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The Chemistry and Fertility of Soils under Tropical Weeds 147

micronutrients using DTPA methods (Baker and Amacher, 1982) and soil pH using
pH-electrode. The results are listed in Table 8.2.
Table 8.2 clearly shows that the soil pH under alang-alang was the highest.
Compared to the initial soil pH of 5.25, it is understood that the presence of alang
alang neutralized the rhizosphere soil pH, meaning that alang-alang roots excreted
-
+
-
more OH and HCO 3 ions than H ions. In a lesser magnitude, C. kyllingia also
-
-
excreted more OH and HCO 3 . On the other hand, A. tricolor and A. spinosus
+
excreted more H and hence showed lower pH that the initial pH. The
+
neutralization of H by alang-alang roots is suspected as the reason for lower
availabilities of Fe, Cu, and Pb in the rhizosphere of alang-alang compared to those
in the root-zones of other weeds and amaranth because the availabilities of heavy
metal cations in general decreased with the increase in soil pH. This phenomenon
is consistent with that reported by Heradilla (1997) that the absorption of Cu and
Pb by alang-alang was lower than those by the other weeds.

Table 8.2. The effect of amaranth and some tropical weeds on the soil pH and
heavy metal availabilities*.

Cyperus
Imperata Amaranthus Amaranthus
Soil kyllingia
cylindica tricolor spinosus
Properties (Green
(Alang-alang) (Amaranth) (Pigweed)
Kyllinga)
pH (H 2 O 1:2) 6.25 5.03 5.01 5.64
-1
Fe (mg kg ) 22.5 80.6 15.2 47.3
Mn (mg kg ) 14.9 187 1.77 2.13
-1
Zn (mg kg ) 8.45 7.68 4.05 11.1
-1
Cu (mg kg ) 0.44 1.00 0.69 1.23
-1
Pb (mg kg ) 0.19 1.13 0.79 0.50
-1

*Adapted from Salam et al. (1997a) and Sriyani and Salam (1998)

Abdul Kadir Salam and Nanik Sriyani – 2019