Agriculture Reference
In-Depth Information
use efficiency. Trees can reduce the unproductive components of the water balance
such as runoff, soil evaporation, and drainage (Ong and Swallow 2003).
11.1.5.2 Weed Suppression
Improved fallows can control
Striga
(
Striga hermonthica
), a parasitic weed that
causes large yield losses in many cereal crops (Barrios et al. 1998; Gacheru and Rao
2001). Although the processes are not well understood, it is suspected that the fallow
species excrete substances that cause suicidal early germination of
Striga
(Jama et
al. 2006). Improved fallows also act as a break crop by smothering weeds, shading
weeds, and outcompeting weeds for nutrients. Increased weed suppression increases
nutrient availability for plant uptake and thus results in increased crop yields.
11.1.5.3 Soil Nutrient Content
System productivity can also be increased by trees through the reduction of nutrient
losses from leaching in deep soil, reduced soil erosion, protection against wind, and
reduction of weed populations and aggressiveness (Leibman and Gallandt 1997).
Trees have the potential to increase overall system productivity by increasing nutri-
ent availability through N fixation. Trees with perennial mycorrhizal associations
may improve the efficiency of phosphorus (P) cycling because of greater absorptive
area and of increasing pools of organic P (de Carvalho et al. 2010). Deep-rooting
trees may pump nutrients from below the crop rooting zone (Harawa et al. 2006),
both those made available by weathering of the bedrock and those leached down
from the upper layers (Young 1989).
11.2
SOIL FERTILITY REPLENISHMENT
11.2.1 S
oil
f
fertility,
r
ePleniShment
with
A
groforeStry
Many smallholder farming areas in southern and eastern Africa have low potential
for agriculture because of poor soil fertility and low SOC content, which also causes
soils to have a low water-holding capacity. The low soil fertility in these soils is
widely recognized as a major factor contributing to low agricultural productivity in
southern Africa (Vanlauwe and Giller 2006). Reduction in soil fertility is driven by
increased human population that has reduced land availability and caused a break-
down of traditional fallow systems that smallholder farmers relied on for soil fertil-
ity replenishment (Jurion and Henry 1969). This has forced people to farm in more
fragile lands. In Zambia, unavailability and inability of most smallholder farmers
to purchase inorganic fertilizers following the removal of government agricultural
subsidies have resulted in a reduction in crop productivity (Howard and Mungoma
1996). A combination of low soil fertility, lack of fertilizer amendments, and poor
rainfall has often resulted in decreased crop production and widespread food short-
ages. This reduction eventually led to a reduction in the ability of most countries in
the region to provide a stable food supply for their people.
Agroforestry is one of the many sustainable options that can assist smallholder
farmers to replenish their impoverished soils (Ajayi et al. 2009). The use of agrofor-
estry technologies has increased crop yields in smallholder farming areas (Kwesiga
