Agriculture Reference
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(Toenniessen et al., 2008), and adopting new policies that encourage input use by farmers
and fair prices for their produce (Sanginga, 2010). Some envisage a tripling of cereal grain
yields through the use of inorganic and organic fertilizers and high-yielding crop variet-
ies, farmer education and empowerment, and improved markets (Sanchez, 2010). Others
emphasize the need for interdisciplinary and participatory system approaches that recog-
nize the importance of socioeconomic and biophysical context, make effective use of local
resources, use well-adapted crop varieties and livestock breeds, and involve judicious use
of external inputs when needed (NRC, 2010; IAASTD, 2009).
Improving soil fertility is a central goal for sustainable agriculture in Africa (IAASTD,
2009; NRC, 2010). Although use of improved cultivars that are more productive, drought
tolerant, and resistant to pests and diseases is an important intervention, it is clear that
poor soil fertility is a major constraint to increased crop productivity (Sanchez, 2010). This
has led some to argue that a primary focus on increasing fertilizer use, together with the
use of improved crop varieties where possible, is needed to improve yields and livelihoods
across many African cropping systems (Vanlauwe et al., 2010; Sanchez, 2010). Others cau-
tion that while fertilizer use may be an important component, an overreliance on pur-
chased inputs makes resource-poor farmers vulnerable to what can be wide fluctuations
in price and availability, and that high prices put fertilizer out of reach for resource-poor
farmers (Denning et al., 2009; NRC, 2010; Sirrine, Shennan, Snapp, et al., 2010; Snapp et al.,
2010). Systems that rely on and enhance locally available fertility resources also need to be
sought.
The reasons for low fertilizer use and the myriad challenges to increasing fertil-
izer use effectively in Africa, especially among the poorest farmers, were aptly described
by Morris et al. (2007). Issues such as low value/cost ratios for fertilizer use, high price
instability, high yield response variability, and high costs of or limited access to credit led
the authors to suggest that low fertilizer use may in fact be a rational response to manag-
ing risk by African farmers (Morris et al., 2007).
Degraded soils are a severe problem in much of sub-Saharan Africa (Bekunda et al.,
2010), and significant declines in soil organic matter (SOM) are well documented (Moebius-
Clune et al., 2011; Joergensen, 2010; Lal, 2006). A review of organic matter in the tropics as a
whole highlighted the lower levels of total soil carbon (C) and nitrogen (N) associated with
arable farming as compared to forest and pastures, with even greater differences observed
in microbial biomass C and N (Joergensen, 2010). Further, the amount of land converted
from forest and savannah to cropland is still increasing in sub-Saharan Africa (Verchot,
2010), a problem exacerbated by degradation of cropland soils.
The loss of SOM affects not only nutrient availability but also other aspects of soil
biological and physical functions that often cannot be rectified by use of fertilizers alone.
SOM levels have an impact on soil physical structure and hence aggregate stability, water
infiltration rates, and water-holding capacity. Indeed, in addition to fertility limitation,
poor soil physical structure, reduced resistance to erosion, and reduced drought resis-
tance are thought to be responsible for low crop yields observed in the region (Lal, 2006).
Organic matter inputs to soil are also critical for supporting a diverse and active soil
microbial community and macrofauna responsible for residue decomposition, carbon
turnover, nutrient cycling, and other functions, such as disease and pest suppression
(Joergensen, 2010; Akinnifesi et al., 2010; Shennan, 2008). While use of inorganic fertil-
izers can increase yields, when used alone they may not provide enough crop residue to
affect SOM positively. Indeed, a meta-analysis concluded that organic resource additions
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