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The Chemistry and Fertility of Soils under Tropical Weeds 103

significantly increases the soil organic C contents. The increase in organic matter
content will be more significant if grass is more frequently used as a rotating
vegetation.

Table 6.8. The differences in organic C and N contents of Vertisols planted
with grasses and food plants * (After Salam, 2014).

Soil 2
Soil 1
Food Grasses
Food Plants Grasses
Plants

7.80
pH 7.27 6.87 8.01
Total Organic C 7.1
7.90 13.4 4.4
-1
(g kg )
Total N 0.9
0.9 1.5 0.7
-1
(g kg )
*Adapted from Studdert et al. (1997)

Salam et al. (2001) also found from a four years observation (1996 – 1999) in a
coffee plantation of West Lampung, Sumatra, Indonesia, that the soil organic C and
total N in coffee plots with Paspalum conjugatum were higher that those with
natural vegetation or control (Table 6.9). A consistent phenomenon was observed
for 3 consecutive years from 1996 through 1999. The local natural vegetation also
showed a consistently higher organic C and total N that that of control coffee
plantation without weeds for the 3 consecutive years.

6.5 Effects of C and N on Soil enzymatic Activities

Organic C in soils is a substrate for enzymatic activities. The more the
substrate, in this case organic C or organic matters, is available in soils, the higher
Abdul Kadir Salam and Nanik Sriyani – 2019