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68 The Chemistry and Fertility of Soils under Tropical Weeds

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time was lower i.e. 18.5 cmol c kg , about the value of subsoil CEC of the primary
forest. The soil CEC of the coffee plantation and the cultivated land were about the
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same, 11.4 and 14.4 cmol c kg , respectively. These values were much lower than
those of the primary and the secondary forests.

Table 4.6. Effects of land-use conversion on some soil chemical properties in Bukit Ringgis,
West Lampung, Indonesia* (after Salam, 2014).

Soil Elev. pH Org. C Tot. N Av. P CEC
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Depth (m) (H 2O 1:1) (g kg ) (g kg ) (mg kg ) (cmol c kg )
(cm)
Primary 0 – 20 4.4 60.4 5.5 4.0 43.2
1,550
Forest 20 – 40 4.8 25.0 2.3 1.5 18.2
Secondary 0 – 20 5.4 41.4 3.4 2.1 18.5
1,400
Forest 20 – 40 4.9 21.7 2.0 1.5 13.7
Coffee 0 – 20 4.9 28.5 2.3 1.5 11.4
1,120
Plantation 20 – 40 4.9 10.1 1.2 1.0 12.5

Cultivated 0 – 20 4.4 15.8 1.7 1.5 12.4
1,100
Land 20 – 40 4.3 7.5 0.8 0.7 1.2

*Adapted from Salam et al. (1998b)

The decrease in soil CEC is related to the decrease in soil organic C and organic
matter contents (Soil Organic Matter = 1.7 x Organic C) (Table 4.6). Organic matter
is one of the important sources of soil negative charges emerged from organic
matter functional groups like carboxylics, hydroxyls, and phenolics. Speeded
organic matter decomposition by land-use conversion might have significantly
degraded these functional groups. Therefore, the degradation of soil CEC is well-
correlated with the soil C content. In relation to this phenomenon that occurred by
organic matter decomposition, the soil total N and available P were also degraded

Abdul Kadir Salam and Nanik Sriyani – 2019