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The Chemistry and Fertility of Soils under Tropical Weeds 97

where  b is soil bulk density, M s is soil mass, V t is soil total volume. The soil bulk
-3
density of mineral soils is about 1.32 g cm . The lower the bulk density of soils
indicates the more porous soils and the more easier for water or O 2 to diffuse into
the soil system.

Table 6.4. The evolution of CO 2 as affected by the substitution of
chemical fertilizers with organic matter *.

-1
No. Treament mg CO 2 -C kg
1 Control 81

2 Organic Matter 0% 210
3 Green Manure 50% 141
4 Green Manure 75% 149
5 Green Manure 100% 154
6 Chicken Manure 50% 293
7 Chicken Manure 75% 268

8 Chicken Manure 100% 412
*Adapted from Yusnaini et al. (2007)

6.2 Effects of Land-Use Conversion

Deforestation transforms most of the primary forest into secondary forest,
plantation, and cultivated land. As shown previously that this transformation shifts
the dominant species of vegetation, including weeds, in West Lampung Indonesia.
Along with this shifting, deforestation also lowers the contents of organic C and
Total N. As shown in Table 6.5, transformation of primary forest lowers its topsoil
organic C content to about 68.5% in secondary forest and to about 47.2% in coffee
plantation, and to even much lower organic C content in intensively cultivated land
to about 28.1%. Similar trend is also shown in subsoils. The total N also shows
similar pattern, with the highest in the primary forest followed by the secondary
forest, coffee plantation, and cultivated land; both in topsoils and subsoils.
Abdul Kadir Salam and Nanik Sriyani – 2019