Relations between morphological color index and soil wetness condition of anthraquic soils in taiwan

The unique hydrologic and soil morphological features of the lowland rice-growing areas of the Chungli Terrace in northern Taiwan derive from diluvium red soils. Soil morphological characteristics and monitoring data provide a long-term record of the hydrological period and are useful for identifying these soils as hydric soils. The objective of this study was to establish relationships between the soil morphology-based soil chroma index and soil wetness conditions, including annual duration of both saturation and reduction. Three transects of a toposequence ranging from 20 to 40 m were selected. Four soils (Plinthaqualfs and Plinthaquults) were located in the toeslope position (20 m), four soils (Plinthudults and Paleudalfs) in the footslope position (30 m), and three soils (Paleudults and Paleudalfs) in the backslope position (40 m) with different redoximorphic features. Because they are seasonally flooded for rice (Oryza sativa L.) production and have perched water tables from February to October, all of these soils are considered to be anthraquic. Seasonally high groundwater levels also occurred during the growing season. Soils in three transects were monitored for groundwater depth, matric potential, and redox potential. The analytical results revealed that the seven soils with A horizons ≤ 25 cm thick were all hydric soils based on redox potential monitoring data. The analytical results indicated significant correlations between the chroma index and saturation time of year at 50-100 cm depth (P < 0.05) and at 100-150 cm depth (P < 0.01). A significant correlation was also found between chroma index and duration of reduction at depths of 50-100 cm (P < 0.01) and 100-150 cm (P < 0. 01). However, no clear correlation existed for depths of <50 cm because of the disturbance associated with plowing and the accumulation of rice straw residual on the soil surface.