akinsanya, olawale - The Federal University of Agriculture, Abeokuta

CHAPTER
FIVE
DISCUSSION
The differences observed in particle size distribution according to the levels of degradation (Table 1)
were due to selective erosion of soil particles by runoff. The severely degraded site had lost most of
its fine particles while these were still retained more on the fairly and moderately degraded strips.
The texture of ~oil, derived from the particle size distribution, determines the potential ability of a soil
to influence crop performance. According to Wild (1993), sandy soils are light and low in nutrient
holding capacity. Ross (1989) stated further that sandy soils are porous and highly vulnerable to
leaching and run-off. Thus, sandy soils are infertile and make fertilizer use inefficient. The
implication of this is that more expenditure in terms of agricultural inputs will be required to
rehabilitate a degraded site. The coefficients of variation for particle size distribution showed that
fine particles (silt + clay) would play a critical role in response of these soils to management
practices as they varied more than sand. More variation was observed under WAS varieties
compared with ITA varieties, suggesting that varieties might have influenced changes in particle size
distribution through differences in canopy coverage of the ground.
Tropical upland soils are inherently low in soil organic matter with usually less than 50 glkg
(David et ai, 1993). The low values of organic matter (Table 1) suggested that the soil, which was
inherenijy low in soil organic matter, was depleted more at various levels of degradation, with the
trend observed from the fairly degraded to the severely degraded strips. Soil extractable P varies
widely in soils of the tropics because of the capacity of tropical soils to fix P and varying capacity of
.crops for P (Sanchez et aI., 1983). The variation in P contents at the various levels of degradation
can be attributed to differences in particle size distribution, particularly the high variations in fine
particles (Tables 1 and 2), and variation in nutrient uptake of the rice varieties.
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