When water levels become inadequate in the soil to support optimum turf growth and performance, we know that irrigation is needed to prevent drought. There is a condition termed localized dry spot however, which refers to turfgrass plants that remain water-deficient in affected areas despite watering. This condition is caused by the development of soil water repellency, also referred to as soil hydrophobicity. In this state, soil repels rather than retains water.
When such a condition occurs, irrigation or rainfall water that hits the turf surface bypasses the water-repellent areas, similar to an electric current that seeks the least resistant route. Ultimately, water deficiency occurs in this area and forms localized dry spot. Sometimes, soil hydrophobicity is evident as slow water infiltration. Many of us have faced this frustration when water stands on the turf surface extensively after a rainfall event or just a light shower. The situation is worse if it happens before play.
To find a proper solution we need to start by understanding the underlying culprit behind soil hydrophobicity. Soil water repellency is a common and universal issue that occurs on all lands. It happens as a natural consequence of soil organic matter decomposition, which is attributed to plant residues and other sources of organic matter in the soil. Over time, decomposition of soil organic matter forms a range of complex organic acids, such as humic acid and fulvic acid, which tend to coat on the surface of soil particles. Once coated with these organic compounds, soil particles become water-repellent and introduced water will not be able to adsorb to the soil surface. Instead, the water molecules simply move away or down deeper in the soil profile under gravity. This resembles wax-coated apples that are waterproof, and water cannot “stick” onto their surface.
Sand that we use to construct intensively managed turf for control of soil compaction, unfortunately, is more prone to develop hydrophobic conditions. By saying this, development of soil water repellency, especially on a sandy soil, is inevitable. An earlier report actually suggested that it might occur as quickly as 6 months after construction of a golf course green, for example.
Once formed, soil water repellency cannot be easily reversed by increasing irrigation water or frequency. As a matter of fact, soil water repellency in summer months typically worsens, as the soil experiences dry-wet cycles between irrigation/precipitation events. This fluctuation in soil water status changes the orientation of those organic acids, resulting in their water-repellent ends pointing outward from the soil particle. During this time, higher temperatures also stimulate production of organic acids, leading to a further progression of soil water repellency.
Fortunately, we can use a powerful tool, wetting agents, to alleviate this condition. Wetting agents are a type of surfactant that contains both oil-loving and water-loving parts in their molecules, just like a household detergent. When applied to a water repellent soil, wetting agents work as a bridge with one end holding onto the water-repellent sand surface and the other end sticking to the water molecule, thus retaining water and consequently wetting the soil. There are numerous wetting agents in the turf market, and early studies aimed at control of localized dry spot found minimal differences among those wetting agents. There are, as a matter of fact, substantial differences when water infiltration and soil rewettability are concerned.
At the University of Missouri, a research team consisted of a turf specialist, a soil physicist and a soil chemist, discovered that wetting agents commonly used on turf exhibit substantial differences in facilitating water infiltration into water-repellent sand. Adapted from a manuscript published in the Agronomy Journal by this team, this research found that wetting agents, such as Cascade Plus, Tournament-Ready, and Hydro-Wet, infiltrated into extremely water-repellent sand at the speed of 84 in/hr or greater at ¼ label suggested rates. Without wetting agents, water alone simply accumulated on the sand surface and never infiltrated, despite maintaining a consistent 4 in ponding depth. Other tested wetting agents, such as Revolution and LescoFlo Ultra, resulted in a slower steady rate at 60 in/hr. The slowest infiltration was found with Surfside 37, which exhibited a steady infiltration rate of 41 in/hr 20 min after ponding. The infiltration with wetting agent solutions, although variable, all surpassed the minimum suggested steady infiltration rate of 6 in/hr, as recommended by the USGA for green construction.
After the wetting agent treated sands went through a total of three dry-wet cycles by oven drying, infiltration of water was again tested and the wetting agents segregated into three groups. The first group contained sands treated with Hydro-Wet and Surfside 37; water infiltration was reduced to the minimal rate after the first run of dry-wet cycle, and after the second dry-wet cycle, no water infiltration was observed. The second group contained sands treated with Revolution and Tournament-Ready, which allowed the water to infiltrate after the first two runs of dry-wet cycle but not after the third dry-wet cycle. The third group included sands treated with Cascade Plus and LescoFlo Ultra, with both maintaining water infiltration at 60 in/hr or above even after three runs of dry-wet cycle.
This lab-based experiment demonstrated the differences among selected wetting agents for water infiltration, and projected their residual effect in facilitating water infiltration under field conditions. Caution is needed, however, because wetting agents are being used for a range of purposes including conservation of irrigation water. An ideal wetting agent would therefore strive for a balance between water infiltration and retention, in order to maintain a firm playing surface and an ideal growing environment for turfgrass plants. The research team at the University of Missouri is also performing field-based experiments; stay tuned for more research findings.
Dr. Xi Xiong is Associate Professor, Division of Plant Sciences, University of Missouri. Xiongx@missouri.edu