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The Chemistry and Fertility of Soils under Tropical Weeds 105

100
90
80
g p-Nitrophenol g -1 h -1 70
60
50
40
30
20
10
0
0 0.5 1 1.5 2 2.5 3 3.5
% C or % N

Organic C Total N

Fig. 6.6. The relationship between the soil enzymatic
activities vs Organic C or Total N (Salam, 2014).

The work of soil enzymes on substrate, measured by p-Nitrophenol per soil
mass per hour, is found to be linearly correlated with the amount of substrate as
organic matter (Organic C and Total N) (Nannipieri et al., 1980; Baruah and Mishra
1984; Tate III, 1984; Salam et al., 1998b; 1999a); the higher the contents of organic
matter the higher the rates of the biochemical reactions. This phenomenon (Fig.
6.6) is easily understood because the substrate is the energy source for
microorganisms producing enzymes.
The positive relationship between the soil enzymatic activities and soil organic
C or organic matter contents is related the role of organic matter as an energy
source for soil microorganisms and macroorganisms. The populations and activities
of microorganisms and macroorganisms increase with the increase in organic
matter contents. Earthworms are found more frequently in topsoils, with higher
soil organic matter contents, than that in underlying subsoils, that have lower
organic matter contents. Earthworms also live better in soils of secondary forest,
Abdul Kadir Salam and Nanik Sriyani – 2019