Page 169 - Ilmu Tanah Book

156 The Chemistry and Fertility of Soils under Tropical Weeds

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of H as a result of the reaction between CO 2 and water molecules. The decrease in
soil pH may release some nutrient elements contained in the structure of the
primary and secondary minerals. The complexing agents may complex parts of the
nutrient elements and cause the nutrient elements to stay dissolved in the soil
solution and, by mass action, may also increase the release of plant nutrient
elements from soil minerals.
Weed roots and microorganism and macrorganims associated with them also
excrete soil enzymes. These biocatalysts are very important in the cycles of some
nutrient elements like C, P, N, and S. The presence of soil enzymes may speed up
the transformation of organic nutrient elements, which are not available to root
absorption, into inorganic nutrient elements, that are readily available to plant
roots. The presence of soil enzymes may accelerate the supplies of nutrients to
plant roots from organic sources.
In addition to their effects on the soil chemical and the soil biological
properties mentioned above, the presence of weeds is also very important to
improve the soil physical properties. The increase in organic C through weed
residues may increase the soil organic C and the soil moisture content and maintain
the soil temperature. The increase in soil CEC may also maintain the soil moisture
content. The increase in the soil organic C is not affected only by weed organic
residues. The excretions of weeds roots may also increase the soil organic matter.
In general, the presence of weeds may improve the soil fertility.

10.1 The Chemistry of Soils under Weed Management

The effects of weeds are significant on the soil chemical properties and also on
the soil biological and the soil physical properties. The soil chemical properties
affected by weeds may include the soil pH, the soil complexing agents, the soil
enzymatic properties, the concentration of CO 2 , the soil organic matter, organic C
and total N, the soil CEC, the availability of various nutrient elements, the soil
exchangeable bases, etc. The soil biological properties affected may include the
population and activity of soil microorganisms, the evolution of CO 2 , the population
of mesoorganisms like earthworms, etc. The soil physical properties affected may
include the soil moisture content and the soil temperature.
The soil pH in the rhizosphere of weeds decreases due to several reasons. The
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first is the excretion of H ions by weed roots. The second is the excretion of
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organic acids by weed roots. The combined excretion of H and organic acids by
Abdul Kadir Salam and Nanik Sriyani – 2019