Page 159 - Ilmu Tanah Book

146 The Chemistry and Fertility of Soils under Tropical Weeds

8.2 Effects of Weed Management

As discussed previously, weeds influence various soil properties which directly
or indirectly affect the magnitude of the respective forms of heavy metal in the soil
environment. The magnitude of the effects on heavy metals depends greatly on
the magnitude of weed root effects on the related soil properties. The presence of
effective weeds may greatly affect the heavy metal forms in the soil environment.
For example, the presence of alang-alang may cause the concentration of dissolved
heavy metals to be lower than do the presence of other weeds since the
rhizosphere of alang-alang show higher pH than other weeds (Salam et al., 1997a).
However, due to the fact that generally the rhizosphere of weeds show lower
+
pH due to the excretion of H , organic acids , and the evolution of respired CO 2 that
+
may increase the soil solution H , the concentration of free ions and dissolved
heavy metals in the soil solution should be higher than in the control soil without
weeds. Hydrogen ions may increase the release of heavy metals from precipitates
and primary minerals. The presence of organic compexing agents may also
solubilize heavy metal precipitates and heavy metal complexes. This process may
intensify the dissolution of heavy metal precipitates (secondary minerals) and
primary minerals. The decrease in soil pH and the increase in the concentration of
complexing agents are a good combination that may accelerate the weathering of
soil minerals and increase the concentrations of heavy metals in soil solution as
free ions, complex ions, and chelates.
The presence of various enzymes excreted by weed roots may also accelerate
the decomposition of various organic matters containing heavy metals. In addition
to increasing the concentrations of dissolved heavy metals, this process may also
enhance the organic matter surfaces with adsorption sites expressed as CEC. The
increase in soil CEC may directly decrease the concentrations of free ions and
indirectly decrease the concentration of complexes and chelates. Therefore, the
organic matter decomposition catalyzed by enzymes may enhance not only the
concentrations of free ions but also the concentration of exchangeable heavy metal
cations.
Salam et al. (1997a) and Sriyani and Salam (1998) planted amaranth
(Amaranthus tricolor) and some tropical weeds including pigweed (Amaranthus
spinosus), green kyllinga (Cyperus kyllingia), and alang-alang (Imperata cylindrical)
in Oxisols from Gedongmeneng, Bandar Lampung, Indonesia. Soil was sampled
after a four-week growth and is analyzed for the soil availabilities of heavy metal

Abdul Kadir Salam and Nanik Sriyani – 2019