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The magnitudes of changes are inversely related to the age of the vegetation cleared. Young
fallow (Chromolaena fallow) produces more ash than forest fallow. Forest fallow produces more
biomass, but this biomass consists largely of trunks and large branches that hardly burn when the
farmer sets fire to the cut vegetative material. Van Reuler (1996) reports that after a short airdrying
period of slashed vegetation, 45% of total biomass could be burnt on a 4 years fallow
against 15% for a 20 years fallow. The amount of fully burned material is then related to the age
of the cleared vegetation and decreases in the following order: CF>BF>FF>=FV. Consequently,
chemical changes in soil are in the same sequence. Fields from CF have a higher pH, and because
of the substantial increase in calcium and available phosphorus, this type of fallow is commonly
used by farmers to grow groundnut (Arachis hypogaea). While FF and FV, because of their
relative richness in decomposable soil organic matter, are preferentially used to grow cucumber
(Cucumeropsis mannii) and other vegetables.
The chemical and nutrient effects of burning are generally considered to be short-lived, with soil
pH values returning to pre-burnt levels within 3 years in low to moderately burned areas
(Chandler et al., 1983). Factors influencing the persistence of these changes include leaching by
rainwater, burn intensity, erosional removal of ash, and crop harvest.
Soil physical property changes. The immediate soil surface heating by burning and tillage have
some direct effect on soil physical properties such as reducing aggregates stability (Table 3.1) and
soil structure. The heat of the fire changes the consistence of topsoil and makes it more friable.
The loose consistence provides a good seedbed. Burning destroys the humic porous topsoil A 1
horizon of old fallow as shown in Figure 3.4.
Soil microbial biomass. Burning contributes to partial sterilisation of the soil although microbial
populations rapidly increase after the heating effect (Jordan, 1985). The heating of the soil
destroys weed seeds and reduces insect, nematodes, and various pathogen populations. This in
turn reduces the soil microbial biomass. On clayey Oxisols of the study area, soil microbial
biomass was 10 to 45% lower under cropped land than under the adjacent fallow land (see Table
3.1).
Conclusion. The results of the study on the effects of clearing and burning on soils properties in
the study area are in line with the actual knowledge (Sanchez, 1976) that the presence of ash from
high intensity burns has several important effects on soil. The amounts of plant-available
nutrients in the soil are increased as a result of the fire-induced release of organically bound
nutrients such as K, Ca, Mg, and P. The dissolution and leaching of white ash results in soil pH
values increasing by 5 to 15%, which promote changes in soil mineralogy, and presumably, other
materials with pH-dependent charge. For example, the step-by-step reaction of hydroxylation of
aluminium may proceed, leading to insolubilization of the toxic Al cation, which occurs in
substantial amounts in low pH soils. This process can lead to the formation of gibbsite (Al
precipitate) and a decrease in soil Al toxicity.
Cropping phase
After slashing and burning, the existing equilibrium between the natural fallow vegetation and
soil is broken and a new cycle is established between disturbed soil and crops / weeds vegetation.
Processes acting in this new system result also on changes in soil status.
Soil physical property status. During the cropping period, considerable deterioration of soil
physical conditions occurs. Erosion, crusting and sealing phenomena contribute to changes in
topsoil compaction, porosity and bulk density (Figure 3.1). These soils become more compact and
denser. Compared to soil under undisturbed forest (FV), the bulk density increases by 10 to 20%
during the cropping period and thus, inversely does the soil porosity. Clay content also decreases
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