Biocalcification and siltation in paddy soils [Philippines]
1987
Asio, V.B.
A study was conducted to identify soils affected by biocalcification and siltation; identify the factors that lead to these processes and evaluate their effects on soil profile characteristics and soil fertility. Detailed studies were conducted in several sites selected during the preliminary survey. Biocalcification which is proposed to be defined as the accumulation of CaCO3 in paddy soils caused by soil fauna or flora, was observed in several paddy soils in the Philippines where it is performed by snails. The process appears to be closely associated with Ca content of irrigation water and soil prolonged wetness either by irrigation, bunding or high water table. Field operations such as puddling hasten the process by crushing the shells. This results in icreased pH and available Ca and Mg and decreased available P and Zn. Indicators of the occurrence of biocalcification are shell accumulation presence of CaCO3 and increased pH in the surface horizons of soils originally non-calcareous or acid. A general model of the process is proposed. Siltation, defined here as the deposition of sediments especially silt in paddy soils due to irrigation, is manifested in many paddy soils studied. Its occurrence is associated with the nature of irrigation system and the quality of irrigation water, the latter being affected by erosion of uplands, bunded lowlands and wastes from mining establishments. In general, areas affectd by siltation were found to have silty texture, decreased CEC [cation exchange capacity] and increased or decreased organic C, Cu and Zn of surface soil layers reflective of the composition of irrigation water. Its occurrence can be detected based on appearance of the irrigation water, surface soil layers, particle size distribution, ratio of coarse silt to fine sand and organic C.
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