You probably didn’t work in turf management for long before realizing that water is pretty important. More specifically, proper soil moisture status is important, especially in stressed situations such as an athletic field.
Proper soil moisture results in healthy rooting, improved nutrient uptake and, in turn, better plant health. Remember that the ideal soil is about 50 percent solids, 25 percent water and 25 percent air. When soil is too wet, the pore spaces fill, resulting in diminished oxygen availability in the root zone. This is deleterious to the grass and can lead to decreased rooting, followed by decreased plant growth. Traffic on wet soil also can lead to accelerated loss of soil structure, which exacerbates drainage and standing water issues.
On the other hand, when soil is too dry we see wilting, decreased rooting, plant loss and susceptibility to disease, all leading to loss of surface quality. In addition to the aesthetics, loss of surface quality increases risk of athlete injury.
Most issues about water on athletic fields concern drainage, or, rather, a native soil field’s relative inability to drain. We’re seeing more mandates to reduce the use of water for irrigating turfgrass, because a lot of our freshwater resources are used for turfgrass irrigation and people have noticed.
Wetting agents, as a class of products, have been around for use in turfgrass management, primarily on golf courses, for many decades and have classically been used to “wet the soil” in order to break up hydrophobic dry spots on putting greens (more on this later). But new types of wetting agent chemistry and new products are being developed that are touted to help soil retain moisture as well. Let’s review what a wetting agent is, what it can and can’t do, and offer some guidance as to the types of products that are available.
As a polar molecule, water has a positive end and a negative end, much like a magnet. Water molecules are attracted to each other, but more so on the surface; these forces cause water droplets to form (hence the term “surface tension”).
The substances that cause hydrophobic dry spots to develop are non-polar. When water comes in contact with a nonpolar hydrophobic substance, it tends to bead rather than penetrate into the substance.
Surfactants are a class of compound that reduces this surface tension, which can occur between two liquids or between a liquid and a solid. Different kinds of surfactants include detergents, wetting agents and emulsifiers. So, a wetting agent is a surfactant, but not all surfactants are wetting agents. Wetting agents used in turfgrass management are a class of surfactants that reduce the surface tension of liquids, helping them to disperse, penetrate and percolate into soil.
Chemically, wetting agents are quite complicated molecules but all of them generally have a polar head (that is hydrophyllic, or “water-loving”) and a nonpolar tail (that is hydrophobic, or “water-hating”). Think of a mix of oil and water (i.e., salad dressing). Oil and water will always separate unless mixed with an emulsifier (another class of surfactant that helps to join two liquids). Wetting agents used in turfgrass don’t act as emulsifiers, but the polar head of the wetting agent can bond with water and the nonpolar tail can bond to hydrophobic soils, thus holding water in place and increasing the opportunity for uptake into the soil. Because of how wetting agents work, the classic applications have sought to improve irrigation efficiency and to reduce dry or hot spots on the turf, but there are other potential applications as well.
To break up hydrophobic dry spots: Sand is used a lot on high-intensity athletic fields to take advantage of rapid drainage. Sands tend to be hydrophobic or to become hydrophobic over time, and so this problem tends to be more severe on sand-based soils.
However, almost all soils will develop some degree of water repellency over time. This is because decomposing organic matter, plant root exudates and fungal hyphae will coat the soil particles. These coatings on the soil particle act to repel water, much like water beads up on the hood of your car after you’ve waxed it.
These hydrophobic areas then appear drier, resulting in either a need for spot watering or increased irrigation over the whole field, which may lead to areas becoming too wet.
Wetting agents help to eliminate these dry spots by breaking this surface tension and increasing the ability of water to absorb into the soil. Some wetting agents break down these hydrophobic substances and redistribute them in the soil profile.
To improve irrigation efficiency: It’s natural to assume that, if water is uniformly applied to the soil surface, penetration of the water through the soil is uniform and the result is spatially consistent wetting of the soil. What is more typical is what is called fingered flow, where, in cross-section, the penetration of water appears as inconsistently spaced channels surrounded by zones of far less penetration by the water.
This is partially because all soils will naturally develop channels, cracks and crevices through which water flows more rapidly. But it also is caused by hydrophobic compounds on the surface of the soil particles. Wetting agents can improve irrigation efficiency by improving the distribution of water penetration into a soil.
Another potential benefit of wetting agents is that, by increasing soil’s ability to take in water, the time the water sits on the soil surface is reduced, minimizing water loss due to evaporation or run-off.
In addition to improving water penetration, certain wetting agents can increase moisture retention in the soil. Increased moisture retention means decreased irrigation requirements. A wetting agent program may help improve uniformity of soil wetting, thus increasing your irrigation efficiency. In some cases, this isn’t trivial. Research has shown that preventive applications of wetting agents prior to drought may reduce irrigation requirements by 50 to 70 percent.
To eliminate wet areas: Some wetting agents have been developed to move water through soil. So, the big question for most athletic field managers who deal with standing water on fields is “can a wetting agent help eliminate wet areas?” Two things determine the answer to this question. One, a large-enough amount of the right type of wetting agent must be present in the soil. But, more importantly, the soil already must have some ability to drain. In other words, if there are layers of compaction, a wetting agent may not help as much. To put it another way, a wetting agent isn’t a substitute for sand.
But, there are field managers who have used wetting agents with success to get fields back into play more quickly after a rain. Also, in wet areas of the field that have a Poa annua problem, wetting agents may help to manage or decrease these populations by reducing the wet soil conditions that favor the weed.
Other potential uses on athletic fields: The effect that wetting agents have on turfgrass disease severity has been studied extensively, with inconsistent results. But, another potential use of retention-type wetting agents is to help retain moisture in the clay used for the infield skin of a baseball field, while improving surface drainage and reducing ponding. There are, in fact, wetting agent-infused calcined clay products on the market for this purpose.
Finally, surface removal of dew may be beneficial on fields with early morning or late evening games. A surfactant applied the day before may reduce surface tension and thus reduce dew formation. This effect typically lasts about 48 hours after application.
Types of wetting agents
Interestingly, when products such as herbicides or fertilizers are discussed, there is almost always a classification system based on chemical properties or appropriate uses/intended targets. Printed classification systems for wetting agents are more elusive.
Unlike pesticides, wetting agents aren’t regulated by the Environmental Protection Agency. That means the manufacturer isn’t required to divulge the active ingredient in the wetting agent. This is further complicated by the fact that many new products introduced in the last few years are combinations of wetting-agent chemistries or may contain other ingredients, such as humic acids.
But sometimes chemistry confuses more than it helps. Just as MCPP and 2,4-D are both phenoxy herbicides but target very different weeds, some wetting agents are chemically related but behave very different in different soils. That said, now is a good time to review what is commonly known about wetting-agent chemistries to understand the intended use or benefit when applied to turf.
- Anionic wetting agents carry a negative charge. They can result in rapid wetting of dry soils but can also be phytotoxic to turf. Also, when applied to clay, they can cause dispersion of the soil particles, which negatively impacts soil structure. Cationic wetting agents aren’t used in the turfgrass industry for the simple reason that, at the concentration needed to be effective as a wetting agent, they’re quite toxic to plant life.
- Most wetting agents used in turfgrass are nonionic; that is, they carry no net charge. The earliest examples of these were called the Polyoxyethylene, or POE, wetting agents that became available in the 1950s. They may help with water repellency issues and may increase water movement into soil. Some of them can be phytotoxic. More recently introduced wetting agents tend to be less phytotoxic. These wetting agents may enhance moisture retention in the root zone or increase water movement in the soil, or both.
- As nonionic wetting agents have evolved, products have emerged that are more effective at causing thinner and more continuous films of water around soil. Also, new products claim to disrupt hydrophobic associations into smaller, more soluble compounds that then redistribute them in the soil profile, which results not only in reducing dry spots but improving moisture retention. Yet other wetting agents have been developed that break down in such a way that the initial breakdown products continue to serve as wetting agents, thus enhancing the products’ longevity in the soil.
How to choose and use
Wetting agents are generally thought to be the most efficient way to deal with hydrophobic soils. Many products on the market will aid in water penetration and distribution, and they’re available in liquid or granular form. Some are injectable and still others can be distributed via an irrigation system. Thus, they’re very easy to use. Some products are considered short-term in their effects (perhaps two to four weeks) while others are long-term, with effects that last one to three months. Depending on product choice and timing of application, some can cause phytotoxicity to the turfgrass.
Not all wetting agents are the same. As sophisticated as the chemistries have become, a reliable method for choosing the correct one is somewhat elusive. As with all products, read and follow the label.
When researching a product, make sure to find field evaluations conducted by university researchers or other third parties. Even with this, there are other variables that come into play, such as rate and timing of application, how much water was applied, turf species used, soil type and soil chemistry.
After choosing a product, it’s a good idea to try it out on small area that includes an untreated check so that you can determine if it is accomplishing your goals. Typically, one application won’t be sufficient. Remember that the use of wetting agents will involve a program of applications per the label direction and may take some time before results are achieved. If you don’t achieve your intended result with a particular product, then research the use of another, perhaps in a different category or class of wetting agents.