Localized dry spot is a common turfgrass problem of which there’s no silver bullet, much to the chagrin of golf course superintendents. At the GCSAA education conference, which is part of the Golf Industry Show in Atlanta, about 80 superintendents attended a seminar about LDS to find out how to manage this problem better. Keith Karnok, Ph.D., a professor in the department of crop and soil sciences at the University of Georgia, was the presenter.
Localized dry spot is the occurrence of an irregular area of turfgrass that, for no apparent reason, begins to show signs of drought stress. Symptoms of LDS on turf include footprinting, blue/green coloration, wilting and death of the plant. There are several causes of LDS, and some include:
· Summer patch;
· Lack of irrigation;
· Hydrophobic soil;
· Mechanical stress;
· Thatch;
· Nematodes;
· Compaction;
· Insects; and
· Disease.
Despite the temptation for superintendents to apply a wetting agent to an area of turf that shows signs of drought stress, Karnok suggests they diagnose the area first. Part of that diagnosis includes testing for hydrophobic soil. If done, Karnok says to make sure the sample is completely dry and done naturally, not dried more quickly by using an oven or microwave. One way to determine if soil is water repellent is to conduct a water drop penetration test. By using the following chart, superintendents can determine how water repellent their soil is.
WDPT (seconds) Degree of repellency
0-5 None
5-60 Slight
60-600 Moderate to high
600-3,600 Severe
More than 3,600 Extreme
Soil with high sand content tends to be more water repellent, Karnok says. Sand particles in soil with LDS have a coating, which is caused by organic matter decomposition, that shields the water molecule when its dry. However, the coating is weakly adhered, but as the coating becomes more uniform, the repellency becomes greater. Because of this, it’s a good idea for superintendents to test topdressing sand to see if it’s hydrophobic before buying it.
Additionally, fair ring organisms can cause LDS because they’re organic matter decomposers, they form a fungal mat, and they produce toxic chemicals. Yet all hydrophobic soils aren’t caused by fairy ring.
Hydrophobic soils relate to push-up greens, too, because they’re modified with sand, and the sand breaks down organic matter; and that decomposition, when its dry, coats the sand, causing hydrophobicity.
Karnok says, according to the microbiologists he’s talked to, there’s no product commercially available that will remove or breakdown the coating because organic matter decomposition can’t be stopped. Sodium hydroxide removes the coating, but it kills the grass. Hydrogen peroxide can be used to dissolve organic compounds, but when used on turf it causes phytotoxicity. Superintendents just have to deal with the organic matter decomposition coating, Karnok says.
Water repellent soil develops eight to 18 months after construction because it takes that long for organic matter to build up. Coarse textured soils are the most prone to becoming water repellent, but the severity of water repellency varies with the seasons – it’s less severe in late fall, winter and early spring.
Clay and organic matter also are related to hydrophobic soil. Hydrophobicity isn’t always related to sand. Although, hydrophobicity is less common in clay soils because they have a much greater moisture holding capacity that overshadows any hydrophobicity.
LDS is more prevalent than it was 20 years ago, partly because of lower cutting heights and more prolific, organic-matter-producing turf varieties. There were hydrophobic soils back then, but the root systems were longer because the cutting height was higher.
Yet the degree of water repellency varies across a green because the soil isn’t uniform. Nonuniformity is caused by undulation; golfer traffic patterns; compaction; and water, fertilizer and pesticide applications. But water repellency isn’t caused by a particular cultural practice.
Also, severe wetting and drying cycles (rainfall, flushing) can increase soil water repellency. The continual maintenance of wet root zone will prevent the symptoms of water repellent soil but the condition will still exist.
Turfgrass species can affect the rate of development of soil water repellency, but all eventually result in varying degrees of hydrophobicity. Even sand with no organic matter added will develop water repellency.
Managing water repellency
Water repellency occurs in the top two inches of the soil profile, and because of that, superintendents need to grow the roots below that two-inch level. Some primary management practices are:
1. Select the best species/cultivars. For example, Tifdwarf Bermudagrass has a better root system than TifEagle, but TifEagle has some better playing conditions than Tifdwarf. Generally, most new grasses have a better root system.
2. Reduce nitrogen. Too much nitrogen decreases root growth.
3. Increase cutting height. Higher heights of cut mean longer roots.
4. Irrigate properly. Water need to be applied more uniformly. One should apply enough water to wet soil at least one inch below the root zone.
5. Implement a strict cultivation and topdressing program. Three percent or less organic matter is needed in the top two inches of soil profile. Five percent or more is bad. Testing topdressing sand for water repellency also is recommended.
Wetting agents
In addition to these cultural practices, the use sand substitutes/soil conditioners and wetting agents can be beneficial. Wetting agents help to alleviate the symptoms of water repellent soil temporarily. All commercially available wetting agents tested at the University of Georgia significantly increased soil water content compared to the water repellent control, according to Karnok. The wetting agents tested, according to the manufacturers’ recommendations, tend to be safe and nonphytotoxic to turf. But differences in phytotoxicity can occur when applied under stress conditions and/or not watered in. Also, rates and the number of applications vary dramatically among products. So the most effective product, rate and number of applications might depend on the degree of soil water repellency.
In most cases, a wetting agent should be applied across the entire green if water repellent soil is indicated. Wetting agents applied to water repellent soil might result in increased water savings. High thatch or organic-matter root zones might tend to dry out slower when treated with a wetting agent. However, applying wetting agents to nonwater-repellent sand root zones offers few advantages. GCN
