Page 170 - Ilmu Tanah Book

The Chemistry and Fertility of Soils under Tropical Weeds 157

weed roots may cause a significant decrease in soil pH. Third is the reaction of CO 2
evolved by weed roots and the associated organisms that may produce high
+
amount of H . These three reasons may eventually lower the soil pH in the
ambient soil mass.
The evolution of CO 2 occurs by at least two important mechanisms. In
obtaining energy for growth and nutrient element absorption, weed roots may
oxidize hydrocarbon. In the presence of O 2 in the soil air, hydrocarbon (CHO) in the
weed roots is oxidized producing CO 2 , H 2 O, and energy. The energy is used among
which to obtain some of the plant nutrient elements in the soil environment.
Similar to plant roots, both microorganism and mesorganism like earthworms also
oxidize hydrocarbon for their food to obtain energy. Energy is immediately used.
The CO 2 released in the respiration by weed roots and macro- and microorganisms
may enter the soil solution. Upon reaction with water molecules, CO 2 may form
+
H 2 CO 3 , which is unstable and quickly dissociate producing H following Eq. 9.1.

+ -
CO 2 + H 2 O  H 2 CO 3  H + HCO 3 ……. Eq. 9.1

+
The reaction of these two compounds (respiration) may form H and HCO 3 -
+
(bicarbonate). The production of H may lower the soil pH and affect several other
important soil properties.
The increase in soil enzymatic activities by weed roots, microorganisms and
mesoorganisms may accelerate the decomposition of organic residues in the soil
environment. In addition to accelerating the cycles of several nutrient elements,
these processes may enhance the availabilities of some nutrient elements. The
2- - 2-
concentrations of several nutrient anions and cations such as HPO 4 , H 2 PO 4 , SO 4 ,
- + 2+
NO 3 , K , Ca , etc increase. The decomposition of weed residues may also increase
organic C and organic matters, and also the soil CEC. The presence of more organic
acid complexing agents may cause some elements like some micronutrient metal
element to stay dissolved in soil water and be available to plant roots.
The increase in organic matter contents may increase the soil water contents
which in turn may accelerate the soil mineral weathering and organic matter
decomposition. The increase in soil water may also increase the population and
activities of microorganisms and mesoorganisms that produce soil enzymes and
organic acids excreted into the soil environment. Therefore, the presence of weed
roots in general may increase the soil fertility.

Abdul Kadir Salam and Nanik Sriyani – 2019