The Soil – Weed Interrelationships
2.3 The Effects of Weeds on Nutrient Element Release from Soil Minerals and Organic Matters
Chemicals released by weed roots may directly and indirectly affect the soil properties that might intensify the releases on nutrient elements from soil solids.
The depletion in soil pH in the root zones of weeds may directly increase the process of soil mineral weathering releasing nutrient elements into the soil water.
Salam et al. (2019) report that several weeds stimulate the release of K from soil minerals. The effectiveness of weed plants in stimulating the release of soil K were:
A. Pintoi > A. gangetica > Widelia sp. > P. conjugatum > P. purpureum. A. pintoi is suggested to be the most effective in stimulating the release of K from soil minerals as proven by the positive values of Exch. K [ Exch. K = Final Exch. K (After Planting) – Initial Exch. K (Before Planting). This phenomenon is related to its higher Root-To-Shoot Ratio compared to others.
The increase in soil water cations is not only attributed to the weathering of soil minerals but also to the decomposition of soil organic matter directly speeded by various soil enzymes excreted by weed roots and their associated microorganisms. Therefore, the increase in soil exchangeable K in the root-zones of A. pintoi reported by Salam et al. (2019) might have also been related to this phenomenon.
Combined with decrease in soil CEC in response to the decrease in soil pH, the increase in nutrient element concentration in soil water may indirectly cause the increase in soil exchangeable cations. However, the presence of weed-roots absorbing cations may cause the real exchangeable cations in soils to decease. This phenomenon is reported by Salam et al. (2019), the soil exchangeable K was lower in the presence of weeds like A. gangetica, Widelia sp., P. conjugatum, and P.
purpureum. A. pintoi is different, demonstrating its effectiveness in stimulating the
Abdul Kadir Salam and Nanik Sriyani – 2019
The Chemistry and Fertility of Soils under Tropical Weeds 33
release of soil mineral K as shown by its value of Exch. K. However, in the long run the presence of weeds may also directly increase the soil C and organic matter content through their dead and decomposing materials. The increase in soil organic matter may increase the soil CEC which might increase the capacity of soils to retain cations. In the short run, the soil organic C and organic matter content are shown not affected by the presence of weeds (Ontia, 2018).
In addition to indirectly affect the soil CEC, organic C, and organic matter, the presence of weeds may also indirectly affect the soil base saturation, which is directly related to the soil exchangeable bases and soil CEC. The increase in soil water concentration of cations may drive the increase in the soil exchangeable cations by the process of adsorption. Combined with the decrease in soil CEC by the decreasing soil pH in the root-zones, these processes may increase the soil base saturation.
Key Questions
1. Explain how weeds cause direct and indirect effects on soil chemical properties like soil pH, CEC, and BS!
2. Explain how the weed-root-excreted H+ and organic acids as well as the evolved-CO2
acidify the weed rhizosphere!
3. Explain how weed roots stimulate the soil mineral weathering and organic matter decomposition!
4. What characteristics of weeds that make them very influential in the soil environment?
Explain!
5. What is the importance of Root-To-Shoot Ratio for competitive vegetation like weeds?
Explain!
6. How does the soil pH affect the soil CEC and soil BS?
7. How do the soil enzymes affect the soil organic matter decomposition? Explain!
8. Explain the relationship between the soil – water – nutrient element and weeds!
9. Explain the mechanisms for the nutrient element losses in the soil environment other than by weed root absorption!
10. Explain all the reactions that balance the nutrient losses in the soil environment! Show them in a diagram!
11. What is the dissolution process? Give an example and explain!
12. Explain the dissolution of Mn-P in the soil environment! Explain all the controlling factors of this reaction!
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34 The Chemistry and Fertility of Soils under Tropical Weeds
13. Explain the weathering of soil primary minerals! Why is it so important for the soil chemical and fertility?
14. Explain the decomposition of soil organic matter? Why is it so important for the fertility of soils? Which nutrients released through this soil chemical process?
15. Explain all forms of nutrient elements in the soil environment! Explain how they relate one to another!
16. Explain the easiness of each nutrient element forms to be absorbed by weed roots!
17. Which forms of the soil nutrient elements in the soil environment readily available to weeds? How are these forms balanced when their concentrations in the soil water are somehow depleted?
18. Which forms of the soil nutrient elements in the soil environment are slowly available to weeds? How do these forms relate to the weed root absorption?
19. Explain the nutrient element complexes, complexation, and complexation constant!
20. Explain the chelation, chelates, and dechelation of nutrient elements!
21. Explain the relationship between all the mechanisms controlling the free ions of nutrient elements!
22. Compare the concentrations of free ions, total dissolved, and total contents of soil nutrient elements!
23. What is cation exchange? Explain all the factors controlling this reaction in the soil system!
24. Explain the dissolution of precipitates of carbonates, sulfates, sulfide, and phosphates!
25. Explain the most important factors controlling the concentrations of free nutrient elements in soil water!
26. Explain the importance of soil pH in controlling the dissolution process in the soil environment!
27. What is soil E? How does this variable control the dissolution of FeS and Fe2(HPO4)3? 28. Explain the weathering of orthoclase! Explain also all the factors controlling this
reaction!
29. Explain the importance of pH in controlling the weathering of primary minerals!
30. Explain how the soil organic matters decompose and release nutrient elements in the soil environment! What nutrient elements are released?
31. Explain all the factors controlling the soil organic matter decomposition!
32. Explain the importance of water and enzymes in the soil organic matter decomposition!
What about other factors like soil pH, soil T, and soil C/N ratios?
33. How does the soil pH affect the activities of phosphatases?
34. Explain how weeds directly and indirectly influence the soil chemical properties!
Abdul Kadir Salam and Nanik Sriyani – 2019