Environmental Engineering Reference
In-Depth Information
In northern Ghana, especially in the north-eastern zone, there is a lack of
information on the relationship between the indigenous tree canopies and soil
fertility. This prompted a study in 1995 and 1996 to evaluate the effects of the
canopies of four indigenous woody species on savanna soil properties. This
chapter reports the findings of that study, which was carried out under the United
Nations University Project on People, Land Management, and Environmental
Change (UNU/PLEC).
Study area
The site lies in the natural Sudan savanna in the Upper East region (UER; Map B).
It is bounded by Bawku, Binduri, and Bugri in the Bawku-East district, and
drained by the River Tamne, a tributary of the White Volta River that has many
small dams and river valleys. The upland soils are mainly luvisol, lixisol,
plinthosol, and fluvisol derived from biotite granite. These have sandy topsoils,
sandy clay subsoils, and low organic matter. The valley bottom soils are gleysol
derived from basic metamorphites (mainly greenstone). They are clayey and rich
in organic matter with high stone content, which increases down to the subsoil
(leptosol and cambisol; Hauffe and Jaensch, 1992). The soil/moisture relationship
is extreme. Annual mean rainfall varies from 645 mm to 1,250 mm with a long
mean of 1,044 mm. Rainfall is limited to five months of the year. The remaining
months are mostly dry. Temperatures range from an annual mean minimum of
22.5°C to a mean maximum of 35.6°C. Phosphorus and nitrogen are scarce (Adu,
1969; Boateng and Ayamga, 1992).
Materials and methods
Four naturally occurring indigenous trees,
F. albida
(
A. albida
),
D. mespiliformis
(ebony tree),
V. paradoxa
(sheanut tree), and
P. biglobosa
(
dawadawa
tree), on
two different soil types (upland soil, luvisol; valley soil, gleysol) were studied.
These trees were chosen because of their occurrence on farmlands and their
importance to the communities, as shown in Table 15.1.
The data in Table 15.1 were obtained in discussions with the farmers during
transect walks across the various topographic lines in the study area. The soils
were sampled from under the tree canopies and from open grassland. At
each site under the tree canopies, samples were taken from four different spots
and hand mixed together to give a bulk sample (about 1.5 m from the tree
trunk). Soil samples were also taken from the open grassland. A total of
48 samples each were collected on luvisol and gleysol. All samples were taken
at a depth of 15 cm from cultivated soils. Leaf samples from the trees were
