Localized dry spots (LDS), also called isolated dry spots or hot spots, are common on golf greens, but not limited to high-sand-content turfgrass root zones. They have also been reported in root zones that consist of a predominately mineral, but finer-textured, soil (such as push-up greens). LDS appears as patches of wilted or dried turf. The patches can be irregularly shaped or circular and may range in size from only a few centimeters in diameter to covering almost an entire green.
The soil or root zone within these patches remains extremely dry, despite heavy irrigation, because the soil environment has become water-repellent, or hydrophobic. The causes of soil repellency are still not completely understood. Research suggests that the microbial decomposition of organic substances (peat, roots, shoots) in combination with fungal activity and frequent wet-to-dry cycles can produce a wax-like, water-repellent coating on sand particles. After this occurs, these soils become extremely difficult to be re-wet. Water will infiltrate into those areas only superficially and most of the irrigation water runs off. The turf dries out and ultimately dies if no countermeasures are taken.
The application of wetting agents or surfactants has become a valuable tool used by turf managers to treat LDS on golf courses and other turf areas. The infiltration of water into these soils can often be made possible and improved by applying a wetting agent. Wetting agents are detergent-like substances that reduce the surface tension of water, which allows it to penetrate and wet the soil more easily. Numerous researchers have studied whether wetting agents can improve the rewetting of dry spots, with most reporting increased infiltration or percolation rates of root zones treated with wetting agents.
Studies to investigate the effects of wetting agents on water retention of soils (soil's ability to store water) have given conflicting results, however. Although previous studies have mostly examined the relationship between wetting agents and moisture content in soils, few studies have focused on understanding the mechanism by which wetting agents improve water availability not only in hydrophobic but also in hydrophilic (water-attractant) soils.
To treat LDS, wetting agents are generally applied to entire greens or other large areas. Thus, not only water-repellent areas, but also hydrophilic areas receive the application. Golf course superintendents and turf managers have reported an increase in water retention in hydrophilic soil areas that have been repeatedly treated with wetting agents. Because changes in water retention in sand-based root zones (or any root zone, for that matter) may have a significant impact on cultural practices, this is a phenomenon that turf managers need to understand better.
To investigate the effects of repeated applications of wetting agents on water retention in hydrophilic root zones, studies were undertaken at New Mexico State University and at Michigan State University. The container experiments were conducted in green houses at the Hancock Turfgrass Research Center at Michigan State University and at the Fabian Garcia Research Center at New Mexico State University. Soil surfactants (Aquatrols' Primer 604, and Midorich wetting agent, which is not currently available in the United States) were repeatedly applied to soil columns in PVC containers. The columns consisted of a sandy root zone that met USGA standards for particle size distribution, overlaying a gravelly drainage layer. The soil columns had no turf cover.
The treatments were applied at the recommended rates of 10 ml per square meter for Midorich and 2.5 ml per square meter for Primer 604. The control columns received only water. Studies differed in length, rates of precipitation and in length of dry down cycles after precipitation. In one experiment, 27 mm of water were applied to each column every second week, while in the other experiment 27 mm of simulated rainfall were applied two times per week.
Time Domain Reflectometry (TDR) probes used to measure soil moisture content were inserted horizontally at 50-mm, 150-mm and 250-mm depths of the soil columns and used for continuous non-destructive soil moisture measurements. Soil moisture readings were taken at respective depths at the end of the dry down cycles, 1 hour before the application of irrigation water.
- Water retention
In Experiment 1 (27 mm of precipitation every second week and dry-down cycles of 1 week), when soil moisture data were analyzed by averaging over all dates and depths, the columns treated with Primer had significantly higher water contents than columns treated with water only. Midorich treated soil columns also had higher moisture contents than the control columns, but the differences were not statistically significant. When the soil moisture data were analyzed separately for each depth, at a root-zone depth of 50 mm, there was no significant difference in soil moisture between treatments. However, Midorich tended to increase soil moisture levels in the upper layer of the root zone relative to other treatments, and Primer tended to increase soil moisture levels at greater depths of the columns. Figure 1 (page Golf 3) shows a typical dry-down cycle in Experiment 1 at a depth of 150 mm. At the end of a 7-day dry-down, soil moisture readings in Primer-treated columns were highest, and lowest in the control columns.
In Experiment 2 (precipitation rate of 27 mm twice per week and dry-down cycles of 4 days), treatment with wetting agents had no significant effect on water retention in root-zone columns. However, the same trends as in Experiment 1 were observed: Midorich increased water retention the most in the upper root zone (50-mm depth), while Primer application affected soil moisture content greatest at depths of 150 and 250 mm.
The overall trends from both studies suggest that the amount of rainfall or irrigation influences the effect of wetting agents on moisture retention. In Experiment 1, a total of 10.5 liters of water was added to each container, which led to a significant increase in moisture retention in wetting-agent-treated soil columns. In the Experiment 2, a total of 47 liters of water was applied to each soil column and differences in moisture retention were marginal. The reason that no statistically significant differences were observed may have been due to the relatively high volume of irrigation water that was applied in a short time. This may have leached parts of the wetting agents out of the root zones.
- Soil physical properties
There are several possible mechanisms to explain how wetting agents increase water retention over time. One study conducted at the University of Georgia showed that water containing a wetting agent percolates through a root-zone profile more uniformly and more slowly than water alone. This indicates that water in a surfactant-treated profile utilizes pore space to a greater extent than water in an untreated profile. Simply put, because of the reduced surfaced tension, water with wetting agents reaches pore space that would not be reached without a surfactant.
A second possibility is that wetting agents may affect moisture retention by reducing leaching of fine soil particles (silt and clay) from soil profiles. A reduction in the leaching of fine soil particles from soil profiles would ultimately increase moisture retention over time compared to profiles that lost silt and clay particles.
Thirdly, some manufacturers of wetting agents suggest that surfactants provide a hydrophilic coating to water-repellent sand particles. This organic hydrophilic coating would reduce or eliminate water repellency, and water that would otherwise run off or percolate through the root zone quickly is held in the soil profile.
In our studies at New Mexico State University, we found evidence of both the reduced leaching of fine particles and of a hydrophilic, organic coating of sand particles. At the end of the studies, soil samples from the columns were analyzed for particle size distribution, soil physical properties and particle density. At the end of Experiment 1, particle density of sand treated with wetting agents was significantly lower compared to sand treated with water only. This might help explain the soil moisture data.
Determination of porosity and particle-size distribution at the end of the study showed that the treatment of different soil columns with wetting agents did not significantly alter their pore-size distribution. Because capillary porosity remained unaffected, the increase in water retention in surfactant treated soils is more likely to be caused by an increase in water bound to a hydrophilic layer than by changes in porosity. This hydrophilic coating, possibly of organic nature, could have caused the decrease in particle density. However, further research would be necessary to determine if a hydrophilic coating of the particles could have caused the increase in water retention in surfactant-treated soil columns.
In Experiment 2, leachate was collected and particle-size analysis of the soil particles in the leachate showed significant differences in silt content between treatments. Silt content was significantly greater in leachate collected from columns treated with water compared to columns treated with Primer. Also, silt and clay content in leachate collected from water-treated columns was always highest compared to Midorich and Primer treated columns. Although treatment differences for slit and clay content were not statistically significant (except for silt content from Primer-treated columns compared to water-treated columns), the leachate data suggest that wetting agents reduce the leaching of fine particles from root zones.
Wetting agents increased moisture retention in columns of a sandy root zone. The type of wetting agent and precipitation rate during the study influenced soil-moisture retention at different depths. Particle-size distribution and soil physical property data do not entirely explain the soil-moisture data. The data further suggest that water retention may be enhanced due to coating of the soil particles with a hydrophilic layer or due to reduced leaching of fine particles from root zones depending on the amount of precipitation.
More research is needed to substantiate these results. However, it seems clear that wetting agents have the potential to significantly affect the water-retention properties sand-based root zones. It's possible that this could be exploited as a cultural management tool for sand-based root zones, not merely to cure LDS, but this is something that hasn't been studied well. At the very least, turf managers should be aware that they may be altering the water relations of their turf when they use wetting agents.
Dr. Bernd Leinauer is a professor of turfgrass soil science at New Mexico State University (Las Cruces, N.M.).
The authors are grateful for the financial support of Aquatrols, CBC (America) Corp., Michigan Turf Foundation, New Mexico State University's Agricultural Experiment Station, Southwest Turfgrass Association and the Rio Grande Golf Course Superintendent Association. Without the generous support from these companies and associations this research would not have been possible.
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