Soil surfactant usage based on solid science
Photo courtesy of AQUA-AID Solutions

Soil surfactant usage based on solid science

Dr. Stan Kostka and Dr. Mike Fidanza offer guidance to help superintendents further understand research and product selection.

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November 4, 2019

New soil surfactant products appear every season. Trying to understand the plethora of products currently being offered as soil surfactants/soil wetting agents can be a daunting task. Type either word into your favored search engine and what do you get? Well, in today’s search, the first was a link to a university fact sheet that wouldn’t open. The second, a link to a website that ostensibly is about soil surfactants but is a platform that offers “wetting agent” products for sale. Accessing correct, current, and complete information can be challenging.  
 
Let’s continue-on through several websites offering information on soil surfactant products. Ah-ha, here’s something from the USGA: “Understanding the different wetting agent chemistries” (http://gsrpdf.lib.msu.edu/ticpdf.py?file=/article/zontek-understanding-7-20-12.pdf). Funny, we even know those authors. If you seek basic information on surfactants, and a logical method to categorize surfactants, it’s a great place to start. Another excellent, more updated resource from the USGA on soil wetting agent uses is: “Factors to consider when developing a wetting agent program” (http://gsrpdf.lib.msu.edu/ticpdf.py?file=/article/jacobs-barden-factors-5-4-18.pdf). And, if you’re really interested, that article includes citations of some of the newly published research. The major take-home message is that these products improve infiltration, mitigate water repellency in soils, enhance hydration, and are now being understood to have some additional effects ranging from enhancing certain soil directed pesticides, improving flushing of excess salts, to helping overwintering survival, and to spring re-establishment/recovery in Bermudagrass.   
 
So, onto some of the newer work. Over the years, there has been considerable debate about products that claim to have effects on organic matter or those that claim to have very specific properties relating to either placement of the surfactant and/or how water behaves in the rootzone. 
 
Can surfactants displace hydrophobic organic compounds?
Recently, researchers at the University of Missouri (Read Research HERE) reported that different classes of wetting agent/surfactants could influence how hydrophobic organic compounds may be displaced from soil particle surfaces. Of the three of the classes of wetting agent/surfactants evaluated, they found that two – OARS (80% polyoxyalkylene polymers and 10% potassium salt of alkyl substituted maleic acid) and Matador (100% alkyl block polymer) – both increased dissolved organic carbon (DOC) in the leachate, which is a means of measuring displacement of carbon. While both materials mitigated soil water repellency, OARS appeared to achieve it by adsorbing onto the surface of those soil particles and hydrophilizing the surface. According to the authors, Matador achieved it by removing the hydrophobic particulate material on particle surfaces. So, there is now research-based evidence that hydrophobic coatings can be modified by surfactants.  What is intriguing is that the results suggest very different modes of action may be in play. By the way, the article differentiates between surfactants and wetting agents, yet all products tested contained chemically related molecules that are by definition surfactants. So, we even have more work to do to expand the knowledge and understanding of the sorts of compounds used in these products.
 
Retainers versus Penetrants
If there isn’t sufficient confusion in the marketplace, the positioning of some surfactant (wetting agent) products can lead one to believe that there is precise knowledge of finely definable modes of action, that specific desired outcomes under diverse environmental conditions can define applicability, that soil responses can be turned on and off simply by changing what product is applied today, or that product “X” can dramatically influence surface firmness. Or, as a former colleague would often state incredulously, “Really?”   
 
These sorts of questions have only been explored for a handful of products. Researchers at the University of Arkansas, University of Wisconsin, Michigan State University and the University of Wageningen (Netherlands) have demonstrated that certain surfactants can increase soil water content under droughty conditions and lower water content during periods of excessive rainfall. However, most of these results come from turf growing on sand or sandy loam. Does this mean those surfactants tested are actually physically holding onto and retaining water in the rootzone, or “pushing” it through the rootzone? Our soil physics colleagues would question that statement. Should the terms “retainer” and “penetrant” even be used for these products?
 
Data remain lacking on the nature and differentiation of products sold as wetters, water retainers and penetrants. There are a preponderance claims based on “reports from the field” or on perceptions, but thorough, documented, replicated field research (either from university or industry researchers) to substantiate these claims is lacking as well.
 
Because of the confusion and lack of data on a range of products and claims relating to product placement, applicability for specific environmental and rootzone conditions, and effects on surface firmness, the GCSAA and a number of its local chapters have funded studies to begin to address these issues under field conditions. Studies are currently underway at Texas Tech, the University of Arkansas and the University of Nebraska.  
 
For example, Dr. Joey Young of Texas Tech shared some of the questions that he is asking in his research:
1. Are there differences between products labeled and marketed as “retainer” and those as “penetrant?”
2. Do penetrants improve movement of water through the soil profile – more rapidly while present after application, but slow movement before reapplication?
3. Will continued application of a penetrant or retaining product create poorer conditions (poor visual quality or soft, “puffy” turf) as seasons and weather patterns change throughout the year?
4. Is surface firmness and ball mark severity and recovery influenced by surfactant application?
5. Do retaining chemistries result in deeper ball marks due to surface moisture retention, and will ball mark injury and time to recovery be reduced?
6. Do penetrating chemistries create less severe ball marks with firmer surfaces, but will injury area be greater and take longer to recover?
 
We look forward to the results from this well-respected group of turf researchers. Click HERE for more information on the University of Nebraska study.
 
What is the importance and value of hydration?  
Historically, a primary use of soil surfactants has been to modify soil water repellency (i.e., hydrophobic rootzone conditions), rewet soil and to improve hydration. Most work-to-date has focused on USGA specification greens or sandy soils where effects have been well-documented by research conducted at the University of Florida, the University of Wageningen, Michigan State University, New Mexico State University and the University of California. However, effects on finer textured mineral soils are conspicuous in their absence.  
 
Research, currently underway at the Pennsylvania State University, has the goal of understanding if seasonal irrigation inputs can be achieved in surfactant treated mature bentgrass (Agrostis stolonifera ‘L-93’) grown on a clay loam rootzone. This study is unique as most have been conducted on sand based rootzones (https://scisoc.confex.com/scisoc/2018am/meetingapp.cgi/Paper/113195). Two surfactants OARS HS (85% Octahydroxy polyoxyalkylene polymer and 7.5% amine salt of alkyl substituted maleic acid) and PBS150 (100% polyoxyalkylene polymers) influenced the amount and frequency of irrigation water needed to maintain a daily volumetric water content (VWC) threshold of 35% during a dry-down period. Surfactants were applied either twice at 1.7 gal/ac (April 27, May 26) or three times (March 30, April 27, May 26) prior to imposition of a dry-down period (June 8, Aug 8). During dry-down, VWC was measured at 9 a.m. daily in each plot. If VWC was ≤ 25%, irrigation was applied to that plot until VWC reached 35%. Two surfactant applications resulted in a 25% reduction in the amount of water that needed to be applied. When surfactants were applied three times, a 36% to 39% reduction in applied water was achieved, respectively for PBS150 or OARS HS. Not only was the amount of water reduced significantly, so too was the frequency of irrigation, all while maintaining acceptable turf quality, with a total of 41 irrigations for the untreated versus a maximum of 14 irrigations for the surfactant treatments.
 
Conclusion
Over the past two decades, soil surfactants have moved from products used to correct localized dry spots to broader applications in water management in fine turf, however, marketing claims often outpace the science. Research continues to build upon our understanding of what these products can and cannot do. Understanding the mode of action, effects on soil physical phenomena, and effects on soil biology and rhizosphere dynamics are the future.  A foundation is being built to support science-based uses of these unique soil amendment compounds.
 
Research says:
1. Soil surfactants or “wetting agents” lead the way for water use and conservation practices on U.S. golf courses.

2. Recent research has quantified the beneficial effects of using soil surfactants, with more work in progress.
3. The marketing terms “retainer” and penetrant” may not be at most accurate or correct way to describe how soil surfactants function with golf course turf.
4.  Research efforts are underway in the U.S. and Europe that will increase our understanding of how soil surfactants affect turf rootzone physical, chemical, and biological properties.

 
Dr. Stan Kostka (stan.kostka@gmail.com) is a visiting scholar, Berks Campus, Pennsylvania State University, Reading, Pennsylvania. Dr. Mike Fidanza (maf100@psu.edu) is a professor of plant and soil science. Berks Campus, Pennsylvania State University.