Soil Repellency Research Articles

Double layer approach to create durable superhydrophobicity on cotton fabric using nano silica and auxiliary non fluorinated materials

Creation of differential superhydrophobicity by applying different non-fluorinated hydrophobization agents on a cotton fabric roughened with silica nanoparticles was studied. Cotton fabric surface has been functionalized with silica nanoparticles and further hydrophobized with different hydrophobic agents such as hexadecyltrimethoxy silane (HDTMS), stearic acid (SA), triethoxyoctyl silane (OTES) and hybrid mixtures of HDTMS/SA and HDTMS/OTES. The cotton fabrics before and after the treatment were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM) and thermogravimetric analysis (TGA). The wetting behavior of cotton samples was investigated by water contact angle (WCA) measurement, water uptake, water repellency and soil repellency testing. The treated fabrics exhibited excellent water repellency and high water contact angles (WCA). When the mixture of two hydrophobization agents such as HDTMS/OTES and HDTMS/SA is used, the water contact angle has increased (145°⿿160°) compared to systems containing HDTMS, OTES, SA alone (130°⿿140°). It was also noted that this fabricated double layer (silica+hydrophobization agent) was robust even after applying harsh washing conditions and there is an excellent anti-soiling effect observed over different stains. Therefore superhydrophobic cotton surfaces with high WCA and soil repellency could be obtained with silica and mixture of hydrophobization agents which are cost effective and environmentally friendly when compared with the fluorosilane treatment.

Soil-water repellency characteristic curves for soil profiles with organic carbon gradients

Soil water repellency (SWR) of soils is a property with significant consequences for agricultural water management, water infiltration, contaminant transport, and for soil erosion. It is caused by the presence of hydrophobic agents on mineral grain surfaces. Soils were samples in different depths at three forest sites in Japan and three pasture sites in New Zealand, covering soil organic carbon (SOC) contents between 1 and 26%. The SWR was measured over a range of water contents by three common methods; the water drop penetration time (WDPT) test, the molarity of an ethanol droplet (MED) method, and the sessile drop method (SDM). The aim to (i) compare the methods, (ii) characterize the soil-water repellency characteristic curves (SWRCC) being SWR as a function of the volumetric soil-water content (θ) or matric potential (ψ), and (iii) find relationships between SWRCC parameters and SOC content. The WDPT, MED, and SDM generally agreed well in predicting the θ range where SWR occurred, and there was close agreement between SWR results determined by average MED and SDM at similar θ. Generally, SWR was only found within the top 20cm of the soil profiles. Six SWR parameters were introduced: (i) the area under the curve (SWR(θ)); (ii) θ at the maximum SWR (θWR-max), (iii) θ where SWR ceased (θnon-WR), (iv) the maximum SWR (CAi-max), (v) pF at the maximum SWR (pFWR-max) and (vi) pF where SWR ceased (pFnon-WR). The relationship between the first three parameters and SOC content were best described with Langmuir type equations (r2 of 0.5–0.7), while the other three parameters changed linearly with SOC contents.

Evaluation of the wear comfort of corporate identity clothes - the influence of water and soil repellency

Corporate identity clothes (CI-clothes) are today more and more important for companies and their identity. One important aspect is the appearance of the clothes over the long lifetime. After several washing and wearing cycles the appearance of such clothes should only show low differences between new and worn clothes. To minimize the signs of use many CI-clothes have a water and soil repellent finishing avoiding contamination. This finishing shows often a negative influence on the comfort of the clothing. The aim of the German research project 16365N was to optimize water and soil repellent finishing in combination with high industrial wash ability and high comfort.

Subcritical soil hydrophobicity in the presence of native and exotic arbuscular mycorrhizal species at different soil salinity levels

Soil salinity and arbuscular mycorrhizal fungi (AMF) influence the soil hydrophobicity. An experiment was performed to determine the effects of soil salinity and AMF species on soil water repellency (SWR) under wheat (Triticum aestivum L.) crop. Six AMF treatments, including four exotic species (Rhizophagus irregularis, Funneliformis mosseae and Claroideoglomus claroideum, a mix of three species), one mix native AMF species treatment and an AMF-free soil in combination with four salinity levels (1, 5, 10, and 15 dS m−1) were used. The soil repellency index (RI) increased with salinity increment ranging from 2.4 to 10.5. The mix of three exotic and native AMF treatments enhanced the RI significantly compared to AMF-free soil in all salinity levels with one exception for native treatment at 1 dS m−1. Among individual AMF species, the C. claroideum treatment at 10 dS m−1 increased the RI by 67% compared to AMF-free soil. The native AMF treatment was more efficient in root colonization, glomalin production and SWR development at 10 and 15 dS m−1, compared to exotic species. In addition to the net positive effect of salinity on SWR, the AMF influences on the RI were greatly dependent on salinity levels.

The effect of dairy sewage sludge amendment on repellency and hydraulic conductivity of soil aggregates from two depths of Eutric Cambisol

AbstractAppropriate management of sewage sludge is an important worldwide issue due to the still growing amount of wastewaters. In the study we examined to what extent the addition of dairy sewage sludge compared with mineral fertilization affects porosity, repellency index, and hydraulic conductivity of variously sized aggregates from two soil depths of Eutric Cambisol derived from loess: 5–15 cm and 25–35 cm. The repellency index was calculated as a ratio of ethanol and water sorptivity. Data on water and ethanol sorptivities of initially air‐dry soil aggregate fractions were obtained from steady state flow measurements using an infiltration device. Hydraulic conductivity was determined by measuring water infiltration at five pressure heads: –8, –6, –4, –2, and 0 cm of water column with the same device as for sorptivity determination. Addition of sewage sludge to the soil decreased the soil repellency index by an average of 27% in topsoil and 32% in subsoil for both aggregate sizes, respectively, and increased hydraulic conductivity about four times in both layers. Smaller aggregates (15–20 mm diameter) from soil amended with sewage sludge, in comparison with larger ones (30–35 mm diameter), had a higher repellency index by 36 and 24% in topsoil and subsoil, respectively. As for aggregates from soil with mineral fertilization, those differences were smaller and equal to 15% in subsoil, in topsoil smaller aggregates even had slightly lower repellency index (by 5%). Aggregates taken from the upper soil layer were more water repellent and had smaller hydraulic conductivity than those taken from subsoil, regardless of soil treatment and aggregate size.

Does water repellency of pyrochars and hydrochars counter their positive effects on soil hydraulic properties?

Adding carbon-rich solids from pyrolysis (pyrochars) and hydrothermal carbonization (hydrochars) to soils can improve their hydraulic properties and increase the plant available soil water capacity (AWC). However, the effects of the specific char characteristics on soil hydraulic properties are still little understood and thus, difficult to optimize for soil amendment. It is often assumed that the water repellency of chars is an important factor influencing the water retention of soils. In using the evaporation method and saturated conductivity measurements, we investigated how changes of hydraulic properties of a loamy sand with 2% (w/w) char addition depend on (i) production processes, (ii) feedstocks, (iii) particle size, and (iv) wetting properties of added chars. Applied chars comprised pyrochars carbonized at 750°C and hydrochars carbonized at 200°C and 250°C from two different feedstocks (digestate and woodchips) and two particle sizes (<0.5mm and 0.5–1.0mm). The wetting properties of the pure chars and soil–char composites were determined by the sessile drop method. To investigate the impact of the water repellency of chars, pure chars and soil–char composites were saturated by capillary water uptake under atmospheric conditions and by saturation from the bottom under vacuum conditions.The chars showed highly variable wetting properties with the contact angles being strongly correlated to the specific surface area of the chars. The saturated hydraulic conductivity was significantly increased after char addition for both vacuum and capillary saturated samples, while being independent of wettability characteristics. The effects for the unsaturated conductivity were negligible. In contrast to expectation, soil amendments with more water repellent chars caused higher increases of AWC (up to 15%), both after vacuum and capillary saturated conditions. Highest increase of AWC occurred for chars from digestate material, particle size fractions <0.5mm, and chars hydrothermally carbonized at 200°C. The increase of AWC only occurred in the low pressure range (pF1.8 to 2.5), whereas it was decreased in the high pressure range (pF2.5 to 4.2). Our study indicates that water repellency of pyrochars and hydrochars have little counteracting effect on the overall improvement of soil water retention and saturated conductivity, and other factors that also depend on production process and feedstock, such as particle size, intra-particle porosity, shape, and plasticity are more important.

Initial water repellency affected organic matter depletion rates of manure amended soils in Sri Lanka

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

Morphology of Rain Water Channeling in Systematically Varied Model Sandy Soils

We visualize the formation of fingered flow in dry model sandy soils under different raining conditions using a quasi-2d experimental set-up, and systematically determine the impact of soil grain diameter and surface wetting property on water channelization phenomenon. The model sandy soils we use are random closely-packed glass beads with varied diameters and surface treatments. For hydrophilic sandy soils, our experiments show that rain water infiltrates into a shallow top layer of soil and creates a horizontal water wetting front that grows downward homogeneously until instabilities occur to form fingered flows. For hydrophobic sandy soils, in contrast, we observe that rain water ponds on the top of soil surface until the hydraulic pressure is strong enough to overcome the capillary repellency of soil and create narrow water channels that penetrate the soil packing. Varying the raindrop impinging speed has little influence on water channel formation. However, varying the rain rate causes significant changes in water infiltration depth, water channel width, and water channel separation. At a fixed raining condition, we combine the effects of grain diameter and surface hydrophobicity into a single parameter and determine its influence on water infiltration depth, water channel width, and water channel separation. We also demonstrate the efficiency of several soil water improvement methods that relate to rain water channelization phenomenon, including pre-wetting sandy soils at different level before rainfall, modifying soil surface flatness, and applying superabsorbent hydrogel particles as soil modifiers.

ChemInform Abstract: Soil Water Repellency: Dynamics of Heterogeneous Surfaces

AbstractReview: 152 refs.

Impact of treated wastewater irrigation on water repellency of Mediterranean soils

Irrigation with treated wastewater (TWW) is gaining importance due to declining water availability in dry regions. TWW irrigation has various potential adverse effects on soil quality such as hydrophobic effects on soil surfaces, reducing initial sorptivity and promoting the formation of preferential flow paths. In May and June 2010, in situ infiltration measurements using mini disk tension infiltrometer were deployed in five different orchard plots in Israel to assess the impact of different irrigation water qualities on the soil water repellency index R. In most plantations, long-term test sites were accessed to compare adjacent plots irrigated with fresh water (FW) or TWW. Topsoil samples were analyzed for selected physical and chemical characteristics. The mean R values increased at all TWW sites, from +15 up to +55 % compared with FW sites. The water drop penetration time (WDPT) increased up to 30 fold at three of five TWW sites compared with FW sites. Subsequent U tests and multilevel analysis indicated an impact of the type of irrigation water on R and WDPT. Moreover, soil electrical conductivity and exchangeable sodium percentage were consistently higher at all TWW sites. These results show that irrigation water quality clearly influences physical and chemical properties of the soil.

Efectos de los incendios forestales en la erosionabilidad de los suelos en Galicia

Soil erodibility exerts a decisive influence on the severity of water erosion resulting from forest fires. This paper reports the results of various experiments conducted on soils inGalicia(NW Spain) with a view to identifying the main soil properties governing soil erodibility, as well as the impact of forest fires on this property and the consequences of the ensuing changes on thehydrological and erosive response of soil. By virtue of their predominantly coarse texture, high contents in organic matter and also high aggregate stability, soil forests inGaliciaare scarcely erodible. Because the impact of forest fires on their erodibility depends on these soil properties, it is also markedly dependent on fire severity. Soil erodibility seems scarcely affected by low-severity fires but increases markedly with increasing severity. The water repellency of soil, whether induced or boosted by fire, is a determining factor for the occurrence of runoff and erosion processes continually observed in burnt areas in NW Spain.

Water repellency of soils with various content of organic matter in north-eastern Poland

Abstract The objective of the work was to investigate hydrophobic properties of soil formations with various amounts of organic matter and occurring in young glacial landscape. The research was carried out in mineral, mineral-organic and organic (slightly and strongly silted mucks, sedge peat, alder wood peat, reed peat) soil formations. Water repellency is very important in soil protection. It favors the formation of stable aggregates and prevents from soil erosion. The study was carried out applying two methods . water drop penetration time test (WDPT) and alcohol percentage test (AP). Among 51 analyzed soil samples in WDPT test, 64.7% of mineral and mineral-organic soil formations were hydrophilic. Among organic soil formations 37.6% was slightly and strongly hydrophobic and they represented strongly silted mucks. Unsilted and slightly silted mucks, weakly and strongly decomposed peats, were very strongly (18.8%) and extremely (43.6%) hydrophobic. AP test showed that strongly silted mucks were moderately and very strongly hydrophobic. Slightly silted mucks, and peats were very strongly and extremely hydrophobic. It can be stated that water repellency decreases simultaneously with the degree of siltation of organic soil formations.

Effect of biochar on soil physical properties in two contrasting soils: An Alfisol and an Andisol

Improving soil physical properties by means of biochar application has been proposed in recent publications. The objective of this study was to investigate to what extent the addition of corn stover (CS) and biochars produced from the pyrolysis of corn stover feedstock (CS) at 350 and 550°C temperatures (CS-350, CS-550) affected aggregate stability, volumetric water content (θV), bulk density, saturated hydraulic conductivity (Ks) and soil water repellency of specific soils. Organic amendments (CS, CS-350, CS-550) were incorporated into a Typic Fragiaqualf (TK) and a Typic Hapludand (EG) soils at the rate of 7.18tCha−1, which corresponded to 17.3, 11.3 and 10.0tbiocharha−1 for the CS, CS-350 and CS-550 treatments, respectively. After 295d of incubation (T295), soils were sampled as (i) undisturbed samples for bulk density and Ks; and (ii) mildly disturbed samples for θV (at −15, −1, −0.3, −0.1, −0.08, −0.06, −0.04, and −0.02bar), aggregate stability and soil water repellency. The θV at time 0 (T0) was also determined at −15, −1 and −0.3 matric potentials for the different treatments. Biochar application significantly increased (P<0.05) aggregate stability of both soils, the effect of CS-550 biochar being more prominent in the TK soil than that in the EG soil, and the reverse pattern being observed for the CS-350 biochar. Biochar application increased the θV at each matric potential although the effect was not always significant (P<0.05) and was generally more evident in the TK soil than that in the EG soil, at both T0 and T295. Biochar addition significantly (P<0.05) increased the macroporosity (e.g., increase in θV at −0.08 to 0bar) in the TK soil and also the mesoporosity in the EG soil (e.g., increase in θV from −1 to −0.1bar). Both biochars significantly increased (P<0.05) the Ks of the TK soil, but only CS-350 biochar significantly increased (P<0.05) the Ks in the EG soil. Biochar was not found to increase the water repellency of these soils. Overall results suggest that these biochars may facilitate drainage in the poorly drained TK soil. However, the present results are biochar-, dose- and soil-specific. More research is needed to determine changes produced in other biochar, dose and soil combination, especially under field conditions.

Soil water repellency: Dynamics of heterogeneous surfaces

Processes controlling the dynamics of the soil–water interfacial properties have a high impact on habitat, filter, buffer, storage, and transformation functions of a soil. Besides surface roughness and chemical heterogeneity, also the dynamics of surfaces properties in soils limit the application of well established methods of wettability determination for solid materials. Numerous studies investigated the influence of changing environmental conditions, like water content, pH and drying and wetting temperature on the repellency of soils. The present paper presents an integral approach linking some of the individual results of several studies. Two hypothetical models are suggested in order to explain differences in the nature of repellency between two types of sites and between wettable and repellent samples within each type of site. The chemical nature dominating the soil water repellency at the one type of sites can be best explained by hydrolysis–condensation reactions. The physico-chemical nature dominating the repellency on the other type of site is probably controlled by micelles- or reverse-micelles-like arrangement of amphiphilic molecules during drying. Wetting properties of the surface layers of organic coating on the soil particles then depend on number and size of amphiphilic molecules, pH and ionic strength in the soil solution. It is concluded that local site properties, e.g., soil-type, climate, or land-use, determine which mechanism controls the dynamics of repellency. Future research has to verify the suggested mechanisms before critical environmental conditions can be identified in order to prevent the negative impacts of soil water repellency.

Impact of biochar and hydrochar addition on water retention and water repellency of sandy soil

Application of biochar (BC) and hydrochar (HTC) in soils is being increasingly discussed as a means to sequestrate carbon and improve chemical and physical properties for plant growth. Especially the impact on physical properties is not well investigated so far.We study the impacts of biochar (BC) and hydrochar (HTC) on water retention characteristics (WRC) as well as on the wettability of sandy soils, using lab and field studies. Sandy soils with different amounts of organic matter were mixed with BCz (feedstock maize) and HTC (feedstock maize silage). Added amounts were 1, 2.5, and 5wt.%, respectively. The mixtures were packed in 100cm3 soil columns. In a field campaign identical amounts of BCf (feedstock beechwood) were added to the soil. Six months after incorporation undisturbed soil samples were taken. For these field samples available water capacity (AWC) was determined. For the packed soil columns the WRC was measured in the pressure head range from saturation to wilting point (−15,848cm). The extent of water repellency was determined for all samples using the water drop penetration time test.Addition of biochar leads to a decrease in bulk density, an increase in total pore volume as well as an increase in water content at the permanent wilting point. An increase in AWC could be observed for all sandy substrates used, except for the highly humic sand. Notable differences in the effects on the AWC could be measured among the three chars used. Particle size distribution of the chars as well as their consistency had different impacts on the pore size distribution of the soil matrix.No direct impact of the chars on the wettability of the soils could be observed. Local spots with hydrophobic character were detected among the samples with hydrochar, attributed to fungal colonisation.

Infiltration, runoff and soil loss in Andisols affected by forest fire (Canary Islands, Spain)

AbstractDepending on the severity of the fire, forest fires may modify infiltration and soil erosion processes. Rainfall simulations were used to determine the hydrological effects of fire on Andisols in a pine forest burned by a wildfire in 2007. Six burned zones with different fire severities were compared with unburned zones. Infiltration, runoff and soil loss were analysed on slopes of 10% and 30%. Forest floor and soil properties were evaluated. Unburned zones exhibited relatively low infiltration (23 and 16 mm h−1 on 10% and 30% slope angles, respectively) and high average runoff/rainfall ratios (43% and 50% on 10% and 30% slope angles, respectively), which were associated with the extreme water repellency of the forest floor. Nonetheless, this layer seems to provide protection against raindrop impact and soil losses were found to be low (8 and 16 g m−2 h−1 for 10% and 30% slope angles, respectively). Soil cover, soil structure and water repellency were the main properties affected by the fire. The fire reduced forest floor and soil repellency, allowing rapid infiltration. Moreover, a significant decrease was noted in soil aggregate stabilities in the burned zones, which limited the infiltration rates. Consequently, no significant differences in infiltration and runoff were found between the burned and the unburned zones. The decrease in post‐fire soil cover and soil stability resulted in order‐of‐magnitude increases in erosion. Sediment rates were 15 and 31 g m−2 h−1 on the 10% and 30% slope angles, respectively, in zones affected by light fire severity. In the moderate fire severity zones, these values reached 65 and 260 g m−2 h−1 for the 10% and 30% slope angles, respectively. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Post-fire rill and gully formation, Schultz Fire 2010, Arizona, USA

The Schultz Fire burned 6100ha on the eastern slopes of the San Francisco Peaks in northern Arizona. The fire burned between June 20th and 30th, 2010, across moderate to very steep ponderosa pine and mixed conifer watersheds. About 40% of the fire area was classified as high-severity, mostly on mountain slopes greater than 30% and in places exceeding 100%. The upper slopes rise to over 3300m and are the source for high energy water, coarse sediments, and woody material. A series of flood events beginning in mid-July 2010 initiated erosion that removed substantial soil from the upper slopes of the watersheds. The second event was characterized by a peak rainfall of 24mm in 10min. Substantial amounts of soil were eroded out of a newly developed rill and gully system, removing the A horizon and much of the B horizon. Prior to the fire there were no rills or gullies as the soil was protected by a thick O horizon. This protective organic layer burned off during the fire leaving the soil exposed to raindrop impact and erosion. There was widespread occurrence of high severity fire, with some watersheds classified as 70% high severity wildfire. This left most of the soils with moderate to severe water repellency. The development of an extensive rill and gully network fundamentally changed the hydrologic response of the upper portions of every catchment. The intense, short duration rainfall of the 2010 monsoon interacted with slope, water repellency and extensive areas of bare soil to produce flood flows an order of magnitude in excess of flows produced by similar pre-fire rainfall events. The rill networks now cover much of the upper slopes and gullies are beginning to cut to bedrock. Sediment delivery to the channel systems is likely to continue unabated for many years and hydrologic response will continue to be flashy.

Relevant effects of vegetal cover and litter on the soil hydrological response of two contrasting Mediterranean hillslopes at the end of the dry season (south of Spain)

AbstractIn Mediterranean regions, hillslopes are generally considered to be a mosaic of sink and source areas that control runoff generation and water erosion processes. These hillslopes used to be characterized by a complex hydrological and erosive response combining Hortonian and saturation excess overland flows. The hydrological response of soils is highly dependent on the soil surface components (e.g. vegetation patches, bare soil, rock fragment cover, crusts), which each one of them is dominated by a certain hydrological process. One of these soil surface components, not widely considered in studies of soil hydrology under Mediterranean conditions, is the accumulation of litter beneath shrubs enhancing water repellency in soils. This study investigates the influence of soil surface components, especially the litter accumulated beneath Cistus spp., in the hydrological and erosive responses of soils on two Mediterranean hillslopes having different exposures. The study was performed by means of rainfall simulation experiments and the Water Drop Penetration Time for measuring water repellency of soils, both techniques being carried out at the end of summer (September 2010) with very dry soils. The results indicate that (i) soil surface components from the north facing hillslope are characterized by a more uniform hydrological and erosive response than those from the south‐facing ones; (ii) the water repellency is more influential on the hydrological response of the north‐facing hillslope due to a greater accumulation of organic rest on the soils as the vegetation cover is also higher; (iii) the south‐facing hillslope seemed to follow the fertility island theory with very degraded bare soil areas, which are the most generated areas of runoff and mobilized sediments; (iv) the experimental area can be considered as a threshold area between the semiarid and subhumid Mediterranean environments, with the south‐facing hillslope being comparable with the former and the north facing one with the latter. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Water quality and surfactant effects on the water repellency of a sandy soil

Differences in irrigation water quality may affect the water repellency of soils treated or untreated with surfactants. Using simulated irrigations, we evaluated water quality and surfactant application rate effects upon the water repellency of a Quincy sand (Xeric Torripsamment). We used a split plot design with two irrigation water qualities, three surfactant application rates, two irrigations, and twelve sampling depths as fixed effects, with four replications. Each water quality x rate x irrigation combination was a main plot and depth was a repeated-measures subplot. A slightly water repellent Quincy soil (average water drop penetration time, WDPT, of 2.5 s) was packed in 25-mm lifts (or layers) to a bulk density of 1.6 Mg/cubic m into 0.15-m-high x 0.105-m-diameter plastic columns. We studied a nonionic surfactant, a blend of an ethylene oxide/propylene oxide block copolymer and an alkyl polyglycoside. We sprayed the surfactant at rates of 0, 9.4, and 46.8 L/ha, diluted with reverse osmosis water (RW) to apply 187 L/ha of solution, onto the soil surface of each packed column. About 24 h after surfactant application, columns were sprinkler irrigated with either RW or well water (WW) at 88 mm/h for 0.25 h without runoff. Well water (pH 7.6) contained 54.90 mg Ca/L, 67.16 mg Na/L, and had an electrical conductivity (EC) of 0.7 dS/m and sodium adsorption ratio (SAR) of 1.7. The RW (pH 5.7) contained 0.882 mg Ca/L, 3.201 mg Na/L, and had an EC of 0.0072 dS/m and SAR of 2.2. After the first irrigation with each water quality, half of the columns were destructively sampled in 12 increments to the 150-mm depth, with water content measured on a subsample and the remainder air-dried. Five days later, sample WDPT was measured. The remaining unsampled columns were oven dried at 30° C for three days to a mean soil water content < 0.04 g/g, cooled at ambient temperatures for 16 h, then irrigated with water of the same quality as before and sampled as previously. After the first irrigation, WDPT at depths from 97 to 117 mm averaged across surfactant rates reached a maximum of 28 s, regardless of irrigation water quality. WDPT was greatest at 117 mm with RW but only at 97 mm with WW. After the second irrigation, maximum WDPT was 1202 s at 139 mm with RW but only 161 s at 117 mm with WW, nearly 7.5 fold less than with RW. WDPT was greatest near the wetting front, irrespective of water quality. We conclude that irrigation water containing modest amounts of electrolytes or salts, in this case mostly salts of Ca and Na, reduces water repellency in the presence or absence of surfactant. Sprinkler irrigating this water-repellent soil translocated repellency-inducing moieties to the wetting front, concentrating them there in either surfactant-treated or untreated soils. Repeated irrigations further concentrated and drove repellency-inducing moieties deeper. When the sprinkler water contained modest amounts of electrolytes, the repellency at the wetting front was reduced nearly an order of magnitude. It appears that irrigating water-repellent soils using water containing electrolytes (probably Ca salts), greatly reduces repellency in the zone where hydrophobic substances accumulate in such soils. In our study, this occurred whether or not the soil was pretreated with surfactant. The results suggest that farmers and managers of turf and other agricultural land may be able to control the soil depth where repellency-inducing substances accumulate. For turf managers, this depth should be well below that of rooting. For farmers, it should be below the depth of rooting or below the depth of annual tillage, whichever is greater. Our experimental results may also help explain erratic surfactant performance under rainfed conditions where neither water quality nor depth of infiltration can be fully controlled.

Water-dependent repellency of model soils as affected by clay

Clay is an important factor that affects soil water repellency. The purpose of this study was to examine the effects of clay content on water-dependent repellency curve using model soils with the clay content ranging from 0 to 20%. Water-dependent repellency was measured using the water drop penetration time (WDPT) test in a drying process. Water repellency appeared (WDPT > 1 s), increased to be extremely repellent (WDPT > 3600 s), and decreased to be wettable again during drying of the model soils. Both the marginal water content at which water repellency appeared with drying (WCWR) and the highest water content at which soils become extremely repellent (HWCE) increased with increasing clay content. In contrast, the lowest water content at which soils remain extremely repellent (LWCE) was found to be independent of the clay content. The water-dependent repellency curve was not entirely shifted, but expanded towards the higher water content by the increase in clay content.