Agriculture Reference
In-Depth Information
with resources; there is often poor access to markets in sparsely populated areas so
that fertilizer costs are high; and trees offer fodder and other tree products that help
households diversify agricultural enterprises and reduce climatic risks.
A recent study from the Sahel (Place and Binam 2013) found that crop yields were
significantly higher in fields where mature trees beneficial to soils were found than
in treeless plots. This was due to the direct effects of the trees and also an indirect
effect by which farmers were more likely to apply manure and fertilizer on fields
on which such trees were found. In quantitative terms, yields of millet and sorghum
were often found to be 15%-30% higher on fields with an average cover of mature
soil-enhancing trees such as
Faidherbia
,
controlling for other inputs, rainfall and
soil type. A similar finding is reported in the case of mature stands of
Faidherbia
in
Malawi, where yields of maize (
Zea mays
) were 15% higher controlling for other fac-
tors (Glenn 2012). Given the low costs involved in establishing and managing trees
through regeneration, the main constraints are related to abiotic and biotic factors
such as aridity, fire, grazing, and the lack of germplasm in the soil. However, one
policy constraint is particularly notable in the Sahel. That is the restriction on cut-
ting, pruning, and transport of trees under the forest codes common in West Africa.
These are intended to protect valued indigenous parkland trees; however, now that
many trees are actually on farms, it has the perverse effect of inducing farmers to
remove young seedlings before they mature (Yatich et al. 2013).
There are more opportunities for using agroforestry to improve soil fertility in
the subhumid and humid regions. There, trees are commonly planted as seeds or
seedlings, and this allows farmers much more control over the densities and species
of trees they wish to use. Growing faster in the more humid areas, the trees can also
provide quicker impacts on yields, which is desirable to farmers. For example, an
improved fallow or dense intercrop practice can already raise yields after 2 years due
mainly to the nutrients from leaf fall and incorporation (Sileshi et al. 2008), while soil
physical and biological properties are slower to improve from trees. In both of these
practices, trees are not allowed to reach mature height as they are either removed after
a short time in the case of fallowing or regularly pruned in the case of intercrops.
The economic return from practicing improved fallows has been well studied for
southern Africa and to some extent elsewhere. In Zambia, improved fallows per-
formed much better than continuous maize production without fertilizer (Franzel
2004; Ajayi et al. 2007, 2009). In the recommended 5-year cycle of 2 years fallow
and 3 years cropping, the net profit from unfertilized maize was only US$130 per
hectare against US$269 and US$309 for maize grown under fallows using
Gliricidia
(
Gliricidia sepium
)
or
Sesbania
(
Sesbania sesban
) species. The use of mineral fertil-
izer for 5 years provided higher net profits; however, returns to labor were similar to
those of improved fallows. Studies of farmer behavior in eastern Zambia revealed
that almost three-quarters of farmers exposed to the practice in the 1990s and early
2000s were still using it by 2009 (Kabwe 2010).
The practice of planting trees for soil fertility is not a traditional farming practice,
probably anywhere in the world. Thus, a major constraint to the practice is awareness
of its potential and understanding of how to put it into practice. A related constraint
is availability of germplasm of trees with soil amelioration benefits. Further work
with farmers has found that even if those barriers are removed, other difficulties or
