i Parkia biglobosa - School of Forest Resources & Environmental ...

3000ppm (Ker 1999). Nitrogen content is also low, the main source coming from
biological nitrogen fixation by vegetation from the atmosphere. Parkia biglobosa,
Faidherbia albiza, and groundnuts, are examples of native leguminous trees and plants in
this area that fix nitrogen. Historically, land use in this area was a traditional shifting
cultivation fallow system. This system allowed areas to lay fallow after two or three
seasons of cultivation and natural vegetation would then regenerate. Vegetative cover
reduces the damage from erosion. However, with increasing human and livestock
population, these shallow soils are continually cultivated and browsed leaving the soil
bare and highly susceptible to erosion from rain and wind without adequate protection
provided by trees and vegetative cover.
There are two distinct seasons in this region, the rainy season and dry season.
The rainy season extends from the months of April through October while the dry season
extends from the months of October through March. During the height of the dry season,
from December through February, the desiccating Harmattan winds blows sand and silt
down from the Sahara until March, when the temperatures rise upwards to 49° Celsius.
The rains are intense throughout the rainy season. Tropical rains on average have six to
ten times more erosive power than temperate rains (Ker 1999). Rates of rainfall intensity
are variable but have been measured at 200 mm/h for brief time intervals. The seasonal
distribution of the kinetic energy load, that is the intensity of the rain averaged over the
season, results in 27-33% of the energy load occurring between April and June. These
early rains in the season can cause substantial erosion since the soil is bare of vegetation
(Quansah 1990). Estimates of soil loss are high in the area and with high erosion the
result is declining soil productivity.
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