Soil moisture retention characteristics of saprock in a weathering profile over rhyolite in Peninsular Malaysia

Abstract

Three broad zones can be differentiated at the weathering profile; an upper 3.9 m thick pedological soil (zone I), an intermediate 21.4 m thick saprock (zone II), and the underlying bedrock (zone III). To determine the soil moisture retention characteristics of saprock, samples were collected at vertical depths of 5.7 m (sample A), 9.1 m (sample B), 12.7 m (sample C) and 16.9 m (sample D). Samples A and B with porosities of 39% and 48%, had 7% and 6% gravel fractions, 48% and 57% sand fractions, 32% and 30% silt fractions, and 13% and 7% clay fractions, respectively. Samples C and D with similar porosities of 46%, had 6% and 17% gravel fractions, 56% and 45% sand fractions, 32% silt fractions, and 6% clay fractions, respectively. Laboratory determinations employing the pressure plate method show increasing suctions from 0 kPa through 0.98 kPa and 9.8 kPa to 33 kPa and 1,500 kPa to result in gravimetric soil moisture retentions of 28.3% through 24.0% and 19.2% to 15.0% and 5.7% for sample A, and from 21.7% through 21.1% and 17.8% to 11.9% and 3.8% for sample B. Similarly increasing auctions yield gravimetric soil moisture retentions of 28.3% through 34.7% and 29.1% to 23.2% and 7.0% for sample C, and from 28.4% to 28.1% and 23.4% to 18.6% and 5.0% for sample D. Regression analyses of gravel, sand and clay fractions plotted against moisture contents retained at large suctions (33 kPa and 1500 kPa) yield variable trends with low correlation coefficients (R2<0.179), though plots involving silt contents yield positive trends with larger correlation coefficients (R2 >0.525). It is concluded that the adsorption of water on the surfaces of silt particles that most likely results in retention of soil moisture by saprock.