Agriculture Reference
In-Depth Information
It is interesting to note that fields classified by farmers as productive versus poor dif-
fered in many of the same measures as discussed, with productive soils having higher
total C and N, labile C, microbial biomass C and N, N mineralization, and soil respiration,
as well as increased effective cation exchange capacity, exchangeable cations, and extract-
able P (Murage et al., 2000). By preferentially applying residues, manures, or fertilizers to
their most productive fields, farmers ensured that at least some of their land had good
SOM and fertility levels (Mtambanengwe and Mapfumo, 2008).
Clearly, legumes can have many beneficial impacts on soil quality, in large part by
impacting nutrient cycling through soil microbial populations, but legume BNF and pro-
ductivity are often suboptimal due to soil fertility or other limitations. There is consider-
able interest in whether soil microbial populations can be altered to enhance different
functions such as BNF, or increase P availability, through the addition of beneficial bacte-
ria or fungi. A publication summarized the current state of use of microbial soil amend-
ments or biofertilizers in the tropics (Uribe et al., 2010). Most of the work on biofertilizers
has focused on Latin America and Asia, with relatively few studies in Africa. The main
types of materials being developed are inoculants of symbiotic BNF bacteria (rhizobia),
nonsymbiotic BNF bacteria, phosphorus-solubilizing bacteria (Yarzabal, 2010), and AM
fungi. All have had success in increasing yields of various crops in experimental trials but
suffered from variable performance in field situations for a variety of possible reasons.
Challenges remain in the production of high-quality, low-cost inoculants, as well as
a lack of understanding of species- and site-specific effects. These can include inoculant
survival issues, competition from indigenous microorganisms, soil type and fertility
effects, and differential interactions based on crop species and varieties. From the discus-
sion, however, given the importance of legumes for helping improve African soils that are
degraded and with low organic matter, further research on biofertilizers is warranted.
One example of a product developed in Africa is PREP PAC, an inexpensive mix of urea,
rock phosphate, legume seeds, and rhizobium, which is being tested in participatory on-
farm trials (Okalebo et al., 2006).
sustainability of production systems
In all probability, increased use of fertilizers, legumes, AF, ISFM, conservation agriculture,
improved seed, and crop/livestock integration will all play a role in improving food pro-
duction and soil quality in Africa (NRC, 2010), but no single technology package is likely
to be broadly applicable across the diversity of systems in the region. Rather, it can be
argued that a systems approach with interdisciplinary research that is participatory and
grounded in the local context and needs is required to develop locally appropriate and
sustainable solutions (NRC, 2010; InterAcademy Council, 2004; Pretty, 2008; IAASTD, 2009;
Snapp and Pound, 2008). To identify what are the most locally appropriate and sustainable
systems among different options requires us to think about how we assess “appropriate-
ness” and sustainability.
Gains in agricultural production must be sustained into the future, yet what is involved
in creating sustainable systems and what makes a system sustainable are complex ques-
tions that merit critical exploration. It is not simply about increasing yields; agricultural
sustainability also encompasses other considerations, such as longer-term impacts on the
environment and resource base (especially soil), as well as on farmers' livelihoods. As dis-
cussed in the NRC report
Sustainable Agricultural Systems in the 21st Century
(NRC, 2010),
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