Bernard Vanlauwe, firstname.lastname@example.org
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 (www.agra-alliance.org). Within the refreshed IITA Strategy 2012â€“2020, ISFM is one of the main pillars of the natural resource management (NRM) research area.
Whats is ISFM?
We defined ISFM as â€œA 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 principlesâ€ (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 farmersâ€™ 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 â€œ4Râ€ Nutrient Stewardship Frameworkâ€”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.
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).
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 farmerâ€™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.
Successful uptake of ISFM practices
Several examples can be identified where ISFM has made a difference to smallholder farmers, including (1) dual-purpose grain legumeâ€“maize rotations with targeted fertilizer applications pioneered by IITA for the moist savannas (Sanginga et al. 2003) and (2) micro-dose fertilizer applications in legumeâ€“sorghum or legumeâ€“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 legumeâ€“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 cerealâ€“grain legumeâ€“livestock systems in the dry savanna. Agriculture, Ecosystems and Environment, 100: 305â€“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 farmersâ€™ income using a strategic application of fertilizers in West Africa. Pages 201â€“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: 337â€“354.
Vanlauwe, B, A. Bationo,Â 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: 17â€“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: 35â€“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.