Environmental Engineering Reference
In-Depth Information
Discussion
A number of studies have reported higher values for soil chemical properties
under tropical tree canopies (Kellman, 1979; Hattan and Smart, 1984; Belsky
et al., 1989; Isichei and Muoihalu, 1992) than in the open grassland, as found in
this study. Because of the accumulation of leaf fall, the higher organic matter con-
tent beneath tree canopies reduces leaching, slows the rate of mineralization as a
result of reduction in temperature, and induces a more favourable microclimate
under the trees. Other advantages include deposition of manure and urine there
by livestock seeking shade or food, nitrogen fixation by the trees (
F. albida
), and
recycling by their deep roots of nutrients from lower soil levels. The high total
nitrogen content under tree canopies is also likely due to the high nitrogen con-
tent of their leaves as compared to the poor nitrogen content of the natural grass
vegetation in grass-dominated savanna soils (Jones, 1973).
As there is a direct correlation between soil organic matter level and cation
exchange capacity, it was not surprising that the cation exchange capacity of the
soils under the tree canopies was higher than in the open grassland. Among the
tree species the high values of chemical properties of soil under
F. albida
are
due to the relatively high nutrient content of its leaves. The low values of N:P
ratio could not have been accounted for by the immobilization of the N and P
under tree canopies. There was no significant difference in available soil phos-
phorus for the gleysol between tree canopies and open grassland. This is because
of the higher clay content of gleysol, which prevents leaching in both circum-
stances, and also the low crop removals of phosphorus from the soil.
Among the two soil types, the soil chemical properties in gleysol were
higher than in luvisol. This might be due to the difference in texture. It has
been reported that in West African savanna (Jones, 1973) and in East Africa,
soil texture is the most important factor controlling organic matter and, conse-
quently, other soil chemical properties. Differences may also be due to the
parent rock material and topography since luvisol are mostly upland and
gleysol are in valleys. Moisture and other relations could also be the result of
their stated differences as well.
Policy implication findings
The results of the study show that indigenous savanna trees have a beneficial
effect on soil nutrient status and, therefore, on soil productivity in Ghanaian
savanna soils. Accordingly farmers need to be encouraged to cultivate and main-
tain these trees on their farms. Also there is need to plant indigenous trees in agro-
forestry systems. It is surprising that indigenous trees have been largely ignored
in agroforestry systems in the savanna areas of Ghana.
