Abdul Kadir Salam and Nanik Sriyani – 2019
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Chapter 9.
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156 The Chemistry and Fertility of Soils under Tropical Weeds
of H+ as a result of the reaction between CO2 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 CO2, 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 CO2, 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 first is the excretion of H+ ions by weed roots. The second is the excretion of organic acids by weed roots. The combined excretion of H+ and organic acids by
Abdul Kadir Salam and Nanik Sriyani – 2019
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 CO2
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 CO2 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 O2 in the soil air, hydrocarbon (CHO) in the weed roots is oxidized producing CO2, H2O, 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 CO2 released in the respiration by weed roots and macro- and microorganisms may enter the soil solution. Upon reaction with water molecules, CO2 may form H2CO3, which is unstable and quickly dissociate producing H+ following Eq. 9.1.
CO2 + H2O H2CO3 H+ + HCO3-
……. Eq. 9.1
The reaction of these two compounds (respiration) may form H+ and HCO3-
(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 concentrations of several nutrient anions and cations such as HPO42-
, H2PO4-
, SO42-
, NO3-
, K+, Ca2+, 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.
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10.2 The Availabilities of Plant Nutrients under Weed Management
The availabilities of macro and micronutrients in soils under weeds may increase significantly. The increase is caused by several mechanisms. The most important sources of nutrients in soil minerals and organic matters may speed up the release of nutrient elements through soil mineral weathering and organic matter decomposition by the decrease in soil pH and the increase in soil enzymatic activities. The supplies of nutrient elements may stay high as long as the soil pH relatively low and the soil enzymatic activities relatively high. The presence of organic acids which may act as organic complexing agents may maintain the availability of these elements
The increase in soil C and organic matter maintains sufficient soil moisture content which may make the dissolved nutrient elements to be available to plants.
In addition, the increase in soil CEC may maintain nutrient reserves available to plants. Higher CEC also makes the mineral weathering continues and maintain the availability of nutrient elements from soil mineral sources. The increase in soil moisture content also maintain higher microbial and earthworm population and activities, and production of enzymes. Higher organic matter decomposition also releases nutrient elements from organic sources. Earthworms also produce faces that contain nutrient elements.
10.3 The Use of Effective Weeds
It is obvious that weed roots may increase the fertility of soils by improving the soil chemical, physical, and biological properties. Therefore, it is probable that weeds are used in the management of soils for crop plants. However, it is important to first select effective weeds to conduct this mission. A little number of weeds are reported to employ in this mission even though further researches must be conducted. Research must not only focus on the effectiveness of weeds in enhancing the soil fertility but also on how the effective weeds are applied in crop production. Such alternatives as to employ weeds in the same time during plant growth or to employ weeds before planting the main crops are one of the choices.
The most important is how to get the benefit of increased soil fertility by the presence of weeds.
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Key Questions
1. How does the soil acidification occur by weed rooting system? What substances and reactions involved in this process? Explain!
2. How does the oxidation reaction in the weed roots producing CO2 occur and acidify the soils? Explain!
3. What are the effects of root excretion of H+ and organic acids? What soil properties are affected by the these substances? Explain!
4. One way of weeds to extract soil nutrients is by excreting enzymes through their roots.
Explain the mechanism!
5. What are the functions of enzymes? Explain it with examples! What is the relationship of this reaction with the soil organic and inorganic sources of soil nutrients?
6. Explain the role of soil enzymes in the nutrient cycles including the C-, N-, S-, and P cycles!
7. Explain the effect of weeds on soil physical properties with some examples! Explain the particular effect of weeds on soil moisture content!
8. Explain the effect of weeds on soil chemical properties with some examples! Explain the particular effect of weeds on soil K!
9. Explain the effect of weeds on soil biological properties with some examples! Explain the particular effect of weeds on soil bacterial and fungal population and activities!
10. Explain the effect of weeds on soil pH with some chemical reactions!
11. Explain the effect of weeds on the availabilities of some plant nutrients! Explain the particular effect of weeds on soil total N!
12. Explain the effect of weeds on soil cation exchange capacity (CEC)! What soil properties directly affect this? Does weed roots affect these soil properties?
13. Explain the effects of weeds on soil organic C and organic matter content!
14. Explain the effects of weeds on soil heavy metal availabilities! What soil properties directly affect this? Do weed roots affect these soil properties? Explain! Explain the particular effect of weed on soil Pb!
15. Can the presence of weeds affect the soil mineral weathering? Explain what soil properties may accelerate this reaction and how! How do weeds affect the changes in these soil properties? Explain!
16. How do weeds accelerate the soil organic matter decomposition? What soil properties affect this reaction? How do weeds affect the changes in these soil properties? Explain!
17. The increase in soil CEC may increase the soil mineral weathering dan organic matter decomposition. How do weeds involve in this acceleration? What chemical reactions involved in this process?
Abdul Kadir Salam and Nanik Sriyani – 2019