1t0SC7l

Greenland, 1960; Sanchez, 1976; Volkoff et al., 1989; Gerold, 1995). In the study area, the
topsoil structure and porosity changes may be the most important soil degradation characteristics
and the main cause of field plot abandonment and shift.
Results show that presently, the shifting characteristic of the cropping system is defined in two
directions:
- The first direction is that of moving within natural fallow plots of different ages. This
constitutes a rotational system, which is common practice nowadays. It concerns mainly food
crop fields. 76% of the food crop fields in the area rely on this rotational system.
- The second direction is that of agricultural fields expanding into undisturbed forestlands. This
is the traditional pure expansive system, which constitutes more threat to forest conservation
and sustainable management system than the rotational system. This system which concerns
24% of the food crop fields is practised mainly for cucumber (Cucumeropsis mannii)
production and for the establishment of semi-industrial farms by the elite. For this system, an
average of 2200 m² of new forest land is opened per household per year in the TCP area. This
average corresponds to about 300 m² per inhabitant. The Ebimimbang plains and Mvié
uplands have the highest rate of forest encroachment per year and per household with values
of 2450 m² and 2650 m² respectively. The higher rates in these two units are probably
because the land is better accessible for agriculture, leading to an increase in population, and
because the areas have been commercially logged several times (Table 4.2).
Table 4.2. Population variation with landform units in the TCP area
Plains and Uplands of
Ebimimbang and Mvié
Hills and Mountains of
Ebom and Nyangong
Population 1593 992
Household 178 137
4.3.2 Relationship between soil degradation and crop yield
Because of the absence of any significant difference between 0-10 cm and 10-20 cm slices,
results of these analyses are presented for the top 0-20 cm soil layer. The analysis took into
account the two main soil types of the study area. Multiple linear regression coefficients from
these analyses are shown in Table 4.3.
Table 4.3. Multiple linear regression equations of various crop yields as functions of physico-chemical soil
properties in 0-20 cm soil slice (at p