Effects of organic content on soil-water characteristic curve and soil shrinkage

Organic matter commonly exists in the subsoil within tree rooting zones. The organic content of soil has been known to affect soil properties. In this study, soil organic contents at various locations around trees were found to be variable due to the ground flora and tree roots. The range of the soil organic content for the current study was between 0.9 and 12.5%. The objective of this study was to investigate the effects of the organic content on the shrinkage and soil-water characteristic curve of soil during rainy and dry seasons when the water content of the soil changes. Soil samples were collected from two locations on Singapore Island at depths of 0.1 and 0.3 m. A centrifuge was used to measure the water content of the specimens at suctions up to 250 kPa, and chilled-mirror psychrometry was used for higher suction. Shrinkage tests were also conducted. The results show that a higher organic content was associated with a greater shrinkage rate and a higher suction corresponding to the residual water content. For finer soils, a higher organic content comes with a lower air-entry value.