Page 46 - Ilmu Tanah

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-CO 2
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!

Abdul Kadir Salam and Nanik Sriyani – 2019