Agriculture Reference
In-Depth Information
were necessary to increase soil organic C in maize-based systems (Chivenge, Vanlauwe,
and J. Six, 2011).
There is an increasing recognition that integrated use of inorganic fertilizer together
with technologies to improve SOM (such as use of legumes as cover crops, relay crops, man-
aged fallows and other agroforestry (AF) systems, and recycling of manure and composts)
may hold the greatest potential for both short-term livelihood gains and longer-term soil
fertility improvements (Sileshi et al., 2008; Sanchez, 2010; Snapp et al., 1998, 2010; Vanlauwe
et al., 2010; Bekunda et al., 2010; Chivenge, Vanlauwe, and J. Six, 2011). Integrated soil fertil-
ity management (ISFM) is now being widely promoted and can be defined as “a set of soil
fertility management practices that necessarily include the use of fertilizer, organic inputs
and improved germplasm, combined with the knowledge of how to adapt these practices
to local conditions aimed at maximizing agronomic use efficiency of applied nutrients
and improving crop productivity” (Vanlauwe et al., 2010, p. 17). In essence, this approach
encompasses the use of any of the organic matter technologies listed, in conjunction with
supplemental fertilizer, depending on the specific characteristics of the area. Conservation
agriculture is also being promoted as a means to improve SOM levels and crop produc-
tivity, which brings in the added dimension of reduced tillage along with maintenance
of permanent soil cover (by crop residues and cover crops) and diversified crop rotations
or plant associations, including legumes (Meyer, 2009). This approach is proving to be
successful in many regions globally, including sub-Saharan Africa (European Technology
Assessment Group, 2009; Naudin et al., 2010), but there are challenges in some areas related
to soil conditions, competing uses for crop residues, labor requirements for weeding, and
access to herbicides (Gowing and Palmer, 2008; Giller et al., 2009).
To what extent can these various integrated approaches and organic matter technologies
improve SOM levels, soil structure, and soil biology and thus sustainably support higher
crop yields over the long term? Here, we focus on annual cropping system diversifica-
tion with legumes and highlight important findings and areas needing more research,
rather than attempt an extensive review of the literature. Various types of legume systems
are being promoted for improved soil fertility and crop production in different contexts.
These include improved fallows, AF, rotation, green manures, and intercropping systems
(Akinnifesi et al., 2010). Most reviews of legume strategies emphasize the evidence for
increased crop yields relative to current farmer practice.
A recent meta-analysis of 94 studies involving legumes concluded overwhelmingly
that the response to legumes is positive, often resulting in two- to threefold yield increases
in low to moderate-yielding sites (Sileshi et al., 2008). Addition of 50% of the recommended
fertilizer dose further increased yields by more than 25% over legumes alone, indicating
that legumes can significantly reduce fertilizer requirements. In a large on-farm trial in
Malawi, Snapp et al. (2010) also demonstrated the benefits of diversification of cropping
systems, with legumes combined with half the recommended fertilizer rates producing
equivalent yields to fully fertilized monoculture systems and with lower yield variability.
Similar conclusions of improved yields and reduced fertilizer requirements were reached
in a review of the effects of various systems that incorporated leguminous trees and shrubs
in maize systems of eastern and southern Africa (Akinnifesi et al., 2010).
There is a variety of mechanisms by which legumes can potentially improve crop pro-
duction. These include increased N provision through biological nitrogen fixation (BNF),
recycling of nutrients from deeper in the soil profile, improved soil structure and physical
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