Page 151 - Ilmu Tanah Book

138 The Chemistry and Fertility of Soils under Tropical Weeds

Heavy metals are trace elements and their natural concentrations in the soil
environment are relatively low. Enhanced concentrations above the natural or the
allowable levels may jeopardize the living things. They may accumulate in living
tissues of the living things and may cause serious health problems. Several
disasters occurred in the 1950s among which was in Minamata Bay Japan caused
by an extremely high level Hg called Minamata Disease. Around the same time also
occurred Itai-Itai Disease by Cd contamination of irrigation water used for paddy
fields. The enhancement of soil heavy metals also occurred in several locations in
Indonesia among which are in Jakarta Bay, Lampung Bay, etc., caused by unwise
disposal of industrial and mine waste into the environment. The problem of heavy
metals may become a great concern since industrialization may intensify the
environmental pollution by heavy metals.
Heavy metals in the soil environment are also affected by the presence of
weeds since the concentrations of heavy metals are influenced by the changes in
soil pH, organic acids, soil organic C, and organic matter contents. These soil
properties are previously discussed to be affected by the presence of weed roots.
The concentrations of soil heavy metals are in general enhanced by the decrease in
soil pH that decreases in the ambience of weeds roots. The increase in soil organic
C and organic matter contents may increase the soil CEC that may decrease the
concentrations of heavy metals in soil solution caused by heavy metal adsorption.
On the contrary, the increase in organic acid complexing agents may solubilize
heavy metals and hence may cause them to stay dissolved in soil solution.
Montiel-Rozas et al. (2016) suggest that the bioavailabilities of heavy metals
can be modified by different root exudates. Among them, low molecular weight
organic acids (LMWOAs) play an important role in this process. The release of
LMWOAs like oxalic acids was proven to be an important mechanism against heavy
metal stress, which is unique to each species and modifiable by means of organic
amendment addition. Therefore, the changes in the land cover may also cause the
changes in heavy metal solubility and concentrations since different vegetation
covers may cause different magnitude of changes in soil pH, soil organic acids, soil
organic C and organic matter contents, and soil CEC.

8.1 The Chemistry of Heavy Metals in Soils

Heavy metals presence in soils may have different forms that may include free
ions, complexes and chelates, exchangeable, secondary minerals (precipitates),

Abdul Kadir Salam and Nanik Sriyani – 2019