Page 84 - Enzymes in Tropical Soils

72 Enzymes In Tropical Soils

5.7). In general it was found that planting soils with cassava decreased the soil acid
phosphatase, alkaline phosphatase, and -glucosidase. Time of cultivation also
tended to lower the enzyme activities. Salam et al. (1999b) also reported that a
similar trend was observed for the changes in Organic C, Total N, Available P, and
exchangeable K.

Table 5.6. Changes in soil organic C and total N in hilly coffee plantation under different
soil conservation techniques*.

Organic C (g kg ) Total N (g kg )
-1
-1
Control NV PG Control NV PG

1996 22.3 28.8 34.7 1.7 2.2 2.7
1997 14.7 17.1 29.1 1.3 1.3 2.3

1998 16.4 18.9 28.9 1.1 1.8 1.1
1999 21.1 23.0 50.0 1.8 2.1 2.4

*Adapted from Salam et al. (2001); NV = Natural Vegetation, PG = Paspalum conjugatum

Soil organic matters consist of Humic and Non-Humic Substances. Non-
Humic Substances are plant residues that have not been decomposed. These
materials are easy to indentify and consist of polysaccharides, lignin, and
polypeptides. Conversely, humic substances are plant residues that have been
decomposed. Humic substances consist of three fractions, each fraction is a
mixture of several undescribed organic compounds. These fraction are Humin,
Humic Acids, and Fulvic Acids (Tan, 1986; 1993).
Humin is the humic substances not soluble in alkaline solution. Humic Acids
are the humic substances not soluble in dilute acids, while Fulvic Acids are the
humic substances soluble or do not precipitate in dilute acids after being separated
from Humic Acids (Fig. 5.4). Table 5.8 shows one example of fractionation of
humic and fulvic acids related to the use of soil tillage system.
Humic acids are the product of organic matter decomposition. These
substances are relatively stable and, therefore, are accumulated in the soil
environment. These substances enable soils to retain plant nutrient elements and,
hence, can buffer their concentrations in soil water. According to Mackowiak et al.

Abdul Kadir Salam - 2014