SportsTurf provides current, practical and technical content on issues relevant to sports turf managers, including facilities managers. Most readers are athletic field managers from the professional level through parks and recreation, universities.
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Irrigation&Drainage | Mica McMillan Water movement and repellency: putting wetting agents to work Editor's note: Water movement in soils is influenced by three processes: chemical, physical and water repellency. Chemical processes include high electrical conductivity, high bicarbonates, high sodium and low calcium. Physical processes include thatch and organic matter, hardpan and com- paction. We will be covering all three in the next three issues; water repellency and wetting agents will be discussed this month. The author is senior research agronomist for Aquatrols. needs to survive and thrive and contributes to waste of water and run-off of soil directed chemicals such as fertilizer and pesticides. Water repellency below the soil surface creates non-uniform distribution of water, and any material applied with water. Prefer- ential flow paths or "fingered flow" are caused by physical and chemical processes but are also strongly associated with water repellency. Due to hydrophobic organic coatings, water molecules are repelled away from soil particles, decreasing access to nu- merous pore spaces. A re-occurring pathway of water flow to the bottom of the soil pro- file is formed. This preferential flow results in non-uniform distribution of water, fertil- izers and pesticides, which can reduce turf quality and enable unsightly localized dry spots to develop. Coarse textured soils go through numer- >> CONDUCTING a water drop penetration test; water drops on the surface of water repellent soil. inability of soil to wet. The causes of soil water repellency are numerous; plant root exu- dates, fungal hyphae, and de- composing organic matter are just a few of the sources of hy- drophobic coating on soil particles. This hydrophobic coating on sand particles pre- vents water from attaching to the particle and may inter- rupt the uniform movement of water through the soil profile. (To determine if soil is water repellent, a water drop penetra- tion test should be performed. Take a core sample down to root depth. Air dry for ap- proximately 2 weeks, then place water drops at one centimeter depths along the core and W 14 SportsTurf | April 2012 ATER REPEL- LENCY or soil hy- drophobicity is the time how long it takes for the water drop to penetrate the soil core. Water repellency is defined: non-wettable [<5 seconds], strongly water repellent [60-600 seconds] and severely water repellent [600-3600 seconds]). In highly managed turfgrass environ- ments such as sports fields, water repellency tends to be more severe at the surface and declines farther along the soil profile. Typi- cally, the top 3 cm of a coarse textured soil are the most hydrophobic. This top 3 cm is enough to significantly disrupt water move- ment. Water repellency at the surface is evi- dent when runoff, puddling and slow infiltration occur. Water repellency signifi- cantly reduces irrigation distribution unifor- mity. Although not visually evident, the delay in water movement into the soil in an arid environment also causes water loss to evaporation. This water repellency and loss prevents your turf from getting the water it ous wet to dry cycles. It is challenging in a turfgrass environment to maintain soil at constant volumetric water content and pre- vent soil from falling below the critical water content or the soil water content where or- ganic acids are prone to displaying hy- drophobic coatings. Rewetting of soil particles is difficult when moisture is severely limiting. Acceptable moisture content, par- ticularly on an in-play sport field, may be below this critical water content. A signifi- cant increase in applied water is needed to overcome the hydrophobic areas and achieve a goal of an even matrix flow of water. Water repellency can be managed. Wet- ting agents are used to alleviate soil water re- pellency and improve water movement through the soil profile. The chemistry of a wetting agent is composed of a hydrophilic end and a hydrophobic end. The hydropho- bic end will attach to the non-wettable or- ganic acid on the soil particle. The hydrophilic or water-loving end of the sur- factant molecule draws the water molecule closer to the soil particle, thereby successfully wetting the soil particle. By doing so, water is retained and soil volumetric water content increased. Plant available water is readily ac- cessible in pore spaces. The wetting ability of the wetting agent helps to reduce preferential flow paths and rewets the soil readily. Ask yourself if you have ever had diffi- culty getting certain areas of a field to ab- sorb water, or if specific areas are always www.sportsturfonline.com By