Effects of Soil Water Repellency on Soil Moisture Retention Curve and S-Index in Two Soils with Different Textures

Water repellent soil is a soil that is not immediately wetted when a drop of water is placed on its surface. Soil water repellency is formed by complex waxy organic compounds. During the decomposition of the organic matters, these waxy compounds cover soil particles and the soil becomes water repellent. Due to the influence of the size and arrangement of the pores on soil water retention curve, the slope of the retention curve at its inflection point that has been defined as soil physical quality index (S), reflects different aspects of soil physical quality such as penetration of water, air and roots and bulk density. In this study, two clay loam and sandy loam soils from the forest and grassland (converted to dryland farming) around the Kaleybar city, East-Azarbaijan province (Iran), were sampled and artificially hydrophobized by stearic acid at different concentrations to obtain five different degrees of water repellency. The purpose was to investigate the effects of water repellency on soil water retention curve, van Genuchten model coefficients and soil physical quality or S index. To achieve these objectives, laboratory-scale studies were conducted in disturbed soil columns (in triplicate). Results indicated that with increasing water repellency (from degree 1 to 5) in both soils, soil water retention curve and van Genuchten coefficients were affected significantly (p<5%). The S index decreased from 0.0592 to 0.0254 in sandy loam soil and from 0.0681 to 0.0537 in clay loam soil with increasing water repellency from degree 1 to 5. Statistical comparison indicated significant difference (p<0.05) in the values of the parameters n and m of van Genuchten model, saturation, field capacity, residual and inflection point moisture between various degrees of water repellency. The value of the a in van Genuchten model varied significantly (p<0.05) among all of water repellency degrees in sandy loam, but in the clay loam only the second degree of water repellency significantly affected the value of a in van Genuchten model.