A review on recent developments in soil water retention theory: interfacial tension and temperature effects

Study of soil physical processes such as water infiltration and redistribution, groundwater recharge, solute transport in the unsaturated zone, compaction and aeration in variably saturated soil hardly is possible without knowledge of the capillary pressure of the soil water as a function of the degree of saturation. Pore space topology, interfacial tension, and temperature probably are the most important physical factors affecting the capillary pressure at a given water content. Despite intensive research in the past decades on the water retention characteristics of soils, our knowledge of their response to varying ambient conditions is far from being complete. Current models of soil water retention as well as of hydraulic conductivity for unsaturated porous media often still use the simplified representation of the pore system as a bundle of cylindrical capillaries. Physical effects, like surface water film adsorption, capillary condensation and surface flow in liquid films, as well as volumetric changes of the pore space are often ignored. Consequently, physical properties of the solid phase surfaces, and their impact on water adsorption and flow, are often not considered. The objective of this contribution is to review various interfacial properties with possible application to the conventional water content — matric potential relation of soils. The ignoring of inter‐facial effects on the water retention of soils is widespread in the literature. The motivation of this paper is therefore to point out some of the more significant deficiencies of our current knowledge on the interaction of solid particle surfaces and the liquid phase in soil. We will first emphasize the impact of the wetting angle on the wetting of dry soil and to present the impact of interfacial tension of the liquid phase in the three‐phase system. At low water content, the transition from capillary‐bound water to adsorbed water and to wetting films is discussed separately, because of its impact on the rewetting process of dry soil. Finally, we discuss the impact of temperature on interfacial tension and water retention of soil as a second important interfacial process affecting directly the water retention of porous media.     

The role of exchangeable cations in the decrease soil moisture energy (pressure) (dedicated to the 110th birthday of A. A. Rode)

Close correlation between different soil properties (the total and capillary pressure of soil water, the content of bound water, the cation exchange capacity, the hydration heat of the exchangeable cations, the wetting heat of the soil, and the energy of the ion detachment from the substances composing the soil solid phase) derived from the mathematical analysis of experimental data confirm the hypothesis about the prevailing effect of exchangeable cations on water binding in soils enriched with clay minerals (of the montmorillonite and hydromica groups) in which the cation exchange capacity of the colloidal fraction is higher than 0.5 meq/g of colloids.     

Factors affecting aggregate instability of Greek agricultural soils

Abstract

The aggregate stability of several surface soils from Thessaly plain, Central Greece, was studied using an instability index which classified the soils according to the stability of their aggregates to water. This index was correlated to the basic soil properties influencing aggregate stability. It was found that soil aggregate stability was positively correlated with clay content, total specific surface area, cation exchange capacity, base saturation, and free silicon oxides content. Silt, very fine and fine sand, and the ratio (sand+silt)/clay influenced negatively the aggregate stability. Organic matter content and sesquioxides have no effect on aggregate stability. It was also observed that the fraction of carbonates with silt dimension influenced negatively the aggregate stability. From the results of this study it was concluded that the factors affecting water aggregate stability were specific surface area and cation exchange capacity of the soils.     

地表覆盖对土壤热参数变化的影响

覆盖条件下土壤热性质的研究对于包气带水热运移及覆盖技术的应用均有重要意义。使用11针热脉冲探头对沙黄土不同深度(6 mm、18 mm、30 mm)的土壤热扩散率、热容量和热导率三个热参数进行测定,并进行地表覆盖(石子覆盖、秸秆覆盖)处理,旨在探究覆盖条件下表层土壤热性质动态变化过程及土壤热参数与水分的内在联系。结果表明:(1)相对于裸土,石子和秸秆覆盖条件下土壤热参数增大,且覆盖对于靠近表层土壤热参数的影响更加明显;(2)随降雨的发生,土壤热参数均增大,在两次降雨期间,土壤热参数逐渐减小,覆盖与裸土热参数差异逐渐增大;(3)三个热参数随降雨的发生,其动态变化过程表现不同,热容量对降雨的响应最为敏感,热导率次之,热扩散率开始减小的时间较热导率和热容量滞后,三个深度滞后时间均在48 h以上,而且覆盖以后热扩散开始减小的时间较裸土推迟(48 h以上)。土壤容重不变的情况下,在频繁干湿交替的过程中土壤水分为土壤热参数变化的最主要影响因素。覆盖条件下土壤热参数与土壤含水量关系研究表明:石子和秸秆覆盖条件下土壤热参数与土壤含水量的变化关系与裸土条件下一致,热导率与含水量呈幂函数增加的趋势,热容量随含水量线性增加,热扩散率随含水量增加先增后减,本研究所用沙黄土热扩散率峰值对应的含水量在0.20 cm3cm-3左右。由以上结果可以发现覆盖对近表层土壤热参数的动态变化有显著的影响,覆盖的保水效应直接影响土壤热参数的变化。     

Adsorption density,mobility and limit values of Cd,Zn, Cu and Pb in acid forest soils

To investigate Cd, Zn, Cu and Pb adsorption in acidified forest soils, six soil samples of the aluminium buffer range were selected and analyzed for their physical and chemical properties. Determination of the specific surface area using ethylene glycol monoethyl ether (EGME) adsorption yielded a characteristic value of the solid phases, which can parameterize the major properties of the various soil constituents with sufficient accuracy.

Traditional adsorption isotherms reveal the relation between the amount of a heavy metal adsorbed and the heavy metal concentration in the soil solution only for the soil under study and can therefore not be applied to other soils. To meet the aim of modelling heavy metal adsorption and mobility also for soils differing greatly in their properties, it was attempted to establish a generalizing adsorption isotherm for soils of entirely different composition of the solid phase. The generalizing adsorption density isotherms introduced in the following provide a useful mathematical model for the quantity/intensity relation of heavy metals in soils that differ greatly in their specific surface area and their composition.

It is also shown that limit values which take into account the major quantities influencing heavy metal adsorption and mobility in acid soils can be established from the regression equation between the adsorption density of a heavy metal (ions/m² specific surface area) and its concentration in the soil solution. In particular in view of the groundwater contamination to be expected if acid rain and, as a result, soil acidification continues, these limit values seem to provide considerably more information than the European limit values, given in mg heavy metal /kg soil, which are presently valid for any soil condition and property.     

Universality of a surface tension—contact‐angle relation for hydrophobic soils of different texture

Wettability of soil affects a wide variety of processes in soils like infiltration, percolation, preferential flow, and surface runoff. Even though efforts have been made to determine contact angles and surface tension or energy of smooth surfaces, the determination on granular materials like soil particles remains unsolved. One objective of this study was to test the consistency of contact angles (CA) measured with the newly modified and easy‐to‐apply Wilhelmy plate method by using solid particles and liquids with defined variations of surface tension. A second objective was to derive basic physical surface properties for the irregularly shaped and chemically heterogeneous soil particles. Advancing contact angles were determined by using model soils varying in texture from clay to coarse sand to check the impact of grain size on the CA measurements. Varying the solid‐surface tension with dichlorodimethlylsilane treatments provided for pure‐water wetting stages ranging from wettable to extremely hydrophobic. The surface tensions of the liquids were varied from 72 mN m^–1 to 25 mN m^–1 by using water or water‐ethanol mixtures. The surface tensions of the model soils were determined with the zero degree–contact angle method following Zisman's critical–surface tension concept. Results show that the measured CA varied continuously with the variations of the surface tension of the liquid and the solid phase, respectively. A general interpretation of the results is possible by using the concept of the Equation of State Approach.     

Aerodynamic method for obtaining the soil water retention curve

A new method for the rapid plotting of the soil water retention curve (SWRC) has been proposed that considers the soil water as an environment limited by the soil solid phase on one side and by the soil air on the other side. Both contact surfaces have surface energies, which play the main role in water retention. The use of an idealized soil model with consideration for the nonequilibrium thermodynamic laws and the aerodynamic similarity principles allows us to estimate the volumetric specific surface areas of soils and, using the proposed pedotransfer function (PTF), to plot the SWRC. The volumetric specific surface area of the solid phase, the porosity, and the specific free surface energy at the water-air interface are used as the SWRC parameters. Devices for measuring the parameters are briefly described. The differences between the proposed PTF and the experimental data have been analyzed using the statistical processing of the data.     

Effects of compaction on soil surface water repellency

Water repellency can reduce the infiltration capacity of soils over timescales similar to those of precipitation events. Compaction can also reduce infiltration capacity by decreasing soil hydraulic conductivity, but the effect of compaction on soil water repellency is unknown. This study explores the effect of compaction on the wettability of water repellent soil. Three air‐dry (water content ～4 g 100 g^?1) silt loam samples of contrasting wettability (non‐repellent, strongly and severely water repellent) were homogenized and subjected to various pressures in the range 0–1570 kPa in an odeometer for 24 h. Following removal, sample surface water repellency was reassessed using the water drop penetration time method and surface roughness using white light interferometry. An increase in compaction pressure caused a significant reduction in soil surface water repellency, which in turn increases the soil's initial infiltration capacity. The difference in surface roughness of soils compacted at the lowest and highest pressures was significant (at P > 0.2) suggesting an increase in the contact area between sessile water drops and soil surfaces was providing increased opportunities for surface wetting mechanisms to proceed. This suggests that compaction of a water repellent soil may lead to an increased rate of surface wetting, which is a precursor to successful infiltration of water into bulk soil. Although there may be a reduction in soil conductivity upon compaction, the more rapid initiation of infiltration may, in some circumstances, lead to an overall increase in the proportion of rain or irrigation water infiltrating water repellent soil, rather than contributing to surface run‐off or evaporation.     

干湿交替过程中土壤胀缩特征的实验研究

土壤胀缩是土壤在干燥、湿润交替变化过程中所表现出的土壤容积随含水量改变的现象。本文通过测定4种土壤脱水和吸水过程中的土柱高度和直径的变化,分析了胀缩过程中土壤容积与含水量的关系及土壤的胀缩特征。结果显示:三直线模型能够很好地拟合4种土壤的收缩、膨胀过程;土壤收缩、膨胀曲线各段的特征值均小于1,这说明土壤胀缩过程中土壤容积的变化速率小于含水量的变化速率;4种土壤胀缩过程中几何因子rs大于1而小于3,且收缩初期几何因子rs等于1,表明土壤的胀缩是各向异性的,并且在土壤收缩的最初阶段只有垂直收缩。     

Swelling of biocrusts upon wetting induces changes in surface micro-topography

In most of non-vegetated areas from drylands, soils are colonized by biocrusts, and although they represent a small fraction of the soil profile, they strongly affect several soil surface properties, such as porosity, available nitrogen and carbon content, hydrophobicity or micro-topography. The influence of BSCs on these soil properties has effects on numerous ecosystem processes, including water retention and runoff generation. Previous studies on the hydrological and erosive response of soils covered by biocrusts have highlighted the role of soil surface roughness as a key variable for understanding the influence of biocrusts on runoff and erosion, but biocrusts' effects on surface micro-topography varies depending on crust water content. Biocrusts can absorb large amounts of water in a short period of time, increasing their volume and modifying surface micro-topography, this change depending on biocruts type and development. A correct characterization of these surface variations may increase the knowledge about hydrological response of biocrusts, and for this reason, accurate ground level measurements of biologically crusted surfaces are essential. The objective of this study is to analyze the effect of wetting on surface micro-topography of biologically crusted soils. To achieve this objective, different crust types were scanned in the laboratory with high resolution laser scanner. Five samples were collected for each of the 4 different crust types (bare soil, cyanobacteria biocrust, and two different lichen biocruts). Two different scans were made in each sample, in dry and wet conditions. Random roughness (RR) was calculated for data from every scan, and the RR indexes obtained before and after wetting were compared. According with our initial hypothesis, an increase in surface height and surface roughness up to 0.24 and 0.20 mm respectively was observed in more developed lichen biocruts, under wet conditions respect to dry ones. These differences, despite being very subtle, could exert strong implications on runoff generation, and water evaporation, and show the complex interactions between biocruts, surface micro-topography and water fluxes.     

Water-facilitated dispersal of inoculant Bradyrhizobium japonicum in soils

Summary Studies were performed to assess the influence of percolating water and an advancing wetting front on the transport of Bradyrhizobium japonicum in sand and silt loam soils, and to assess the influence of clay content on water-facilitated dispersal of these bacteria in a sand amended with various amounts of kaolin. The data obtained showed that movement of B. japonicum in soil was dependent upon water movement and that both percolating water and an advancing wetting front readily transported bacteria in coarse-textured soils. Percolation with the equivalent of 10 cm of rainfall dispersed B. japonicum throughout 40-cm columns containing sand and silt loam soils. Percolation with 5 cm of water was sufficient to disperse B. japonicum throughout 20-cm columns of these soils but did not transport these bacteria below the surface 4 cm of a sand amended with 12% kaolin. Our finding that cells of B. japonicum are readily transported by an advancing wetting front indicates that non-saturated flow of soil water contributes to dispersal of inoculum in soils.     

可耕种坡地的土壤水力参数非均质性变化

The spatial variations of the soil hydraulic properties were mainly considered in vertical direction. The objectives of this study were to measure water-retention curves, θ（ψ）, and unsaturated hydraulic conductivity functions, K（ψ）, of the soils sampled at different slope positions in three directions, namely, in vertical direction, along the slope and along the contour, and to determine the effects of sampling direction and slope position of two soil catenas. At the upper slope positions, the surface soils （0-10 cm） sampled in the vertical direction had a lower soil water content, 0, at a certain soil water potential （-1 500 kPa 〈 ψ 〈 -10 kPa） and had the greatest unsaturated hydraulic conductivity, K, at ψ 〉 -10 kPa. At the lower slope positions, K at ψ〉 -10 kPa was smaller in the vertical direction than in the direction along the slope. The deep soils （100 110 cm） had similar soil hydraulic properties in all the three directions. The anisotropic variations of the hydraulic properties of the surface soils were ascribed to the effects of natural wetting and drying cycles on the structural heterogeneity. These results suggested that the anisotropy of soil hydraulic properties might be significant in influencing soil water movement along the slope and need to be considered in modeling.     

Carbon storage in loess derived surface soils from Central Germany: Influence of mineral phase variables

The influence of the soil mineral phase on organic matter storage was studied in loess derived surface soils of Central Germany. The seven soils were developed to different genetic stages. The carbon content of the bulk soils ranged from 8.7 to 19.7 g kg^—1. Clay mineralogy was confirmed to be constant, with illite contents > 80 %. Both, specific surface area (SSA, BET‐N₂‐method) and cation exchange capacity (CEC) of bulk soils after carbon removal were better predictors of carbon content than clay content or dithionite‐extractable iron. SSA explained 55 % and CEC 54 % of the variation in carbon content. The carbon loadings of the soils were between 0.57 and 1.06 mg C m^—2, and therefore in the ”︁monolayer equivalent” (ME) level. The increase in SSA after carbon removal (ΔSSA) was significantly and positively related to carbon content (r² = 0.77). Together with CEC of carbon‐free samples, ΔSSA explained 90 % of the variation in carbon content. Clay (< 2 μm) and fine silt fractions (2—6.3 μm) contained 68—82 % of the bulk soil organic carbon. A significantly positive relationship between carbon content in the clay fraction and in the bulk soil was observed (r² = 0.95). The carbon pools of the clay and fine silt fractions were characterized by differences in C/N ratio, δ¹³C ratio, and enrichment factors for carbon and nitrogen. Organic matter in clay fractions seems to be more altered by microbes than organic matter in fine silt fractions. The results imply that organic matter accumulates in the fractions of smallest size and highest surface area, apparently intimately associated with the mineral phase. The amount of cations adhering to the mineral surface and the size of a certain and specific part of the surface area (ΔSSA) are the mineral phase properties which affect the content of the organic carbon in loess derived arable surface soils in Central Germany most. There is no monolayer of organic matter on the soil surfaces even if carbon loadings are in the ME level.     

The effect of temperature‐induced soil water repellency on transient capillary pressure–water content relations during capillary rise

Long‐term capillary rise experiments (0 to about 89 000 hour) were performed at 19°C on homogenized and heat‐treated podsolic forest top‐ and subsoil samples. These were packed into columns, the bases of which were then partially immersed, at constant depth, in water reservoirs to simulate a constant water table. Selected columns were equipped with tensiometer and TDR probes. Other columns were removed at prescribed times and divided into 2‐cm horizontal segments whose volumetric water contents were determined. The degree of saturation was then estimated by comparison with the capillary rise in duplicate arrangements of samples immersed in ethanol. It was found that the heat treatments conferred increased water repellency (WR) on the soil, which increased with temperature (significantly so at greater than 60°C). This had a profound effect on the capillary rise characteristics and development of water content in the soil behind the wetting front, indicating an effective, albeit slow, reduction in effective WR. This has implications for hydraulic modelling of soils with significant WR and demonstrates that sub‐surface WR exerts a significant influence on capillary rise from a water table and suggests that commonly used indicators of surface WR using droplet tests may not be useful for such modelling purposes.     

土壤初始含水率对膜孔灌湿润体特征的影响

为了研究膜孔灌中土壤初始含水率对湿润体特征及累积入渗量的影响,首先通过室内试验验证HYDRUS模拟西安粉壤土膜孔灌湿润体形状以及含水率分布的可靠性,然后基于HYDRUS模型模拟在不同初始含水率条件下膜孔灌湿润体的变化过程。结果表明:基于HYDRUS模型模拟的累积入渗量和湿润锋运移距离与室内试验结果的R~2均接近1,标准偏差绝对值均小于10%,拟合良好,表明HYDRUS模型模拟入渗过程的可靠性。膜孔灌湿润锋形状可采用椭圆方程表示。当初始含水率较小(不大于0.1 cm~3/cm~3)时,湿润体半径的含水率分布可采用椭圆方程表示;从膜孔中心到湿润锋表面,随着初始含水率的增大,湿润体内的含水率梯度减小,湿润体半径的含水率分布曲线由椭圆曲线逐渐转变为平缓曲线。基于湿润体含水率分布规律建立了考虑初始含水率的累积入渗量模型,累积入渗量与湿润体半径的三次方呈正比,湿润体半径可表示为湿润锋水平运移距离和垂向运移距离的几何平均值;对于不同的膜孔半径(1～5cm),模型计算累积入渗量与HYDRUS模拟值的R~2为0.99,标准偏差绝对值小于10%;对于粉土、粉壤土和壤土,当初始体积含水率不大于0.25 cm~3/cm~3算累积入渗量与HYDRUS模拟值的R~2为0.99,标准偏差绝对值小于10%,结果表明该模型对不同土壤质地和膜孔半径的适用性良好;该模型在计算作物灌水需求量方面优于Kostiacov模型等传统的经验模型。该研究揭示了不同初始含水率下的膜孔灌湿润体特征,并建立了累积入渗量模型,可为膜孔灌灌溉水量的计算提供参考依据。     

Simultaneous Sorption of Cd,Cu, Ni,Zn, Pb,and Cr on Soils Treated with Sewage Sludge Supernatant

Disposal of sewage sludge creates the potential for heavy metal accumulation in theenvironment. This study assessed nine soils currently used as Dedicated Land Disposal units(DLDs) for treatment and disposal of municipal sewage sludge in the vicinity of Sacramento,California. Adsorption characteristics of these soils for Cd, Cu, Ni, Zn, Pb, and Cr were studiedby simultaneously mixing these elements in the range of 0-50 µmol L-1 with sludgesupernatant and reacting with the soil using a soil:supernatant ratio of 1:30, pH = 4.5 or 6.5, andconstant ionic strength (0.01 M Na-acetate). The concentration of metals in the supernatant wasdetermined after a 24 hr equilibration period. Adsorption isotherms showed that metal sorptionwas linearly related to its concentration in the supernatant solution. The distribution coefficientKd (Kd = concentration on solid phase/concentration in solution phase) was computed as theslope of the sorption isotherm. The distribution coefficients were significantly correlated to soilorganic matter content for Ni, Cu, Cd, and Pb at pH 4.5 and for Ni, Cu, Zn, and Cd at pH 6.5.There was also a correlation between Kd and soil specific surface area but no relationship to othersoil properties such as CEC, clay content, and noncrystalline Fe and Al materials. Therefore, soilorganic carbon and surface area appear to be the most important soil properties influencing metaladsorption through formation of organo-metal complexes. The Kd values for all elements werehigher at pH 6.5 than at 4.5. Selectivity between metals resulted in the following metal affinitiesbased on their Kd values: Pb>Cu>Zn>Ni>Cd≈Cr at pH 4.5 andPb>Cu≈Zn>Cd>Ni>Cr at pH 6.5.     

Do surface active substances from water repellent soils aid wetting?

Soil water repellency is usually unstable, as exemplified by the common method of quantifying repellency degree – the water drop penetration time (WDPT) test. Dynamic penetration and infiltration of water into repellent soils is generally attributed to either reduction of the solid‐liquid interfacial energy (γ_SL) or reduction of the liquid‐vapour interfacial energy (γ_LV), or both. The reduction of γ_SL can result from conformation changes, hydration, or rearrangement of organic molecules coating soil particle surfaces as a result of contact with water, while the reduction of γ_LV can result from dissolution of soil‐borne surface active organic compounds into the water drop. The purpose of this study was to explicitly test the role of the second mechanism in dynamic wetting processes in unstably repellent soils, by examining the drop penetration time (DPT) of water extracts from repellent soils obtained after varying extraction times and at different soil : water ratios. It was indeed found that soil extracts had lower surface tensions (γ_LV approx. 51–54 mN m⁻¹) than distilled water. However, DPT of the soil extracts in water repellent soils was generally the same or greater than that of water. Salt solutions with the same electrical conductivity and monovalent/divalent cation ratio as the soil extracts, but lacking surface active organic substances, had the same DPT as did the extracts. In contrast, DPT of ethanol solutions prepared with the same γ_LV, electrical conductivity, and monovalent/divalent cation ratio as the soil extracts, was much faster. Ethanol solutions are usually used as an agent to reduce γ_LV and as such, to reduce DPT. It is concluded that the surface‐active, soil‐derived organic substances in aqueous soil extracts do not contribute to wetting dynamics, and as such, this mechanism for explaining kinetics of water penetration into water repellent soils is rejected. It is also concluded that the rapid penetration of ethanol solutions must be due not only to changes in γ_LV, but to also to changes in either or both γ_SL and the solid‐vapour interfacial energy (γ_SV). These results stand in sharp contrast to well‐accepted logical paradigms.     

非饱和土壤导水参数的推求

本文根据一维非饱和土壤水分的垂直入渗再分布和水平扩散再分布,采用土壤湿润剖面平均湿度和湿润锋湿度之间函数关系的三种形式,分别推导出非饱和土壤导水率,水分扩散率,比水容量的解析表达式,解析表达式中仅有四个独立参数,均可通过实验数据的简单拟合而得到.与其它方法相比,这种新的推求方法具有花费少、准确度高和测定范围大等特点.     

间隔覆盖法对塿土坡面土壤水分调控效果

利用室内人工模拟降雨试验,分析研究间隔覆盖条件下坡面土壤水分空间分布及蓄存与利用情况。结果表明：各面积比处理的湿润锋分布形状基本相同,但其深度差别较大,湿润锋在入渗区上部的分布形状体现了间隔覆盖法的特色;各处理入渗区表层土壤含水率沿坡不同位置点略有差异,坡面含水率分布的差异程度有随着覆盖区面积的增大而降低的趋势。各处理剖面含水率分布则符合一般规律,即由表层向深层逐层减少;覆盖面积比及降雨强度是影响入渗区雨水可利用率的重要因素。入渗区的雨水可利用率随面积比增大呈降低趋势,但其入渗量、单位面积承接及蓄存水量均呈增加趋势,这对入渗区植物生长发育及抵抗干旱是有利的。而这些指标随降雨强度增大,则表现出相反的趋势,间隔覆盖法在小雨强下的实施效果较好。对于在不同试验条件下,如何确定适宜面积比进而提高雨水可利用率,以及该方法在不同土壤及降雨条件下的实施效果仍是需要进一步研究的问题。     

Pore shape and organic compounds drive major changes in the hydrological characteristics of agricultural soils

A small increase in soil organic matter (SOM) content can change soil hydrological properties from a completely wettable to a partially water‐repellent state. Although considerable research describes hydrophobic compounds as a primary driver of this shift, the influence of pore shape has only been considered in a few studies and none of these has emphasized the role of different carbon compounds. Using a capillary bundle model of non‐cylindrical (wavy) capillaries, we described measured hydrological properties of five agricultural soils that have a small degree of water repellency and textures ranging from coarse sand to heavy clay. To isolate the influence of SOM, it was removed by combustion to provide an SOM‐free treatment. Water and methanol sorptivities quantified infiltration rates and soil‐water wetting angles in packed soil cores. Different cores were sectioned to measure wetting profiles and calculate diffusivity. The results from natural soils were supplemented by measurements carried out on model ‘soils’ consisting of quartz particles (50–200 µm) with four different hydrophobic states. Soil organic matter removal increased water sorptivity from about 60% for a coarse sandy soil (Haplic Arenosol) to about 290% for a heavy clay soil (Haplic Leptosol), corresponding to a decreased apparent wetting angle of 20–30°. Application of the wavy pore model suggests that the apparent wetting angle resulting from SOM removal can be several times smaller than its Young value. Generally, SOM removal increased water diffusivity values by one to two orders of magnitudes. The SOM components having the greatest impact on contact angle were hexanedioic acid and heneicosanoic acid (both hydrophilic) and docosane (hydrophobic).