Agriculture Reference
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it from decomposition. After an improved fallow of 1-1.5 years with various spe-
cies in Kenya, Albrecht and Kandji (2003) found an increase in SOC in the top
30 cm by 1.69-2.15 Mg ha −1 C in coarse soils, and by 2.58-8.34 Mg ha −1 C in fine-
textured soils. Because agroforestry increases biomass addition to SOC, it increases
soil aggregation, while woody cover and leaf litter from agroforestry protect topsoils
from erosion, increasing the SOC that remains in the soil profile.
11.3.2.3 Intercrops, Improved Fallows, and Woodlots
Smallholders in eastern and southern Africa manage agroforestry species as inter-
crops with food crops such as maize or as improved fallows to replenish soil fertil-
ity and/or provide timber and fuelwood (Akinnifesi et al. 2010). Three of the most
widely used management patterns for agroforestry in eastern and southern Africa are
intercrops, improved fallows, and rotational woodlots.
Intercropping systems are most appropriate for smallholders in relatively
population-dense areas with small landholdings, as no land has to be removed from
cereal production to include the agroforestry species. Intercrop populations will vary
across different cropping systems and ecological conditions. Gliricidia , for example,
was planted in alternating planting ridges, at 0.9 m distance between trees within a
ridge, and 0.75 m distance between ridges in Malawi, with maize planted in the ridges
at 44,400 plants ha −1 (Makumba et al. 2007). These Gliricidia populations may also
be used in minimum tillage systems. Makumba et al. (2007) reported 123 and 149
Mg ha −1 of soil C (Table 11.4) in 10 and 7-year intercrops in Malawi (Makumba et
al. 2007). Soil C in the intercrop was roughly double that in unfertilized sole maize,
at 64 and 73 Mg ha −1 after 7 and 10 years, respectively. The 7-year study also accu-
mulated 1.2 times as much soil C under the maize- Gliricidia intercrop as under an
adjoining grass fallow. Unfertilized maize in this study represents the loss of soil C
when such soils are cropped continuously to maize with no inputs.
A 10-year intercrop in Zambia with three agroforestry intercrop species ranged
from 225 to 245 Mg ha −1 of soil C (Kaonga and Bayliss-Smith 2009), similar to the
245 Mg ha −1 found in natural fallow, but 1.3-1.6 times as much soil C as found in
fertilized maize and miombo treatments. These differences may be, in part, due to
a decline in soil C in the fertilized maize treatment. Thus, including agroforestry
intercrops in a cropping system can maintain and increase soil C compared with
the natural miombo vegetation and to cropping systems with only mineral fertil-
izer added. In another study from the same location, Kaonga and Coleman (2008)
reported an increase from an initial soil C of 26.2 Mg ha −1 up to 37.4 Mg ha −1 during
10 years in the upper 20 cm of the soils.
Improved fallow agroforestry is appropriate for smallholders in low-population-
density areas, where landholdings are larger and land can be spared for improved
fallows. The trees are established and left for 1.5-2 years to develop leaf biomass that
is incorporated into the soil before the crop planting phase. Kaonga and Coleman
(2008) reported that Tephrosia , pigeon pea, and Sesbania were established at 1 m by
1 m spacing, and maintained for 2 years. During land preparation for the third year,
the trees were cut to 10 cm, the stems and branches harvested for fuelwood, and the
leaves and twigs incorporated into the soil. Maize then replaced the trees in a 2-year
cropping phase.
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