[en] Landscape Ecological Survey of the Bipindi-Akom II ... - ITTO

The pH2 of the Nyangong Nyangong topsoils topsoils and subsoils subsoils is in the range of 3.5 to 4.5 (extremely (extremely acid).
Khanna and Ulrich (1984) (I 984) define the pH range 3.8-4.2 as the aluminum buffer range, which
means that aluminum is exchanged against hydrogen with decreasing pH.
The pH of the Ebom soils is 3.5-5 in the topsoils and 4-5 in the subsurface horizons (very
strongly to extremely acid). The pH values of the Ebom soils are only slightly higher than those
of the Nyangong soils, and they are intermediate between the Nyangong and Ebimimbang soils.
The pH values of the Ebimimbang topsoils are 1-1.5 units higher than those of the Nyangong
and Ebom soils. The topsoil pH is 5-6 (medium to strongly acidic), and the subsurface soil pH
is 4.5-5.5 (strongly to very strongly acid). This marked gradient in pH with depth is
characteristic for soils in the cation exchange capacity buffer (4.2-5). Cations are exchanged
against hydrogen to buffer changes in pH. Soils with pH values within this range are
characterized by sharp differences in chemical characteristics (Khanna and Ulrich, 1984), which
is confirmed by the base saturation data of Ebimimbang topsoils and subsurface horizons.
The Valley Bottom soil has pH values that resemble those of the Ebimimbang soil. The topsoil
pH is between 5 and 6 and the subsurface pH is between 4.5 and 5.5.
All subsoils have pH values between 3.5 and 5.5. In these pH ranges manganese (Mn),
aluminum (AI), iron (Fe) and trace elements as copper (Cu), zinc (Zn) and borium (B) are
available for plant growth (Euroconsult, 1989; Landon, 1991). In general the aluminum
concentrations of the soils in the TCP research area are high and exchangeable potassium (K),
magnesium (Mg) and calcium (Ca) concentrations are very low in the soil solution (see Table
5.4b.). Crops grown on these soils may experience nutrient deficiencies. High aluminum
concentrations in the soil solution are toxic to many crops. Low pH values as found in the
Nyangong soils, facilitate the formation of iron and phosphate aluminium compounds. This
phosphate phosphate is not available to plants. The somewhat higher topsoil pH values might be explained
by recent agricultural activities which include the burning of vegetation by which alkaline ashes
are are formed. formed.
5.4.2 Organic carbon and total nitrogen
The organic carbon and related total nitrogen contents of the topsoils differ significantly
between between the the four four soil soil types. types. The The observed observed organic organic carbon carbon range within within the survey survey area area is 2%-9% 2%-9%
and the total nitrogen contents range from 0.15%-0.5%. The topsoils have significantly higher
organic organic carbon carbon and and total total nitrogen nitrogen contents contents than the subsoils. subsoils. Fig. 5.2. shows shows the the relation relation between between
organic carbon and total nitrogen concentrations for for all all samples. The The CIN value calculated with
regression regression analysis analysis on all data is 13 (R2 = 0.94).
The Nyangong topsoils have organic carbon contents between 4% and 9% and total nitrogen
contents between 0.25% and 0.4%. The CIN values of the topsoils are between 12 and 18. The
subsurface horizons have organic carbon contents between 1 1% % and 3 3% % and total nitrogen
contents between 0.1 % and 0.15% (Table 5.4a.). In the subsoils the organic carbon content drops
rapidly to below 0.5%. The CIN values of the subsurface horizons are in the range of 10 to 14.
2 With pH, we mean pH(H 20), 20), unless otherwise indicated.
48