vision discussion papers 4, International Food Policy Research Institute. Deckers J (1993) Soil fertility and environmental problems in different ecological zones of the developing countries in sub-Saharan Africa. In: The role of plant nutrients and sustainable food production in sub-Saharan Africa H van Reuler and W.H. Prins (Eds.) The Netherlands, pp. 37-52. Doberman, A. and Cassman, K. G. 2002. Plant nutrient management for enhanced productivity in intensive grain production systems of the United States and Asia. Plant and Soil 247: 153 - 175. Eicher CK, Kupfuma B (1998) “Zimbabwe’s Maize Revolution: Insights for Closing Africa’s Food Gap.” In: CK Eicher and JM Staaz (eds) International Agricultural Development, John Hopkins University Press. FAO (2001) Farming systems and poverty: Improving farmers' livelihoods in a changing world. Food and Agriculture Organization of the United, Rome, Italy Giller KE, Rowe E, de Ridder N, van Keulen H (2006) Resource use dynamics and interactions in the tropics: Scaling up in space and time. Agricultural Systems 88:8–27.
Tittonell P, Corbeels M, van Wijk MT, Vanlauwe B, Giller KE (2008). Targeting nutrient resources for integrated soil fertility management in smallholder farming systems of Kenya. Agronomy Journal. 100:1511–1526. Vanlauwe B, Bationo A, Chianu J, Giller KE, Merckx R, Mokwunye U, Ohiokpehai O, Pypers P, Tabo R, Shepherd K, Smaling E, Woomer PL, Sanginga N (2010) Integrated soil fertility management: Operational definition and consequences for implementation and dissemination. Outlook on Agriculture, 39: 17-24. Zingore S, Murwira HK, Delve RJ, Giller KE (2007) Soil type, historical management and current resource allocation: three dimensions regulating variability of maize yields and nutrient use efficiencies on smallholder farms. Field Crops Research 101:296–305.