Evaluation of Two Soil Surfactants for Soil Water Management of Creeping Bentgrass on a Wettable Clay Loam Rootzone During an Imposed Dry-down Period¹

Soil surfactants, or more commonly known as “wetting agents” in the ornamental horticulture industry, have become an important and heavily relied-upon tool for water conservation with managed amenity and sports turfgrass ecosystems. In a field study on a wettable clay loam rootzone, two soil surfactants (OARS HS or PBS150; both from AquaAid Solutions, Rocky Mount, NC) were applied as sequential monthly applications to fairway-height creeping bentgrass (Agrostis stolonifera L. ‘L-93’) test plots. During an imposed 63-day dry-down period, test plots were irrigated based on a specific soil volumetric water content target. Turfgrass plots treated with OARS HS required 39.5% less irrigation water needed versus irrigating untreated turfgrass plots. PBS150-treated turfgrass required 35.6% less irrigation water versus untreated plots. For example, as extrapolated from this field study, 10 ha (25 acres) of creeping bentgrass fairways would require 15,383,853 L (4,063,984 gal) water for irrigating untreated turfgrass during that 63-day period. Turfgrass treated with OARS HS would use 9,307,231 L (2,458,710 gal) or achieve 6,076,621 L (1,605,274 gal) in water savings, and turfgrass treated with PBS150 would use 9,907,201 L (2,617,206 gal) or achieve 5,476,652 L (1,446,778 gal) in water savings. The economic benefit of reduced electrical energy consumption associated with a reduced need for operating the irrigation system was not determined. This study represents the first replicated field research investigation of turfgrass to quantify both a reduction in the amount of irrigation water inputs and a reduction in the number of irrigation events that may be accomplished with the use of soil surfactants. This study also illustrates the effectiveness of evaluating soil surfactant treatments and applications utilizing an imposed dry-down method.

Soil surfactants, also referred to commercially and to the practitioner as “wetting agents”, have become an important component in turfgrass management (Fidanza et al. 2019, 2023, Kostka 2000, Kostka and Fidanza 2017, 2018, 2019). Soil surfactant chemistries and product formulations have been reviewed by Zontek and Kostka (2012) and Fidanza et al. (2020). Golf course superintendents and sports turf managers routinely incorporate the use of soil surfactants into their agronomic maintenance programs (Bauer 2017, Jacobs and Barden 2018). With turfgrass management, soil surfactants are typically utilized to improve irrigation water use efficiency, as a component of an overall water conservation program, and to treat localized dry spot (or dry patch) and troublesome hydrophobic or water-repellent soil conditions (Cisar et al. 2000, Dekker et al. 2019, Kostka et al. 2007, Mitra et al. 2006, Moore et al. 2010, Soldat 2010).

Water-repellent field soils typically alternate between wettable and non-wettable status, and these conditions can occur during a particular season-of-the-year or between seasons due to changes with temperature, precipitation, or both (Crockford et al. 1991, Dekker and Ritsema 1994). Kuhnt (1993) noted that a soil surfactant’s performance or efficacy depends on the soil’s wettability, how the soil surfactant was applied, and the soil surfactant’s effects on the surface tension and liquid-solid (i.e., water on the soil particle surface) contact angle. Lehrsch et al. (2011) noted, however, that soil surfactants applied to wettable soils result in unknown outcomes.

In a national survey of golf course superintendents, the number one water conservation method employed was the use of soil surfactants (Gelernter et al. 2015, Lyman et al. 2007, Throssel et al, 2009). However, there is an insufficient amount of documented information that quantifies irrigation water savings in managed turfgrass as-a-result-of using of soil surfactants (Gelernter et al. 2015, Soldat et al. 2010). Therefore, the purpose of this investigation was to treat turfgrass maintained on a wettable soil rootzone with soil surfactants followed-by an imposed dry-down period, and during that dry-down period measure the frequency and amount of irrigation water inputs needed to maintain a predetermined soil volumetric water content.

The study was conducted at the Joseph Valentine Turfgrass Research Center located at The Pennsylvania State University (University Park, PA). The study site was placed under an aluminum framed hoop-house of 6.1 m (20 ft) width × 12.2 m (40 ft) length × 6.1 m (20 ft) vertical height. In mid-August 2015, during site preparation and prior to seeding with creeping bentgrass (Agrostis stoloniferous L. ‘L-93’), a 4 mL (0.15 inch) thick × 30 cm (11.8 inch) width of black plastic was placed along the entire soil perimeter of each predetermined test plot. A narrow trench was carefully hand-dug using a flat-spade to provide space for the plastic to be inserted fully just below the turfgrass surface to avoid being damaged from mowing. The plastic provided a physical barrier or “buffer zone” in-between all plots to keep rootzone water from moving or migrating into adjacent plots.

This field study was conducted inside an aluminum framed hoop-house in 2016 on eight-month-old and fully established creeping bentgrass maintained at a 12.7 mm (0.5 inch) height-of-cut, similar to golf course fairways in the Mid-Atlantic USA region (Thoms and Lindsey 2023). Turf was mowed three times per week with a reel mower, with clippings removed. Soil particle size analysis of rootzone soil samples measuring 1.9 cm (0.75 inch) diam × 10.2 cm (4 inch) depth were conducted at the Agricultural Analytical Services Laboratory (University Park, PA). Soil texture was determined as a clay loam, with 36.5% sand, 36.3% silt, and 27.2% clay (Fig. 1). The rootzone was considered wettable, since three random soil cores of 1.9 cm diam (0.75 inch) × 10.2 cm (4 inch) depth were removed prior to starting the field study for the purpose of conducting an in-field water droplet penetration test (Dekker et al. 2009), and water drop infiltration times measured ≤ 5 sec at the 1, 2, 3, and 4 cm (0.4, 0.8, 1.2, and 1.6 inch) depths.

Soil surfactant treatments were as follows: OARS HS (contains 85% octahydroxy polyoxyalkyene polymers, 7.5% amine salt of alkyl substituted maleic acid, and 7.5% inert ingredient; AquaAid Solutions, Rocky Mount, NC) applied at 15.9 L·ha⁻¹ (5 fl oz·1000 ft⁻²) at 28-day intervals for three applications on 30 March, 27 April, and 26 May, or PBS150 (contains 100% polyoxyalkylene polymers; AquaAid Solutions, Rocky Mount, NC) also applied at 15.9 L·ha⁻¹ (5 fl oz·1000 ft⁻²) at 28-day intervals for three applications on 30 March, 27 April, and 26 May. Two non-treated (i.e., no soil surfactant applied) treatments were included and identified as Untreated-Water and Untreated-Control (Table 1).

All treatments were arranged as a randomized complete block design with three replications. Individual plot size measured 0.9 m (3 ft) × 1.5 m (5 ft). All soil surfactant treatments were applied using a CO₂-pressurized (275 kPa [40 psi]) back-pack sprayer, calibrated to deliver 815 L·ha⁻¹ (2 gal·1000 ft⁻²) water carrier using a 11008E flat-fan nozzle resulting in 100% plot coverage. Immediately after all treatments were applied on all application dates, all soil surfactant-treated and non-treated plots received post application irrigation of approximately 76 to 88 mm (3 to 3.5 inch) water to “rinse-in” all treatments. Of note, the entire trial site was periodically treated with fungicides for the prevention of the occurrence of turfgrass diseases so as not to confound the study.

On 6-7 June 2016, the entire site was irrigated to achieve soil field capacity. On 7 June 2016, the hoop-house was covered with clear plastic to simulate a rain-out shelter effect, thereby covering the entire trial site, and preventing natural rain from interfering with the study. A dry-down period was imposed from 8 June through 9 August 2016 (63 consecutive days or nine weeks., Therefore, both soil surfactants were applied at two or three times prior to the dry-down period. Each day during the dry-down period, at approximately 0900 hr, each individual plot was measured for percent volumetric water content (VWC) to a rootzone depth of 7.6 cm (3 inch) using a Spectrum FieldScout 150 (Spectrum Technologies, Aurora, IL). A mean of three VWC readings was obtained from random locations within each plot. A rootzone VWC of 25% was set as the minimum threshold, and 35% VWC was set as the maximum threshold, which reflects managing turfgrass on mineral or native soil (Hummel 1993, Kammerer 2019). Also, the volumetric water content for a typical clay loam soil is listed at 16% for permanent wilting point, and 36% field capacity (https://www.specmeters.com/assets/1/22/Soil_postcard.pdf). With each soil surfactant-treated plot or Untreated-Water plot on each day during the dry down period, if mean VWC measured ≤ 25%, irrigation water was slowly added with a 3.78 L (1 gal) volume watering container to return VWC to 35% as indicated by periodically measuring the VWC, and the total quantity of water added was recorded for that plot. If the VWC measured > 25%, no water was added to that plot. The watering event or frequency for each plot on each day also was recorded as needed. With each Untreated-Control plot, water was added daily or as needed to maintain 35% VWC for each day during the dry-down period.

Turfgrass quality was evaluated at the start, half-way, and at the end of the dry-down period for creeping bentgrass maintained as a typical golf course fairway (Kammerer 2019). Turfgrass quality was determined on a visual 1 to 9 scale, with 9 = best quality (color, uniformity, density, plant health), 5 = minimum acceptable quality, and 1 = worst quality.

All data (i.e., frequency of watering per plot, amount of water added per plot, and turfgrass quality) were subjected to analysis of variance using Agricultural Research Manager (Gylling Data Management; Brookings, SD). Treatment means were compared using Fisher’s protected least significant difference test at p ≤ 0.05 (Mead et al. 2003).

Turfgrass plots treated with either soil surfactant (i.e., PBS150 or OARS HS) applied three times prior to the imposed dry-down period required significantly less water input (range of 127.9 to 134.3 L [33.8 to 35.5 gal]) versus Untreated-Water or Untreated-Control plots (range of 211.4 to 220.2 L [55.8 to 58.2 gal]) during the dry-down period (Fig. 2). Water use during dry-down was statistically similar when comparing PBS150 (134.3 L [35.5 gal]) to OARS HS (127.9 L [33.8 gal]), or when comparing Untreated-Water (211.4 L [55.8 gal]) versus Untreated-Control (220.2 L [58.2 gal]). When comparing soil surfactant treatments to the Untreated-Control during the dry-down period, plots treated with PBS150 or OARS HS resulted in a range of 39.0 to 41.9% reduction in irrigation water consumption versus the Untreated-Control, or a range of 36.5 to 39.4% reduction in irrigation water consumptions versus Untreated-Water. The plots irrigated by threshold (i.e., Untreated-Water as previously described in the Materials and Methods section) only resulted in a 3.9% reduction in irrigation water consumption versus plots irrigated daily (i.e., Untreated-Control).

Therefore, irrigating turfgrass during the dry-down period with water only (i.e., soil surfactants not applied to Untreated-Water or Untreated-Control plots) required significantly more water use versus turfgrass treated with three applications from either soil surfactant. Thus, irrigating soil surfactant-treated turfgrass during the dry-down period resulted in a measurable reduction of water use (i.e., irrigation water savings). This is the first documented, replicated field study with mineral soil to quantify an actual amount of irrigation water utilized for soil surfactant-treated turfgrass, as well as quantify a significant reduction in water amount utilized with soil surfactant-treated turfgrass.

It is not evident from this field trial as to the specific plant-soil relationship or mechanism responsible for this reduction or optimization in water use from soil surfactant-treated turfgrass (Ahmadi et al. 2017, 2018). Lehrsch et al. (2011) speculated that soil surfactant-treated soil particle surfaces within the rootzone resulted in a decrease in the liquid-solid contact angle of water (i.e., those surfaces became wettable), and therefore more water or films of water would be retained within soil pore spaces. Ahmed et al. (2016) investigated rhizosphere wetting kinetics and determined that a soil surfactant application may influence plant transpiration and root rehydration functions. Since soil moisture meters or sensors would therefore measure a higher amount of volumetric water content in the rootzone, the turfgrass practitioner may perceive this as the soil or rootzone is “holding” water (Kostka and Fidanza, 2019). Soil scientists, however, would refer to this soil water property as “retention” (Weil and Brady, 2017). With water retained in the soil, the root-to-soil connectivity within the rhizosphere could be improved and maximized, particularly during abiotic and biotic stress (Carminati and Vetterlein 2014; Carminati et al., 2010, 2011, 2016). O’Brien et al. (2023).

Turfgrass plots treated with either soil surfactant (i.e., PBS150 or OARS HS) applied three times prior to the imposed dry-down period (i.e., 63 days) required a significantly lower number of irrigation events (range of 13 to 14) versus Untreated-Water (24) or Untreated-Control plots (43) (Table 2; Fig. 3). Untreated-Control plots required more frequent irrigation (43 of 63 days) but with lower amounts of water needed with each irrigation event at a mean of 5.1 L (1.3 gal) compared to all other treatments. Compared to Untreated-Control plots, the Untreated-Water plots required less frequent irrigation (24 of 63 days) and a mean of 8.8 L (2.3 gal) of water with each irrigation event. Thus, Untreated-Control plots required a frequent (i.e., occurrence) and light (i.e., amount) irrigation strategy.

Plots that received either soil surfactant treatment required less frequent irrigation occurrence (range of 13 to 14 of 63 days) but a higher amount of water with each irrigation event (range of 9.1 [2.4 gal] to 10.3 L [2.7 gal]). Therefore, soil surfactant-treated plots required an infrequent (i.e., occurrence) and heavier (i.e., amount) irrigation strategy. Thus, turfgrass treated with a soil surfactant or more specifically treated with a soil surfactant program should be a consideration with the turfgrass practitioner when developing an irrigation water use conservation or best management practices strategy (DaCosta and Huang 2005, Gross 2004, Moeller 2013, Whitlark 2022). This is the first documented, replicated field study with mineral soil to quantify a significant reduction in the frequency of irrigation events needed with soil surfactant-treated turfgrass. The potential economic benefit of reduced electrical energy consumption associated with a reduced demand for operating the irrigation system was not determined.

Turfgrass quality was considered acceptable for all plots and all treatments throughout the duration of this field study, and therefore turfgrass quality was not considered to be a confounding factor with measuring irrigation water inputs during the dry-down period (Table 3). Overall turfgrass quality may have been slightly reduced during the dry-down period since the structure over the study site was covered with plastic (i.e., to prevent natural rain from reaching the plots). Of note, no quantifiable measurements relating to daily light integral or photosynthetic efficiency were conducted during the dry-down period.

In conclusion, this is the first field study to quantify a reduction in irrigation water use and frequency (i.e., irrigation events) with turfgrass treated with a soil surfactant. For example, as calculated from this field study, 10 ha (25 acres) of creeping bentgrass fairways would require 15,383,853 L (4,063,984 gal) water for irrigating untreated turfgrass during the 63-day dry-down period. Turfgrass treated with OARS HS would use 9,307,231 L (2,458,710 gal) water for 6,076,621 L (1,605,273 gal) in water savings, and turfgrass treated with PBS150 would use 9,907,201 L (2,617,205 gal) water for 5,476,652 L (1,446,778 gal) in water savings. Soil surfactant-treated turfgrass was irrigated 13 to 14 times versus 24 times with untreated turfgrass, representing a 42 to 46% reduction in irrigation frequency required while maintaining acceptable turfgrass quality. The “dry-down” experimental method employed in this field study could be a useful technique for future research on evaluating soil surfactants for water conservation and utilization in turfgrass management (Schiavon and Serena, 2023).