Abstract
Methods to evaluate soil water repellency (SWR) require extensive studies on field soils and are subject to the heterogeneity of SWR throughout the soil profile as well as plant/soil interactions. The objectives of this study were to develop a synthetic method to create hydrophobic sand, and to determine if that hydrophobic sand would affect the establishment of bermudagrass (Cynodon dactylon L. Pers. × C. transvaalensis Burtt-Davy, cv. Tifeagle) sprigs. Two techniques were developed to render sand hydrophobic: soap:sand method (hydrophobic sand; HSS) and sand:peat method (hydrophobic sand and read sedge peat; HSP). Both HSS and HSP remained severely hydrophobic at 0 cm depth for only 7 d, and at the 1- to 6-cm depth for 77 continuous days, as determined by water drop penetration time. Bermudagrass establishment, root growth, or shoot growth in two greenhouse experiments with four root zone substrates–HSS, HSP, WSAND (wettable sand), and WSP (wettable sand and reed sedge peat)—were not consistent. In conclusion, both HSS and HSP were shown to be safe and effective methods to synthetically produce hydrophobic sand for potential use in laboratory research, but further evaluation is needed to determine the feasibility of using HSS and HSP for turfgrass growth evaluations.
The global distribution of subcritical and severe SWR is widespread, occurring in sand dunes (Dekker et al. 2001), agricultural lands (Cooley et al. 2009), golf course putting greens (Cisar et al. 2000), fairways (Oostindie et al. 2008), fire-affected soils (Robichaud 2003), forests (Orfanus et al. 2008), and pastures (Johnson et al. 2005). SWR may be referred to as soil hydrophobicity and defined as soil that repels water or the inability of soil particles to accept or attract water. Within the aforementioned environments and ecosystems, the spatial variability of SWR may be extreme. Heterogeneity of severity of water repellency or hydrophobicity in soils is extensive and dependent on several factors: soil moisture, fire, seasons, temperature, vegetation, management practices, and soil texture (Dekker and Ritsema 2003; Hallett et al. 2001; Johnson et al. 2005; Keizer et al. 2005; Leighton-Boyce et al. 2005; Orfanus et al. 2008; Woche et al. 2005). This degree of SWR variability in a field site warrants the development of a laboratory method to induce hydrophobicity on soil for critically testing and evaluating not only amelioration practices but also studying the impact of SWR on germination, plant establishment, and growth parameters. The method must be consistent for repeatability of research experiments as well as practical for ease of use.
Current methods to produce hydrophobic sands include soil treatment with an organosilicone material commonly used in the semiconductor industry known as octadecyl trichlorosilane (OTS). Bauters et al. (1998) and Song et al. (2014) applied OTS to sand at varying percentages to study water movement, water infiltration, and soil surfactant efficacy on sands. This method resulted in synthetically derived hydrophobic sand that performed similar to naturally occurring water repellent sand. However, OTS is listed as a Category 1 hazardous material and may cause eye and skin damage as well as being a respiratory hazard (Fisher Scientific 2015). Furthermore, OTS has not been studied as a water repellent substrate for evaluating plant responses, and plants typically will not establish in OTS-treated substrates/sands (Kostka S, personal communication).
A potentially nontoxic option other than OTS could be stearic acid, which is a long-chain saturated fatty acid derived from many animal and vegetable fats and oils (Fisher Scientific 2022; NCBI 2022). Stearic acid has hydrophobic properties and is used in the manufacture of soaps, food additives, cosmetics, lotions, shampoos, and detergents (Boyle and Leifer 2002; Fidanza et al. 2020). Shorter-chain fatty acids may prove harmful to plants, whereas the longer-chain fatty acids may act as insecticides (Cloyd 2022). Another nontoxic option could be reed sedge peat, a material with naturally derived hydrophobic properties. Reed sedge peat typically is harvested from bogs containing well-decomposed reeds, sedges, and grass species. Peat has been shown to contribute to water repellency, and peats in general, when dry, are hydrophobic (Moody et al. 2009; Perdana et al. 2018). Therefore, the objectives of this study were to 1) determine if soap or reed sedge peat could induce water repellency on sand and remain stable under controlled environment temperature and irrigation conditions, and 2) determine if synthetically prepared water repellent sand would affect vegetative bermudagrass establishment.
Materials and Methods
Location.
This study was conducted at the University of Florida’s Institute of Food and Agricultural Sciences, at the Fort Lauderdale Research and Education Center, in Davie, FL. Experiments one and two were initiated in May 2013 and October 2013, respectively.
Soap method.
Ivory® bar soap [Procter & Gamble Corp., Cincinnati, OH; ingredients: sodium tallowate and/or sodium palmate, water, sodium cocoate and/or sodium palm kernelate, glycerin, sodium chloride, fragrance, coconut acid*, palm kernel acid*, tallow acid*, palm acid*, tetrasodium EDTA (*contains one or more of these ingredients)] was used to make a soap or stearic acid solution to hydrophobize sand. Five grams of Ivory® bar soap and 500 mL of deionized water were placed in a bowl and heated inside a laboratory microwave (1000 W) for 140 s to completely dissolve the soap. An additional 500 mL of deionized water was added to the microwaved stearic acid solution. One 22.6-kg bag of coarse sand (Florida Silica Sand, Miami, FL) was poured into a small, portable cement mixer followed by the 1000-mL stearic acid solution being poured onto the sand. The sand and solution were blended for 10 minutes until the sand appeared thoroughly and completely wetted. The coated sand was poured into 30-cm length × 23-cm width × 10.25-cm depth aluminum pans until half full, then oven dried for 4 hours at 65 °C. After cooling to ambient temperature, the coated sand was tested for water repellency. Ten random 25-mL samples were collected from the pans and placed on the laboratory bench. Next, a 35-μL pipette was filled with deionized water, and one drop placed on the surface of all samples. A stopwatch was used to record length of time for the entire drop to penetrate completely into the sand surface, and water drop penetration time (WDPT) was recorded in seconds according to the procedure described in Dekker et al. (2009). Of note, WDPT measurements were halted at 600 s and time was recorded. The severity of repellency was based on classification by Dekker et al. (2009) (Table 1). The sand was found to be severely water repellent (WDPT ≥600 s). The final product was hydrophobic soap coated sand (HSS).
Classification system for water repellent soils based on water drop penetration time.
Sand and reed sedge peat method.
A 90%:10% v/v blend of coarse sand (Florida Silica Sand) and reed sedge peat (Dakota Peat and Equipment, Inc., Grand Forks, ND) were placed in a small cement mixer and blended until thoroughly and uniformly mixed. The sand and peat mixture was placed into 30-cm length × 23-cm width × 10.25-cm depth aluminum pans until half full. The pans were placed in 180 °C oven for 2 hours. After this mixture cooled, it was tested for water repellency as described for the soap method and found to be severely water repellent. The final product was a hydrophobic sand and reed sedge peat mixture (HSP).
Duration of hydrophobicity.
Plastic containers (3.75 cm × 20.65 cm) were filled with either HSS (n = 33) or HSP (n = 33) and placed inside a greenhouse on a bench under misters that irrigated the containers with 62.5 mm water per day. Each week, for 11 continuous weeks, three containers of each HSS and HSP were removed to measure water repellency using a laboratory test for WDPT (Dekker et al. 2009).
Bermudagrass establishment.
For each of two greenhouse experiments conducted, poly-vinyl chloride lysimeters (7.6-cm diameter × 30.5-cm length) were assembled and filled with either two hydrophilic (wettable) root zone substrates, or two hydrophobic (water repellent) root zone substrates. The wettable root zone treatments were WSAND (100% wettable sand; Florida Silica Sand) or WSP (90%:10% v/v wettable sand:peat). The hydrophobic root zone treatments were HSP or HSS. Representative substrate samples were sent to a laboratory (Hummel and Company, Trumansburg, NY) for physical and chemical analysis (Table 2).
Chemical and physical characteristics of substrates.
Root zone substrates were firmly packed into the lysimeters and sprigged at a rate of 20.0 gm2 bermudagrass (cv. Tifeagle) per column. A complete liquid fertilizer (24N–8P2O5–16K2O) (Miracle-Gro Water Soluble All Purpose Plant Food, Scotts-MiracleGro, Marysville, OH) was applied weekly at a rate of 2.4 g·m−2 N. During the establishment phase, 25.0 mm of total irrigation throughout the day was applied via pop-up misters. Experiment one was initiated on 16 May 2013, and visual assessment of percent surface area covered with bermudagrass was recorded about every 10 d. Experiment two was initiated on 8 Oct 2013, and percent cover ratings were recorded every 7 to 14 d for 132 d. Visual bermudagrass cover ratings ceased when all treatments reached ≥90% surface area covered.
At the conclusion of both experiments, columns were deconstructed to determine bermudagrass shoot counts (i.e., total number of shoots per surface area of column) and roots were analyzed using WinRHIZO PRO v. 2009 (Regent Instruments Inc. Ottawa, ON, Canada). Also, for both experiments, water repellency was determined via WDPT. The recorded WDPT for each root zone substrate was the average of five measurements obtained from a composite sample of the 0- to 15-cm root zone depth.
Experimental design and statistical analysis.
For the duration of hydrophobicity experiment, all samples were arranged as a randomized complete block with four replicates, and WDPT data were subjected to analysis of variance (ANOVA) using PROC GLM (Statistical Analysis Software, v. 9.3; Cary, NC, USA) with treatment means compared using Tukey’s multiple range test at P ≤ 0.05 (Mead et al. 2003). For establishment experiments, treatments were arranged in a complete randomized block design with four replications, and all data were subjected to ANOVA using PROC GLM (Statistical Analysis Software, v. 9.3) and treatment means compared using preplanned orthogonal contrasts at P ≤ 0.05 (Mead et al. 2003).
Results and Discussion
Substrate chemical and physical characteristics.
All four root zone substrates—WSAND, WSP, HSS, and HSP—are soils with less total surface per area on a volume basis or coarse textured sand soils that are more likely to be water repellent then clay soils with larger surface area (Cisar et al. 2000; Ma’shum et al. 1988). Substrate pH was considered suitable for turfgrass (Turgeon and Kaminski 2019) at 7.2, 6.4, 6.9, and 6.3 for WSAND, WSP, HSS, and HSP, respectively (Table 2). Organic matter content was 0.04% and 0.06% for WSAND and WSP, respectively, and 0.81% and 0.68% for WSP and HSP, respectively (Table 2). For all substrates, bulk density ranged from 1.67 to 1.69 g·cm−3, and particle density ranged from 2.63 to 2.65 g/cm3 (Table 2). Saturated hydraulic conductivity was higher with WSAND and HSS at 129.3 and 119.6 cm·h−1, respectively, compared with 73.1 and 91.4 cm·h−1 for WSP and HSP, respectively (Table 2). Of note, hydrophobic sand (∼HSS) exhibited similar infiltration or measured saturated hydraulic conductivity as wettable sand (∼WSAND) (Dekker et al. 2001). For all substrates, total porosity ranged from 36.4% to 36.7%, and aeration porosity ranged from 27.5% to 32.1% (Table 2). Cation exchange capacity (cmol⋅kg−1) for WSAND, WSP, HSS and HSP ranged from 1.10, 3.03, 1.68, and 3.63) (data not presented).
Substrate duration of hydrophobicity.
With both HSS and HSP, WDPTs were not recorded for more than 600 s (Table 3) as that is considered severely water repellent (Dekker et al. 2009). At the start of the evaluation period (i.e., week zero), HSS and HSP were severely water repellent at all depths including the air-surface interface of 0-cm depth (Table 3). At week 1, WDPT at 0 cm was severely water repellent for HSS and strongly water repellent for HSP, at 600 and 479.7 s, respectively (Table 3). At weeks 2 and 3, WDPT at 0 cm was slightly water repellent for both HSS and HSP (Table 3). At week 4, HSS was wettable and HSP was still slightly water repellent (Table 3). At weeks 5 through 11, WDPT at 0 cm was considered wettable (WDPT < 5 s) for both HSS and HSP (Table 3). At only week 6, 8, and 9, HSP was significantly more hydrophobic compared with HSS; however, WDPTs for both were ≤5.8 (Table 3). At the 0-cm depth (i.e., surface interface), the wettability or lack of water repellency may be explained by the repeated impact of fine water drops from overhead irrigation that perhaps degraded the hydrophobic coating (Dekker et al. 2009). At depths of 1 through 6 cm, HSS and HSP were severely water repellent as maintained under greenhouse conditions for the duration of the 11-week evaluation period (Table 3).
Duration of hydrophobicity by root zone depth for hydrophobic soap coated sand (HSS) and a hydrophobic 90:10% v/v blend of coarse sand and reed sedge peat (HSP) as determined by water drop penetration time (WDPT).
The WDPT results indicate that under greenhouse conditions with overhead irrigation supplied to bare or exposed HSS or HSP, both the HSP and HSS at 0 cm were severely to strongly water repellent for the first 2 weeks, slightly water repellent for the next 2 weeks, and wettable for the remaining 7 weeks (Table 3). The severely water repellent WDPTs for both HSS and HSP at the 1- through 6-cm depths were similar to extremely water repellent WDPTs measured on both a 10 g·kg−1 and 30 g·kg−1 stearic acid–treated silica sand (Dlapa et al. 2004). Of note, HSP had not been evaluated before this evaluation for longevity of water repellency. Thus, both HSS and HSP maintained severe hydrophobicity under daily irrigation and greenhouse conditions at the 1- to 6-cm depths for 11 continuous weeks (i.e., 77 d).
Bermudagrass establishment.
Bermudagrass cover for all substrates was 2% at the start of Expt. 1 (Table 4). Bermudagrass cover in WSAND was not significantly different vs. HSS throughout the experiment, with a final cover of 33.1% for WSAND and 29.4% for HSS (Table 4). Bermudagrass cover for WSP was significantly higher compared with HSP from 15 through 52 d of the experiment, with a final cover of 72.2% for WSP and 45.9% for HSP (Table 4).
Bermudagrass cover by root zone substrate over time in Expt. 1.
After Expt. 1, it was determined that HSS and HSP were still slightly or strongly water repellent, respectively (Table 8); therefore, the second trial ran for longer to determine if cover would reach 100% and if so, were the sands still water repellent. In Expt. 2, bermudagrass cover for all substrates was 2% at the start (Table 5). Cover assessment did not start until most substrates exhibited ∼20% cover with the objective of determining which treatment or treatments would reach 100% first (Table 5). Bermudagrass cover for WSAND was not significantly different vs. HSS throughout most of the experiment, except at day 35 with 46.3% vs. 32.5% cover for WSAND and HSS, respectively (Table 5). Final cover for WSAND was 92.5% and 91.3% for HSS (Table 5). Bermudagrass cover for WSP was not significantly different vs. HSP during most of the experiment, except at days 27 and 35 (Table 5). On day 27, cover for WSP was significantly higher vs. HSP at 56.3% vs. 41.5%, respectively (Table 5). On day 35, cover for WSP also was significantly higher vs. HSP at 73.8% vs. 48.7%, respectively (Table 5). Final cover for WSP and HSP was 100.0% (Table 5).
Bermudagrass cover by root zone substrate over time in Expt. 2.
In both experiments, perhaps the higher organic matter content, cation exchange capacity, and the lower saturated hydraulic conductivity in WSP and HSP provided greater nutrient and water retention than WSAND and HSS (Table 2). Establishment and cover results demonstrate HSS and HSP did not injure or kill the bermudagrass sprigs, and therefore were not considered phytotoxic to bermudagrass as established from sprigs. As reported in previous research, SWR deterred establishment of perennial ryegrass (Lolium perenne L.) seed and rangeland grasses mainly by reducing water infiltration and moisture availability to the plant which also may explain low establishment in WSAND (Osborn 1968). Murphy et al. (2005) also reported that “L-93” creeping bentgrass (Agrostis stolonifera L.) establishment and quality was significantly lower on straight sand compared with amended sand greens. Because substrate root zone moisture could not be evaluated during the establishment experiments, it is likely that the water repellent or hydrophobic properties of HSS and HSP were contributing factors to reduced initial bermudagrass cover. As reported by Steenhuis et al. (2001), as little as 3% w/w of hydrophobic particles may reduce wetting of the soil and increase the likelihood of preferential flow path formation. As seen by the lack of infiltration in both HSS and HSP (Figs. 1 and 2), preferential flow paths may have formed on the sides of the columns (Fig. 3) as a result of the hydrophobic properties of those two substrates.
Example of water remaining on the surface and not infiltrating into HSS (hydrophobic soap coated sand) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Example of water remaining on the surface and not infiltrating into HSS (hydrophobic soap coated sand) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Example of water remaining on the surface and not infiltrating into HSS (hydrophobic soap coated sand) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Example of water remaining on the surface and not infiltrating into HSP (hydrophobic sand and reed sedge peat mixture) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Example of water remaining on the surface and not infiltrating into HSP (hydrophobic sand and reed sedge peat mixture) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Example of water remaining on the surface and not infiltrating into HSP (hydrophobic sand and reed sedge peat mixture) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Example of a preferential water flow path exhibited in HSS (hydrophobic soap coated sand) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Example of a preferential water flow path exhibited in HSS (hydrophobic soap coated sand) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Example of a preferential water flow path exhibited in HSS (hydrophobic soap coated sand) substrate.
Citation: HortScience 58, 12; 10.21273/HORTSCI16963-22
Therefore, this lack of water infiltration throughout the profile may have initially hindered or slowed bermudagrass establishment in the severely water repellent HSS and HSP when compared with the wettable WSAND and WSP (Dekker and Ritsema 2003). It was not feasible for root zone volumetric water content data to be collected during the experiment. It is possible the low moisture-holding capacity of sand root zones and their high infiltration rates (Waddington 1992), whether in a hydrophobic or hydrophilic substrate, negatively affected bermudagrass establishment.
Differences among the hydrophobic substrates may be attributed to their higher organic matter content (Table 2). As reported by Cann (2000), the addition of clay increases surface area of sand grains and may cover hydrophobic areas on the sand grain, rendering the sand hydrophilic and also increasing water retention. A benefit of adding peat to a sand root zone is to increase moisture retention (Waddington 1992). Murphy et al. (2005) also reported improved creeping bentgrass (Agrostis stolonifera L.) establishment in organic-amended root zones when compared with inorganic-amended root zones. The higher organic matter content (Table 2) in WSP and HSP may have provided more plant available moisture to the bermudagrass vs. WSAND and HSS. Thus, the organic matter content may have provided an additional benefit to the bermudagrass grown in WSP and HSP as faster establishment and 100% cover was achieved by the end of Expt. 2 (Table 5).
Bermudagrass establishment experiments—root analysis.
Root length, surface area, diameter, density, volume, and weight were all measured at the conclusion of each experiment (Tables 6 and 7). In Expt. 1, bermudagrass root length, surface area, density, volume, and total dry weight were significantly higher in WSAND vs. HSS (Table 6). Karnok and Tucker (2001) reported similar findings, indicating water repellent soil and soil moisture stress negatively affect turfgrass rooting. The possible reduction of water and nutrient retention in WSAND and HSS could negatively affect root growth in those two substrates and the additional stress of SWR in HSS was most likely another contributing factor to poor root growth. No significant differences with root measurements, however, were observed with WSP vs. HSP (Table 6).
Root analyses and shoot count of bermudagrass for each root zone substrate at the conclusion of Expt. 1.
Root analyses and shoot count of bermudagrass for each root zone substrate at the conclusion of Expt. 2.
In Expt. 2, bermudagrass root length was the only root measurement significantly higher in WSAND vs. HSS (Table 7). Root length, surface area, density, volume, and total dry weight were all significantly lower in WSP vs. HSP (Table 7). Although some pronounced effects with root zone substrate and root measurements were observed in Expt. 1 and Expt. 2 (Tables 6 and 7), the results were not consistent between the experiments. The root measurements from the two experiments may have been confounded by the duration of the study, and this reflects some of the challenges to produce an artificial system for a naturally occurring phenomenon. It is not understood whether changes in SWR in some of the substrates is significantly influenced by irrigation, reorientation of hydrophobic compounds, plant species, and plant establishment, or other factors (Schlossberg et al. 2005).
Bermudagrass establishment experiments—shoot analysis.
Turfgrass shoots and stand density are considered a reflection of turfgrass quality and health (Turgeon and Kaminski 2019). In Expt. 1, shoot growth was not significantly different in WSAND vs. HSS, or in WSP vs. HSP (Table 6). In Expt. 2, shoot growth again was not significantly different in WSAND vs. HSS, but shoot growth was lower in WSP vs. HSP (Table 7).
Bermudagrass establishment experiments—WDPT.
At the end of Expt. 1 (duration = 52 d) and Expt. 2 (duration = 132 d), WDPTs confirm WSAND and WSP remained wettable (WDPT < 5 s), and HSS was slightly water repellent (WDPT 5–60 s). HSP remained strongly water repellent (WDPT 60–600 s) in Expt. 1 and slightly water repellent (WDPT 5–60 s) in Expt. 2 (Table 8).
Water drop penetration time (WDPT) for root zone substrates as determined at the conclusion of Expt. 1 and Expt. 2.
Conclusion
Both synthetically prepared HSS and HSP were hydrophobic at the surface under irrigated conditions in a greenhouse environment for ∼3 and 4 weeks, respectively. At the 1- to 6-cm depths, however, the substrates remained severely hydrophobic for 77 d when no vegetative cover was present. Therefore, both HSS and HSP represent valid methods that could be used to produce hydrophobic or water repellent sands and sand mixtures for use in research.
When using these hydrophobic root zone substates (i.e., HSS or HSP) to test the effects of water repellency in the laboratory or greenhouse, consideration should be given to specific parameters to evaluate. For example, HSS and HSP could be used to evaluate water repellency effects on plant growth, and to evaluate amelioration strategies by testing experimental soil surfactants, biostimulants, and cultural practices. Although the hydrophobicity of HSS and HSP lasted for ∼77 d under daily irrigation while inside a greenhouse, the hydrophobic duration is unknown if a plant is introduced into the substrate or if maintained under different growing or environmental conditions. It is implied that the “manufactured” water repellency of HSS or HSP lasts for a finite period and is not permanent.
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