A fourth principle is required to define Conservation Agriculture in sub-Saharan Africa: The appropriate use of fertilizer to enhance crop productivity
Introduction
In recent years, Conservation Agriculture (CA) has been promoted to intensify smallholder farming system sustainably in sub-Saharan Africa (SSA) (Benites and Ashburner, 2003, FAO, 2011). Conservation Agriculture is commonly defined around a set of three principles: minimum tillage, soil surface cover, and diversified crop rotations. One of the main justifications for promoting CA in Africa is its widespread use in large-scale farming in various parts of the world (Bolliger et al., 2006, Kassam et al., 2009), with some adoption by smallholders, e.g., in southern Brazil and Paraguay (Evers and Agostini, 2001). This has been partly driven by the presence of an enabling environment, including the availability of herbicides. It is important to note, however, that definitions of ‘smallholder farmers’ are not consistent. In southern Brazil, a smallholder is classified as farming less than 50 ha, whereas in SSA, smallholder farmers commonly have access to less than 2 ha. While some of the initially hypothesized benefits of CA including soil carbon sequestration and increased crop yields have not been unequivocally confirmed (Govaerts et al., 2009, Luo et al., 2010), CA often results in more stable and economically favourable yields, usually after a number of years after conversion from conventional agriculture (Knowler and Bradshaw, 2007, Rusinamhodzi et al., 2011).
Uptake of CA by smallholders in SSA remains limited (Kassam et al., 2009, Andersson and D'Souza, 2013) and a number of important constraints to widespread adoption have been highlighted. The lack of organic resources to provide sufficient surface mulch consistently ranks amongst the top constraints (Erenstein, 2002, Giller et al., 2009), especially in areas with high livestock feed requirements (Valbuena et al., 2012). Minimal tillage without surface mulch usually results in depressed yields (Verhulst et al., 2011, Baudron et al., 2012, Thierfelder et al., 2013), partly because mulch provides the necessary conditions for reduced run-off and soil moisture conservation, notably in drier climates (Mupangwa et al., 2012). Mulch also ensures that the physical conditions of the topsoil are conducive for seed germination and initial crop growth.
In this paper we argue that a fourth principle – the appropriate use of fertilizer – is required to define CA to enhance both crop productivity and produce sufficient crop residues to ensure soil cover under smallholder conditions in SSA. This paper is not meant to advocate CA but argues that without acknowledging this fourth principle the chance that CA will achieve success, especially with smallholder farmers, is limited. Fertilizer application is proposed as a separate principle for CA in contrast to other agronomic practices, including planting time, spacing, and weeding regime, because fertilizer is essential for CA to work, whilst the sub-optimal implementation of other crop management practices do not lead to the failure of CA as such. Moreover, given the reframing of CA in SSA from a resource-saving to a productivity-enhancing paradigm (www.fao.org, Andersson and D'Souza, 2013), and recognizing that fertilizer use is essential to raise crop productivity on most African soils (e.g., Mwangi, 1997, Abuja Fertilizer Summit, 2006), the proposed fourth principle is fully aligned to this reframing of CA. Although this principle also applies to other continents, we chose to focus this paper on SSA since this is the continent where CA is commonly promoted in areas with little or no availability or use of fertilizer.
Section snippets
The origin of CA and its three principles
The CA revolution in the tropics started in the 1970s with large-scale farmers in Brazil (Landers, 1999), and spread to other countries in Latin America and certain parts of South Asia (e.g., the Indo-Gangetic basin; Gupta and Sayre, 2007). Minimum or no-tillage with mulching was advocated to reduce the impact of rain on exposed soil and consequent soil erosion losses (Landers, 1999, Roose and Barthes, 2001). An important benefit of minimum or no-tillage was the energy saved by eliminating
Smallholder farming and CA
Smallholder farming conditions in SSA differ from large-scale farming situations in many aspects relevant to implementation of CA. First of all, smallholder crop yields are often poor due to the limited use of agro-inputs and labour (e.g., Tittonell and Giller, 2013), with little crop residue produced as a consequence. Secondly, in many smallholder farming systems in SSA, there are competing demands on available crop residues, especially for livestock feed (Giller et al., 2009, Valbuena et al.,
The quest for organic resources
Before the widespread promotion of CA, many attempts were made to enhance the availability of organic resources in smallholder farms. These were mainly driven by the search for low-input agricultural practices and the widespread belief that fertilizers were beyond the reach of African smallholder farmers. Examples include alley cropping systems (e.g., Kang et al., 1981), integration of herbaceous legumes (e.g., Carsky et al., 2001), improved legume tree fallows and biomass transfer systems
Appropriate fertilizer use as a fourth principle for CA in smallholder systems in Africa
The lack of organic resources is a major constraint to adoption of CA by smallholder farmers. Appropriate use of fertilizer results in substantial increases in crop productivity and in the availability of crop residues, across within-farm soil fertility gradients, and more so on mid-distant and remote fields, which constitute the majority of the fields in smallholder farming systems (e.g., Fig. 1b). These findings suggest that crop production in SSA would greatly benefit from fertilizer
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