skip to main content

Key Takeaways

  • There are no established standards for what defines a “penetrant” or “retainer” wetting agent, and each manufacturer can apply those labels to whichever products they choose.
  • Research on sand-based putting greens found no significant differences in volumetric water content between penetrant and retainer wetting agents.
  • At the active ingredient level, wetting agents demonstrated an ability to function as both a penetrant and retainer in response to changes in environmental conditions.
  • Expectations for how a wetting agent will function should be based on data and experience, not marketing terminology.
  • Site-specific factors such as weather conditions, cultural management practices and organic matter content play an important role in wetting agent performance and need to be considered to maximize the effectiveness of any product.
     

When it comes to managing water on golf course putting greens and other playing surfaces, few tools are as versatile or as valuable as wetting agents. These products, also known as soil surfactants, can be applied for a number of different reasons including preventing localized dry spot (LDS), improving moisture uniformity, increasing water infiltration, prolonging rootzone moisture retention, reducing winter injury, and enhancing the efficacy of fertilizers and pesticides (Abagandura et al., 2021; DeBoer et al., 2020; Hutchens et al., 2020; Jacobs & Barden, 2018). Yet for all their uses and importance, wetting agents remain one of the least understood inputs used on golf courses.

It can be difficult for golf course superintendents and turf managers to differentiate between penetrants, retainers and the multitude of other wetting agent products available because their respective chemical compositions are often unclear. This leads to questions such as: “Why are many wetting agent labels so confusing?” and “Is there a difference between a penetrant and a retainer?” While we won’t be able to clear up all of the confusion, the goal of this article is to address common questions regarding differences between wetting agents marketed as penetrants and retainers. Specifically, we will look at where these terms come from, how they correspond to basic wetting agent chemistry, and the results of field research comparing penetrants and retainers in sand-based putting greens.

Sources of Confusion

Wetting Agent Terminology

Simply put, wetting agents are not as well-understood as other golf course inputs. Because they are not subject to the same registration process or labeling requirements as pesticides, wetting agents arrive in the marketplace accompanied by far less data about what they are and how they work. That’s not to say that there are zero regulations for wetting agents, or that companies are intentionally misleading with label information. It’s simply a fact that when superintendents select a wetting agent, they must do so with less information regarding chemical composition and product performance as they would with a fungicide or other control product.

Previous efforts to classify wetting agents provide helpful guidelines but are limited by the fact that wetting agent active ingredients are not always known (Zontek & Kostka, 2012). A recent analysis of the U.S. wetting agent market reported 192 different branded formulations. For 32 of those, the active ingredient was not disclosed or unknown, while an additional 33 were listed as blends of multiple chemical classes (Fidanza et al., 2020). In the absence of research data and formulation details, anecdotal evidence and marketing terminology have filled the void.

First and foremost, penetrant and retainer are marketing terms. That alone does not make them inaccurate, but it does highlight the fact that there are no established standards and each manufacturer can decide what they want to call a penetrant or a retainer. There are no benchmarks, and consequently no way of ensuring consistency among the terms. Going back to our original reasons for applying a wetting agent, a penetrant would, theoretically, increase water infiltration, while a retainer would prolong or maintain rootzone moisture. The question then becomes, how exactly can wetting agents accomplish one effect more than the other?

Structure-Function Relationship

Claims that wetting agents are either penetrants or retainers is derived from depictions of the simplistic, two-part structure of wetting agents themselves. There is an old adage with wetting agents: “You either treat the water or treat the soil.” To understand where that idea originated and how it plays into the penetrant-retainer debate, it’s helpful to briefly review basic wetting agent chemistry.

Turfgrass wetting agents are part of the much larger, diverse category known as surfactants (surface active agents). Surfactants are amphiphilic, meaning they have two distinct parts: a hydrophilic “water-loving” portion, and a hydrophobic “water-fearing” portion. The extent to which the hydrophile interacts with water, and the hydrophobe interacts with soil (or sand) particles, form a basis for defining wetting agent function. Adjusting the ratio or placement of hydrophile to hydrophobe (Figure 1) to emphasize one of these interactions over the other, forms a conceptual basis for creating wetting agents that are perceived as either penetrants or retainers.

Penetrants tend to focus on hydrophile-water interactions. Think about the difference between a water droplet and a thin film of water. The droplet holds its shape because the water molecules are highly attracted to each other. This important property of water, known as cohesion, leads to strong surface tension. Wetting agents effectively “get in the way” of water molecules bonding to each other, so that they no longer hold together as tightly. Consequently, surface tension is reduced and the droplet spreads out into a film, and presumably penetrates more readily into the ground and through soil pores.

Retainers emphasize interactions between the hydrophobe and the soil (McCarty & Kerns, 2016). Here, it’s helpful to consider how sand-based putting greens develop hydrophobicity from the buildup of waxy organic compounds on sand particles (Song et al., 2019). There needs to be something that “builds a bridge” between the waxy coatings on the sand and the water. This relates to another essential property of water – adhesion. Where the waxy coatings inhibit water from attaching to the sand, wetting agents help overcome this. The hydrophobic part of the surfactant “holds on to” the hydrophobic coatings on sand particles, leaving the hydrophile available to “hold on to” water in the rootzone.

The key point is, although there is some basis in wetting agent chemistry, penetrant and retainer are effectively marketing terms. So, what does it look like when penetrants and retainers are used side by side on a putting green? What are the measurable differences?

Research Results: Comparing Penetrants and Retainers

In 2019, researchers at the University of Arkansas compared eight different wetting agents marketed as “penetrants” and “retainers” from four different wetting agent companies (Table 1). Volumetric water content (VWC) was measured twice per week using a FieldScout TDR 300 portable moisture meter at four depths: 1.5, 3.0, 4.8 and 8.0 inches. The trial was conducted on both ‘L-93’ creeping bentgrass (Agrostis stolonifera L.) and ‘TifEagle’ ultradwarf bermudagrass (Cynodon transvaalensis (L.) Pers. x Cynodon dactylon (Burtt-Davy)  sand-based putting greens at the Milo J. Shult Agricultural Research and Extension Center in Fayetteville, Arkansas. The trial sought to compare these products across a range of moisture conditions from extremely wet to extremely dry.

Over the eight weeks of the trial (data not shown), there were no significant differences in VWC among wetting agents on any date, at any depth (Karcher et al., 2020). Put another way, if you were calling one product a penetrant and one a retainer, it meant you were looking at the label, not the data. During dry-down periods, both penetrants and retainers had better VWC uniformity than nontreated turfgrass. However, a clear-cut separation between these two categories of wetting agents was not observed.

In 2021, another study at the University of Arkansas included the same eight wetting agents applied monthly to a sand-based ‘Flagstick’ creeping bentgrass green. This time, VWC was measured at 0.5, 1.5, 3.0, and 4.8 inches weekly. The season-long results reinforced notions of inconsistency and variability among different penetrants and retainers. At all depths, both categories contained individual products with VWC greater than, and also less than nontreated turfgrass (Figure 2). There were also penetrants with greater VWC than retainers at all depths. When comparing each individual company’s product marketed as a penetrant to their product marketed as a retainer, the season-long differences in VWC averaged less than 2.5% across all depths. Differences between penetrant and retainer products for three of the four companies were less than 1%, and one was actually negative – meaning the penetrant held more water in the rootzone than the retainer.

So, with these research results in mind, how should we really think about the terms penetrant and retainer?

Asking the Right Questions

One of the key takeaways is that it matters where you look. When focusing solely on surfactant chemistry – a wetting agent’s ability to reduce the surface tension of water in a laboratory environment – drawing a distinction between penetrants and retainers is plausible (Fleetwood & Xiong, 2021). In terms of the science behind it, it could be said that the chemistry of a wetting agent can affect the physics of water. However, when you add the complex biology of putting greens into the equation, each with their own unique chemical and physical properties, VWC doesn’t always correspond with marketing terminology. Which begs the question, where do you need a wetting agent to be a penetrant or retainer – in the lab or on the putting green?

To help answer that question, superintendents should focus on the verbs more than the nouns. Rather than relying on marketing terms to tell us what a wetting agent is supposed to do, the results of this research suggest taking a more open-minded approach and observing how water behaves differently in the rootzone as a result of applying a particular product. How much a wetting agent improves infiltration, retention and uniformity of rootzone moisture, regardless of any penetrant or retainer designation, is what’s most important (Kostka et al., 2022). Taking additional VWC or firmness measurements before and after wetting agent applications is an excellent way to document actual wetting agent performance. Allowing data to lead the way will help superintendents find the right product to strike the desired balance between plant health and playability.

"Taking additional VWC or firmness measurements before and after wetting agent applications is an excellent way to document actual wetting agent performance."

A final question worth asking is, “How important is it to practitioners that wetting agents are classified as either penetrants or retainers?” Is anyone disappointed if a product does both? The dual (amphiphilic) nature of wetting agents, with both hydrophile and hydrophobe, means they can affect both cohesion and adhesion of water. Bringing it back to the old adage about either treating the water or the soil, perhaps too much emphasis has been placed on the “either-or” aspect of that statement. Penetrant and retainer have become effective marketing terms, but have they also restricted the way we think about individual wetting agent performance? One final study suggests that at the active ingredient level, the answer may be yes.

Research Results: Wetting Agent Active Ingredients

To address the fact that we don’t always know what’s in a wetting agent, another 2021 research study at the University of Arkansas took a step back from penetrants, retainers and commercial wetting agents entirely. Instead, it investigated wetting agent active ingredients themselves, independent of commercial products. For all that remains unknown about turfgrass wetting agents, what we do know is the majority are non-ionic, block copolymers (Fidanza et al., 2020; Zontek & Kostka, 2012). This research took four block copolymer active ingredients (Poloxamers), each with a different hydrophile-hydrophobe ratio, and applied them as stand-alone wetting agents alongside a nontreated control and a commercial standard (Revolution, Aquatrols Corp.) on a sand-based ‘Pure Distinction’ creeping bentgrass putting green.

In 2021, the early portion of the research study was marked by mild temperatures and recurring rainfall, during which time all wetting agent treatments produced VWC less than nontreated turf as would be expected from a penetrant (Figure 3). In August, when the weather became hotter and drier, an upward shift in VWC was observed, so that by the end of the trial all wetting agents resulted in VWC greater than nontreated turf, as would be expected from a retainer. It is important to emphasize that many commercial wetting agents are blends of multiple active ingredients, including chemistries other than block copolymers. However, this research tells us that on a basic active ingredient level, the most common class of turfgrass wetting agents has the ability to function as both a penetrant and a retainer, depending on the conditions. Which leads to the ultimate take-home message in all of this.

Conclusion

Wetting agent function cannot be understood exclusively in terms of chemistry; it has to consider chemistry within the context of conditions. Wetting agent research has a history of variability, both year to year and across locations (Throssell, 2005). While that has limited researchers’ ability to draw definitive conclusions about individual products, perhaps a better way to interpret inconsistent results is to recognize how much they underscore the massive influence of site-specific conditions and management practices on wetting agent performance. In addition to weather, irrigation practices and organic matter content have long been associated with wetting agent efficacy as well. So, to expand on our initial statement, developing an effective wetting agent program becomes chemistry within the context of cultural practices.

Getting the most out of a wetting agent program starts with moving beyond the unknowns and confusion surrounding marketing terminology. Wetting agent chemistry is inherently versatile, so understanding the roles that weather and cultural management play in their performance is key to maximizing their benefits. For sand-based greens, have a target VWC range in mind, but don’t rely on wetting agent applications alone to achieve that goal. Consistently track VWC and evapotranspiration, making the necessary adjustments to wetting agent application rates, timings and irrigation as needed. Finally, appreciate that organic matter is the gatekeeper to success for so many putting green management practices, wetting agent applications included. Considering all these factors together will help refine wetting agent programs and help achieve the goals of infiltration and retention alike.    

About the Authors

Daniel O’Brien is a graduate research assistant and Ph.D. student at the University of Arkansas in Fayetteville, Ark.

Mike Fidanza, Ph.D., is a professor of plant and soil sciences at the Pennsylvania State University, Berks Campus in Reading, Pa.

Stan Kostka, Ph.D., is a visiting scholar of plant and soil sciences at the Pennsylvania State University, Berks Campus in Reading, Pa. 

Mike Richardson, Ph.D., is a professor in the horticulture department at the University of Arkansas in Fayetteville, Ark.

References

Abagandura, G. O., Park, D., Bridges Jr, W. C., & Brown, K. (2021). Soil surfactants applied with 15N labeled urea increases bermudagrass uptake of nitrogen and reduces nitrogen leaching. Journal of Plant Nutrition and Soil Science, 184(3), 378-387.

DeBoer, E.J., Karcher, D.E., McCalla, J.H., & Richardson, M. D. (2020). Effect of late-fall wetting agent application on winter survival of ultradwarf bermudagrass putting greens. Crop, Forage and Turfgrass Management, 6(1), 1-7.

Fidanza, M., Kostka, S., & Bigelow, C. (2020). Communication of soil water repellency - causes, problems, and solutions of intensively managed amenity turf from 2000 to 2020. Journal of Hydrology and Hydromechanics, 68(4), 306-312.

Fleetwood, M., & Xiong, X. (2021). Improving water retention in hydrophobic soil. 2021 Turfgrass Field Day Online Golf Session. August 12, 2021. University of Arkansas System Division of Agriculture and University of Missouri Extension.

Hutchens, W. J., Gannon, T. W., Shew, H. D., Ahmed, K. A., & Kerns, J. P. (2020). Journal of Environmental Quality, 49(2), 450-459.

Jacobs, P., & Barden, A. (2018). Factors to consider when developing a wetting agent program: A one-size-fits-all approach to developing a wetting agent program is not possible. USGA Green Section Record, 56(9), 1-6.

Karcher, D. E., Richardson, M. D., & O'Brien, D. P. (2020). How do various wetting agents affect water movement and retention in sand-based putting green profiles?. 2020 ASA, CSSA and SSSA International Annual Meeting. ASA-CSSA-SSSA.

Kostka, S., Fidanza, M., & Bigelow, C. (2022). The science behind wetting agent applications. Greenkeeper International, March 2022, 74-75.

McCarty, L.B., & Kerns, J.P. (2016). Best Management Practices for Carolinas Golf Courses. Carolinas Golf Course Superintendents Association. 63-66.

Song, E., Pan, X., Kremer, R. J., Goyne, K. W., Anderson, S. H., & Xiong, X. (2019). Influence of repeated application of wetting agents on soil water repellency and microbial community. Sustainability, 11(16), 4505.

Throssell, C. (2005). GCSAA-USGA wetting agent evaluation. Golf Course Management, 73(4), 52-91.

Zontek, S. J., & Kostka, S.J. (2012). Understanding the different wetting agent chemistries: A surfactant is a wetting agent but a wetting agent may not be a surfactant. Surprised?. USGA Green Section Record, 50(15), 1-6.