Agriculture Reference
In-Depth Information
and other natural water sources such as lakes and rivers (Schroder, 1985; Spalding and Exner, 1993;
Davies and Sylvester-Bradley, 1995; Krichmann and Bergstrom, 2001). Krichmann and Bergstrom
(2001) defined organic farming as a form of agriculture based on the following principles: (i) prohibit
the use of synthetic fertilizers and pesticides, and rely upon crop rotation, animal manures, crop
residues, and green manure to maintain adequate soil fertility; (ii) enhance and improve the biologi-
cal conditions for symbiotic N
2
fixation; (iii) emphasize recycling of animal manures; and (iv) create
a balance between the number of animals and the cultivated area. Krichmann and Bergstrom (2001)
reviewed literature on organic farming and nitrate leaching to lower soil profiles or natural water
sources and concluded that lower yields due to lower nutrient inputs in organic cropping systems are
often followed by lower N leaching loads than in more intensive conventional systems.
2.9.12 a
doptInG
a
GroforestrY
In
C
roppInG
s
Ystems
Agroforestry (AGF) is defined as an intensive land management practice that optimizes the ben-
efits (physical, biological, ecological, economical, and social) arising from biophysical interactions
created when trees and/or shrubs are deliberately combined with crops and/or livestock (Garrett
et al., 1994). Including AGF in cropping systems improves internal drainage, enhances infiltration,
reduces runoff, traps sediments and nonpoint source pollution, and creates wildlife habitat and
connective travel corridors (Omernick et al., 1981; Schmitt et al., 1999; Udawatta et al., 2002; Qiu,
2003; Schultz et al., 2009; Gold and Garrett, 2009). The inclusion of trees in AGF buffers integrates
long-term environmental benefits of forest ecosystems such as improved soil physical properties,
moisture storage capacity, soil quality, and carbon sequestration into agricultural land (Seobi et al.,
2005; Kumar et al., 2008; Jose, 2009; Anderson et al., 2009; Senavirante et al., 2012).
Adopting AGF system is an important strategy in reducing soil degradation, improving crop
yields, and N use efficiency (Dossa et al., 2012). AGF systems that combine trees or shrubs with crops
can potentially provide organic inputs and improve nutrient recovery and use efficiency (Mafongoya
et al., 2006). The mechanisms include nutrient recovery from subsoil layers not exposed by the crop
that are deposited on the surface layer through litter input and root turnover, reduced nutrient loss
(Young, 1989; Gathumbi et al., 2004), and improved soil, physical, chemical, and biological proper-
ties (Buresh and Tian, 1998). Hartemink et al. (2000) in Kenya showed that
Sesbania sesban
during
fallow periods can retrieve considerable subsoil inorganic N in highly weathered Alfisol and Oxisol.
This N was shown to be effectively recycled for subsequent crops.
More than 95% of farmland in sub-Saharan Africa is rain-fed, and crop yields are generally
low and variable as a consequence of variable rainfall, drought, and land degradation (Wani et al.,
2009). Corn accounts for >50% of the cropped area and the calories consumed in many countries
in sub-Saharan Africa (Sileshi et al., 2010). Growing corn in association with legume trees in AGF
arrangements has been shown to increase yields in many parts of sub-Saharan Africa (Sileshi et al.,
2012). Sileshi et al. (2011) also reported that integration of legume trees into cropping systems is one
option to mitigating land degradation because they add considerable amounts of organic matter and
N to the soil (Snapp et al., 1998; Mafongoya et al., 2006; Akinnifesi et al., 2007; Beedy et al., 2010).
More than two decades of AGF research in southern Africa has shown that organic matter added
to the system increases the structural stability of the soil, resistance to rainfall impact, infiltration
rates, and faunal and microbial activities (Sileshi and Mafongoya, 2006; Beedy et al., 2010).
Dossa et al. (2012) reported that the indigenous shrub,
Guiera senegalensis
, coexists with crops
to varying degrees in farmer fields throughout the drier Sahel (Africa), as an integral part of the
cropping systems. The presence of this shrub in the cropping systems improved crop yields as com-
pared to plots with no shrub (Dossa et al., 2012). These authors further reported that the improve-
ment in crop yields in association with
Guiera senegalensis
shrub was related to improved nutrient
availability and higher soil quality measured in terms of particulate organic matter content. Dossa
et al. (2012) further reported that the positive crop response in the presence of shrubs in a dry year
suggests that shrubs were not significantly competing with the crop for water. This was indeed
