The accuracy and the reproducibility of water repellency measurements can be improved with the use of proper measurement techniques. The purpose of this study is to compare the contact angles obtained using three different methods and to examine the relationship between contact angle and water drop penetration time (WDPT) using non-repellent to extremely repellent sands. Fine silica sand, artificially hydrophobized with stearic acid, was used for the measurements. Contact angles were estimated using the molarity of an ethanol droplet (MED) test, the capillary rise method (CRM) and the sessile drop method (SDM). The CRM was effective for soils with contact angles < 90°. The MED test was suitable for soils with contact angles ≥ 90° using 10 s as the ethanol drop penetration time. The SDM was effective for soils with contact angles ranging from very low to very high (11-105°). Directly measured contact angles using the SDM were in good agreement with indirectly obtained contact angles using the MED test and the CRM. The WDPT was < 1 s for contact angles ranging from 11 to 69°, gradually increased from 69 to 93°, and exceeded 3600 s beyond that. The WDPT was most sensitive for contact angles within the range of 88-93°. Extreme repellency was observed where the solid surface free energy was ≤ 45 mN m -1 .

Soil water repellency is related to organic matter and clay, and varies non‐linearly with soil water content. The purpose of this study is to assess the combined effects of organic compounds, water content and clays on water repellency of a model sandy soil under wetting and drying processes. Hydrophobic stearic acid and hydrophilic glucomannan were used as the organic compounds, and kaolinite or montmorillonite was used as the clay conditioner. Water repellency was estimated using the water drop penetration time test. Repellency did not appear in samples free of stearic acid. Samples containing both stearic acid and glucomannan showed higher repellency compared with samples containing stearic acid alone during the wetting process. Glucomannan with stearic acid increased the critical water content and widened the range of water content at which soils showed slight repellency. During the wetting process, the repellency of most samples increased with increasing water content under relative humidity conditions ranging from 33 to 94%. During the drying process, repellency appeared, reached a maximum and then decreased in samples containing stearic acid. Maximum repellency was observed not at oven‐dried but at air‐dried water content. Repellency was highly sensitive to water content at around air‐dried condition. The effects of organic compounds and clay on the water repellency of sandy soils were negligible in oven‐dried condition. Repellency tended to increase with the addition of a small amount of clay (1–2%) as a dry mix during the wetting process. Once wetted, repellency disappeared with the addition of montmorillonite, but not with kaolinite. The higher the kaolinite content, the higher the critical water content.

Soil water repellency is a transient soil property varying with soil-water contact time. The purpose of the present study was to determine the time dependence of the sessile drop contact angle and its relation to repellency persistence estimated using the water drop penetration time (WDPT) test with hydrophobized sand. The contact angle decreased exponentially and almost reached apparent equilibrium after 20 min of soil-water contact time. Time dependence of the contact angle can mainly be attributed to the adsorption of water molecules onto low-energy hydrophobic organic matter surfaces. Contact angles initially greater than 90°decreased to less than 90°within about 40 s. However, the WDPT of these samples was longer than 3600 s. The WDPT responded to the initial contact angle, but not to the contact angle decreased with soil-water contact time. This was considered to be caused by differences in the surface free energy between the surface and the lower layers. Repellency persistence, or the WDPT, can be considered to be the time taken to increase the surface free energy to overcome water repellency, not only on the surface in contact with the droplet, but also in the adjacent layers below the surface.

The purpose of this study was to determine the effects of ambient relative humidity (RH) on contact angle and water drop penetration time (WDPT) using a series of sand samples hydrophobized with stearic acid. The contact angle was estimated using the sessile drop method. The contact angle increased first sharply and then slightly with increasing stearic acid content up to about 5.0 g kg -1. The contact angle did not change with increasing stearic acid content above 5.0 g kg -1. The contact angle increased with increasing RH from 33 to 94%. The RH did not affect the contact angle of samples with a stearic acid content above 5.0 g kg, where the particles were considered to be completely coated by hydrophobic organic material. The contact angle increased with increasing RH in sands partially coated with hydrophobic material, which might have resulted from an increase in adsorbed water molecules on the remaining high-energy mineral surfaces. The WDPT increased with increasing RH. The higher the RH, the lower the stearic acid content at which extreme repellency (WDPT ≥ 3600 s) appeared. Samples with a contact angle below 75°o r with surface free energy above 72 mN m -1 (equivalent to the surface tension of water) were non-repellent (WDPT < 1 s). Samples with a contact angle above 90° or with surface free energy below 50 mN m -1 were extremely repellent. There was no effect of RH on the relationship between WDPT and contact angle (or surface free energy), indicating that the effect of RH was comparable on both measurements.

Lowered stability of soil aggregates governed by insufficient organic matter levels has become a major concern in Sri Lanka. Although the use of organic manure with water repellent properties lowers the wetting rates and improves the stability of soil aggregates, its effects on soil hydrophysical properties are still not characterized. Therefore, the objective of this study was to examine the relation of water repellency induced by organic manure amendments to the water entry value and water retention of a Sri Lankan Ultisol. The soil was mixed with ground powders of cattle manure (CM), goat manure (GM), Gliricidia maculata (GL) and hydrophobic Casuarina equisetifolia (CE) leaves to obtain samples ranging from non-repellent to extremely water repellent, in two series. Series I was prepared by mixing GL and CE with soil (5, 10, 25, 50%). Series II consisted of 5% CM, GM, and GL, with (set A) and without (set B) intermixed 2% CE. Water repellency, water entry value, and water retention of samples were determined in the laboratory. Soilwater contact angle increased with increasing organic matter content in all the samples showing positive linear correlations. Although the samples amended with CE showed high soil-water contact angles in series I, set A (without 2% CE) and set B (with 2% CE) in series II did not show a noticeable difference, where >80% of the samples had soilwater contact angles <90°. Water entry value (R 2 = 0.83-0.92) and the water retention at 150 cm suction (R 2 = 0.69-0.8) of all the samples increased with increasing soil-water contact angles showing moderate to strong positive linear correlations. However, set A (without 2% CE) and set B (with 2% CE) in series II did not differ noticeably. Water entry value of about 60% the samples was <2.5 cm. Mixing of a small amount (2%) of hydrophobic organic matter with commonly used organic manures slightly increased the water repellency of sample soils, however not up to detrimental levels. It did not generate adverse effects on water entry and increased the water retention. It was clear that intermixing of small quantities of hydrophobic organic manure with organic manures commonly used in Sri Lankan agriculture, would not generate unfavorable impacts on soils.

Soil water repellency affects the hydrological functions of soil systems. Water repellency is associated with the content and the composition of soil organic matter. In the present study, we examined the effects of hydrophobic and hydrophilic organic matter contents, the hydrophobic ⁄ hydrophilic organic matter ratio and the total organic matter content on water repellency using model sandy soils. Stearic acid and guar gum were used as the hydrophobic and hydrophilic organic compounds, respectively. Water repellency was estimated using the sessile drop method. Hydrophobic organic matter content was found to be the dominant factor affecting soil water repellency. Hydrophilic organic matter was found to increase the contact angle to some extent without the presence of hydrophobic organic matter. With the presence of both hydrophobic and hydrophilic organic matter, the effects of the hydrophilic organic matter content on contact angle were found to be dependent on the hydrophobic organic matter content of the soil. This relationship was explained by the differences in the surface free energies of different organic matter and mineral surfaces. The contact angle increased with increasing hydrophobic ⁄ hydrophilic organic matter ratio when the hydrophilic organic matter content was constant. When the hydrophobic organic matter content was constant, contact angles were roughly comparable, irrespective of the hydrophobic ⁄ hydrophilic organic matter ratio. The contact angles were not comparable at each total organic matter content. Accordingly, the hydrophobic ⁄ hydrophilic organic matter ratio and the total organic matter content in soil may not provide satisfactory information about soil water repellency.

Abstract:The wetting rate of soil is a measure of water repellency, which is a property of soils that prevents water from wetting or penetrating into dry soil. The objective of the present research was to examine the initial water repellency of organic manure amended soil, and its relation to the soil organic matter (SOM) depletion rates in the laboratory. Soil collected from the Wilpita natural forest, Sri Lanka, was mixed with organic manure to prepare soil samples with 0, 5, 10, 25, and 50% organic manure contents. Locally available cattle manure (CM), goat manure (GM), and Casuarina equisetifolia leaves (CE) were used as the organic manure amendments. Organic matter content of soils was measured in 1, 3, 7, 14, and 30 days intervals under the laboratory conditions with 74±5% relative humidity at 28±1°C. Initial water repellency of soil samples was measured as the wetting rates using the water drop penetration time (WDPT) test. Initial water repellency increased with increasing SOM content showing higher increasing rate for hydrophobic CE amended samples compared with those amended with CM and GM. The relation between water repellency and SOM content was considered to be governed by the original hydrophobicities of added manures. The SOM contents of all the soil samples decreased with the time to reach almost steady level at about 30 d. The initial SOM depletion rates were negatively related with the initial water repellency. However, all the CE amended samples initially showed prominent low SOM depletion rates, which were not significantly differed with the amended manure content or the difference in initial water repellency. It is explicable that the original hydrophobicity of the manure as well has a potentially important effect on initiation of SOM decomposition. In contrast, the overall SOM depletion rate can be attributed to the initial water repellency of the manure amended sample, however, not to the original hydrophobicity of the amended manure. Hydrophobic protection may prevent rapid microbial decomposition of SOM and it is conceivable that hydrophobic substances in appropriate composition may reduce organic matter mineralization in soil. These results suggest the contribution of hydrophobic organic substances in bioresistance of SOM and their long-term accumulation in soils.

The soil-water contact angle is used as a measure of the surface hydrophobicity of soils. The contact angle for particular solid-liquid combination is considered to vary with the drop size. In this paper, we focused on examining the drop size dependence of contact angle on soil surfaces compared with homogeneous solid surfaces, and determining its relation to the droplet geometry and line tension. The contact angle estimated using geometric parameters of the droplets ( G ) showed decreasing trend with increasing drop size from 5 to 50 mL irrespective of the deformations in the droplet shape in larger drops. This was considered to be a result of the corresponding deviations of the geometric parameters of the droplets. The directly measured contact angle ( A ) first decreased and then increased with increasing drop size from 5 to 50 mL. The drop size at lowest A for hydrophobized silica sand with 1 g kg -1 stearic acid (SA) and the acryl surfaces was 20 mL, whereas that for hydrophobized silica sand with 5 g kg -1 SA and siliconed paper was 30 mL. The decrease in A with increasing drop size was explained as a result of the line tension effect using the modified Young's equation. Despite the surface heterogeneity, all the surfaces tested in this study showed positive line tensions on the order of 10 mJ m -1 . Irrespective of the heterogeneity of the surfaces, the A in this experiment agreed with the modified Young's equation for drop sizes up to about 20-30 mL, where the A and G were also in good agreement. Drop size dependence of contact angle was independent of the level of surface hydrophobicity. The A on all the examined surfaces started to increase with increasing drop size when the deformation index, I d , exceeded 5%, where the wetting radius, R exceeded the capillary length. The increase in A with increasing drop size was attributed to the deformations of water drops due to the effect of gravity.