The Effect of Soil Conditioners on Water Characteristic Curves and Adsorption Strength of Red Loam Soil

Abstract

【Objective】 Amending degraded soils with soil conditioners is a technique widely used to improve soil quality. Different conditioners are commercially available but their effect on soil release curve is less understood. The purpose of this paper is to compare change in water characteristic curves of a red loam soils amended using different soil modifiers. 【Method】 The soil was amended by glutinous rice gum, wood fiber, and their mixture at different ratios. We measured the change in matrix suction (ψ) and volumetric soil water content (θv) of each soil sample. The internal friction angle (φ) of the soil at saturation was determined using the direct shear test method, from which we calculated the adsorption strength (τψ) of the soil using the Fredlund double stress variable shear strength. 【Result】 ① Glutinous rice gum and wood fiber both increased the saturated soil water content (θs), while their mixture reduced θs. Glutinous rice gum and its mixture with wood fiber reduced the residual soil water content (θr), while the wood fiber working alone only reduced the residual water content when the amendment ratio was low or high. All three modifiers increased the air-entry value (ψa), with the increase ranked in the order of the mixture > glutinous rice gum > wood fiber. Glutinous rice gum reduced the residual matric potential (ψr), while the wood fiber increased ψr, with the mixture in the between with its ψr higher than that of the non-amended soil. The matric potential decreased fastest as soil water content increased in the glutinous rice gum treatment, followed by the mixture and wood fiber. ②All conditioners increased the adsorption strength τψ, and the shear strength decreased with the increase in ψ. For soil water content in the range of 23.73%~30.76%, amending the soil with 2.5%~5.0% of wood fiber gave the highest adsorption strength, followed by non-amended soil, the amendment by 2.5%~5.0% of mixture, and the amendment by 0.5%~5.0% of glutinous rice gum. 【Conclusion】 ①Glutinous rice gum mainly reduced the ability of the soil to hold water. Wood fiber influenced θs, θr and ψr most and improved the ability of the soil to hold water. The mixture of wood fiber and glutinous rice gum increased ψa most, improving the ability of soil to hold water in the wet-end, but reduced water-holding capacity of the soil in the middle range. ②The effect of different modifiers in increasing τψ is ranked in the order of wood fiber > glutinous rice gum > their mixture.

Keywords

• soil amendments
• red loam
• matric suction
• soil-water characteristic curve
• the adsorption strength