Determining soil hydraulic properties with different wetting liquids : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Soil hydraulic properties such as hydraulic conductivity, water retention and plant water relationships dominate the water movement processes. For example, the infiltration, evaporation and water flow within the soil. As a widespread phenomenon in soil, soil water repellency alters the soil hydraulic functions by resisting the water flow into the soils. Moreover, the hydraulic properties not only depend on the pore system of soil but also are affected by the physicochemical properties of the pore surfaces. Compared to water, ethanol is not affected by the water repellency due to its lower surface tension. The purpose of using ethanol is to eliminate the effect of soil water repellency on hydraulic properties measurements. However, applying pure ethanol into natural soil can degrade the soil organic materials and lead to changes in physical properties. Therefore, the aqueous ethanol solutions can be an alternative option. Nevertheless, the feasibility of using aqueous ethanol for determining soil liquid retention curve remains unknown. This study aims to test the feasibility of the aqueous ethanol solutions (0, 10 %, 20 %, 30 %, 40 %, 50 % and 100 % v/v) for establishing liquid retention curve under a small range of applied pressure in coarse sand samples with different wettability and organic amended natural soil (Templeton silt loam (TSL) with mushroom compost, biochar, dairy effluent, and municipal compost, the application rate was 15 % by weight). The result showed that the desorption of testing aqueous ethanol solutions compared to water was quicker by approximately 1 to 3x based on the liquid content at the same applied pressure and the time for reaching each equilibrium. The outcomes from the sand experiment suggested that when the concentration of aqueous ethanol solution was below 50 %, the losses via evaporation were less than 1% and the relative differences between testing liquid and water was lower than 10 %. However, the evaporation losses were 11.06 % and 24.6 %, and the relative differences were all around 15 % and 30 % in 50 % and 100 % ethanol solution, respectively. In contrast, the organic amendments in Templeton silt loam not only increased the water retention capability but also reduced the evaporation losses. The evaporation losses in TSL soil and TSL with organic amendments soil were with 1 % in solutions with less than 50 % ethanol content. Evaporation losses of 50 % and 100 % ethanol were 2.8 % and 2.9 %, respectively. The relative differences between water and testing liquid were similar to the sand experiment when the testing liquid concentrations were below 50 %. However, the relative differences in 50 % and 100 % ethanol solution were smaller than in the sand sample with around 10 % and 15%, respectively. Additionally, the analysis of relative difference suggested that almost all aqueous ethanol solutions had lower volumetric liquid content at the same supplied pressure compared to water, which was a reflection of the aqueous ethanol solutions desorbed faster than water under equivalent pressure. Therefore, 10 % to 40 % of ethanol solutions could be considered an alternative for determining soil hydraulic properties due to their faster desorption rate and not being affected by soil water repellency. Due to the evaporation and dissolution of organic material, aqueous ethanol solutions above 50% concentration were not suitable for coarse sand or medium-textured soil.