Can soil surfactants and PGRs reduce turfgrass water requirements?

By Matteo Serena, PhD & Bernd Leinauer, PhD

Soil surfactants, also called wetting agents, are materials that decrease interfacial tension between hydrophobic and hydrophilic surfaces of soil particles. In turfgrass, soil surfactants are commonly used to prevent and treat localized dry spots (LDS) and areas of water repellency in the soil. These products are either injected into the irrigation water or sprayed on the soil surface. A report by Throssell (2009) stated that an average of 92% of the golf course facilities in the US use wetting agents as part of their spray plan, while 34% inject soil surfactants though their irrigation systems.

Currently, there are more 120 different brand name products sold as a soil surfactants or wetting agents and most of these products share similar characteristics. These products can be separated into different classes depending on how they affect soil moisture. For example, products categorized as “penetrants” can increase movement and percolation, “water holders” or “retainers” improve retention; both groups may help distribute water more evenly throughout the soil profile. Applications of these products may help in implementing water conservation strategies.

In addition to wetting agents, products categorized as plant growth regulators (PGR) may have also the ability to reduce water use. Plant growth regulators are chemical substances that restrict essential plant growth by inhibiting either cell division (Type I) or gibberellic acid synthesis (Type II). They act like an herbicide in that they enter and move through the plant and interfere with specific enzymes inside the plant cells, which can slow or interrupt a specific metabolic process. The use of PGRs has developed into a standard management practice in the turf industry in order to maintain high quality turfgrass with improved playability.

The primary reason these substances are used is to decrease mowing frequency and prevent scalping at times during which turfgrass growth quickly (i.e. rain, warm temperatures, fertilization). In the past, greenhouse research trials have demonstrated that PGR-treated plants displayed reduced evapotranspiration (ET) compared to untreated plants due to a reduction in the surface area of the leaves and overall plant size.

The turfgrass research group at New Mexico State University has been investigating the use of soil surfactants in turfgrass for more than a decade. More recently, our efforts have also focused on how these products can be implemented for water conservation strategies in an arid and semi-arid environment. Research trials initially focused on the applications of soil surfactants to treat and prevent LDS on greens. Later, the research began to focus more on how surfactants applied to golf course fairways affect the overall quality under reduced irrigation. We focused on fairway turf because it is the largest playing surface on a golf course with the greatest impact for conserving water. Additionally, we compared applications of PGRs in conjunction with wetting agents to evaluate a potential synergistic potential. During a 3-year study on warm season turfgrasses, both bermudagrass and seashore paspalum were irrigated at 50% of replacement ET and were treated with one of two soil surfactants and one PGR and compared against a non-treated control.

In general, the turfgrasses that were treated with a PGR had higher overall turfgrass quality when compared to the wetting agent and to the non-treated control. Moreover, the turfgrass plots treated with a soil surfactant had higher turfgrass quality than the non-treated control. During the study, we also found that wetting agents resulted in higher volumetric soil water content along with better moisture uniformity. Also, the PGR increased chlorophyll content in the plant, which produced a greener canopy and resulted in longer color retention during fall months, by having a healthier plant during the fall months.

After examining the results of the first research project, we decided to investigate the effects of combining (tank-mixing) both the wetting agents and PGR treatments in a second 3-year study. This study was conducted only on Princess 77 bermudagrass but we included three drought levels as treatments. We observed that the combination of a PGR with a soil surfactant resulted in higher turfgrass quality at 50% of ET compared to the non-treated areas.

Now, over the next 3 years, we will test multiple surfactants and PGRs to find out if there is a difference between some of the products on the market and with the intent to further improve overall turfgrass quality under limiting water supply. At this point, based on the results of our previous research, soil surfactants combined with a PGR offer a strategy to conserve irrigation water while at the same time improve turfgrass quality on limited inputs.

Matteo Serena, PhD, is a Research Assistant Professor in Turfgrass Science at New Mexico State University; Bernd Leinauer, PhD, is a Professor and Extension Turfgrass Specialist at New Mexico State.