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The Chemistry and Fertility of Soils under Tropical Weeds 57

plants. Paspalum conjugatum was also observed to give higher effects that did
natural vegetation. The same effect but with lower values was also observed in
subsoils (Salam, 1996; Salam et al., 2001). The natural vegetation comprises some
dominant vegetation or weeds i.e. Chromolaena odorata, Clibadium surinamense,
Cludemia hirtam, Imperata cylindrica, Melastoa affine, Mikania micrantha, and P.
conjugatum (Oki et al., 1999).

Table 4.2. The changes in soil pH, organic C, total N, and available P of
cassava plantation in Gunung Batin Central Lampung Indonesia*
(After Salam, 2014).

Period of
Organic C Total N Available P
Cultivation pH -1 -1 -1
(g kg ) (g kg ) (mg kg )
(Years)
0** 4.7 43.0 2.60 5.03
1 - 5 4.5 20.0 1.00 24.7

6 - 10 4.4 12.8 0.80 7.70

*Adapted from Salam et al. (1999b); **Bushes adjacent to the plantation.

4.2 The Effects on Soil Enzymatic Activities

To obtain energy from organic matters, soil organisms including
microorganisms, macroorganisms, and plant roots produce some enzymes. These
enzymes work on the bio-cycles of some nutrients in the soil – plant system
(Tabatabai, 1982; Tate III, 1987). Like any other enzymes, as biocatalysts these soil
enzymes accelerate the soil biochemical reactions that change reactants to form
new products without themselves involve in the reactions. After any enzymatic
reactions, the soil enzymes may return to their original forms and may re-
participate in the same biochemical reactions. There are several enzymes
produced by microorganisms, macroorganisms (for example earthworms), and
plant roots (Hayano, 1973; Alexander, 1977; Ross and Cairns, 1982; Frankenberger,
Jr. and Dick, 1983; Baruah and Mishra, 1984; Satchell et al., 1984; Satchell and
Abdul Kadir Salam and Nanik Sriyani – 2019