Agriculture Reference
In-Depth Information
Africa retain indigenous trees on their farm fields after converting original
woodland to cropland because they know that trees modify microclimates and
thus improve agricultural crop production, and that they protect fields that are
susceptible to water and wind erosion. This type of land-use practice is referred to
as an agroforestry parkland system (Boffa, 1999). Thus, maintaining trees on the
farm is traditionally the rule rather than the exception in the drylands of Eastern
Africa. In the absence of trees, mineral fertilization alone is not sufficient to
sustain crop production in dryland agriculture because of the lack of organic
matter in the soil. Commercial fertilizers are beyond the economic means of
many farmers.
Trees in agroforestry parkland systems have been reported to influence the
fertility of dryland soils significantly by maintaining soil organic matter (Young,
1997). Evidence of the influence of trees in agroforestry parkland systems on
soil fertility comes from studies that compare soil fertility status with the
productivity of crops under tree crowns and in the open. Studies based on
chemical analysis of soils beneath some of the priority indigenous fruit trees
discussed above have shown a common pattern of superior soil fertility under
isolated canopies of trees than in areas distant from the trees. For example,
Kater et al . (1992) and Bayala et al . (2002) found higher levels of carbon, total
phosphorus and potassium under Vitellaria paradoxa crowns than in the open.
Higher levels of nitrogen, phosphorus, potassium and calcium under crowns of
Adansonia digitata than in the open were also reported by Belsky et al . (1989).
The generally higher soil nutrient status under tree canopies is also reflected in
the mineral content of understorey herbaceous species. For example,
Bernhard-Reversat (1982) found that the nitrogen content of aerial herb parts
was higher under Balanites aegyptiaca than in the open.
Several factors contribute to the higher fertility of soils under tree crowns
than in the open. These include the greater soil microbial activity under trees
than in the open as a result of the high level of nutrient accumulation under
trees through litterfall, root decay and exudation; washout and leaching of
nutrients stored in tree canopies; nitrogen fixation by trees; dung deposition by
livestock that use the trees as shade; and faeces dropped by birds that nest in
the trees (Belsky et al ., 1989; Tomlinson et al ., 1995; Boffa, 1999).
Increases in organic matter and improved microclimatic conditions under
trees enhance soil microbial activity, organic matter decomposition and soil
physical characteristics. For example, Belsky et al . (1989) reported 35-60%
higher soil microbial biomass carbon, a lower bulk density of topsoil and higher
water infiltration rates under Adansonia digitata crowns than in the open.
Fine soil particles lost through wind erosion are intercepted by trees and
deposited by throughfall and stemflow. For example, Roose et al . (1974, cited in
Boffa, 1999) reported that rainwater collected under Vitellaria paradoxa
canopies had higher concentrations of nitrogen, phosphorus, potassium, carbon,
calcium and magnesium than in the open. Evergreen trees such as Balanites
aegyptiaca , Cordeauxia edulis , Tamarindus indica and Ziziphus mauritiana may
also play an important role in dust deposition as they retain their leaves during
the dry season, when strong, moist, soil-laden winds prevail in drylands.
However, no studies have been reported on these trees in this regard.
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