Water scarcity, prolonged drought and rising water costs are driving forces for turfgrass reduction or elimination in arid and semi-arid areas. Because of turf’s economic importance, efforts continue to reduce usage of potable irrigation water while maintaining turfgrass quality and functionality.
Managing turfgrass in areas where periodic and/or prolonged drought conditions or stringent irrigation ordinances exist can be challenging. Turfgrass irrigated with insufficient water or with poor irrigation coverage can often result in localized dry spots (LDS).
Different classes of products are available in the market aimed at increasing soil water retention and water-use efficiency, hence reducing water consumption under drought conditions (1). Soil surfactants, also called “wetting agents” by turf practitioners, are commonly used to manage water repellency in soil, prevent and alleviate LDS, and improve turfgrass quality (2, 3).
Different classes of chemistries define their mode of action with water and soil, which directly impacts their function and capability. In recent years, discussions have addressed manufacturer claims of labeling products as “retainers,” which were marketed as having properties to help the soil retain water, or “penetrants,” which aid infiltration into soil and eventually help with drainage (2). However, data are lacking among wetting agent products on various soil types in replicated, nonbiased field research. The objective of this study was to identify which commercially available nonionic surfactants could help sustain deficit-irrigated hybrid bermudagrass performance throughout the summer in arid environments.
A two-year study was conducted at the University of California, Riverside turfgrass research facility from May 24 to Oct. 31 in both 2018 and 2019. Soil type was a Hanford fine sandy loam (70.4% sand, 19.8% silt, 9.8% clay) with 0.6% organic matter. The research was performed on mature Tifway II bermudagrass (Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy) established in 2017. During the research period, plots were maintained under golf course fairway conditions and were mowed three times per week at a height of 0.5 inches (1.3 centimeters) using a reel mower with clippings returned. Verticutting was performed in April each year prior to treatment initiation. Fertilizer (21-7-14, Yara International ASA) was applied at 0.5 pound nitrogen/1,000 square feet (24.4 kilograms/hectare) monthly from April to November.
Nonlimiting irrigation (75% reference evapotranspiration, or ETo) was applied by means of sprinklers from November to June 6 each year as needed to avoid drought stress or for preconditioning of treatments during two weeks after initial application. From June 7 to Oct. 31 each year, to maximize the uniformity of water distribution, plots were hand-watered at either 45%, 55% or 65% ETo based on the previous week’s ETo determined via an on-site weather station located approximately 300 feet (91.44 meters) from the study site. Irrigation occurred three times per week, with hose and nozzle calibrated weekly following ETo calculation.
Ten wetting agent treatments were selected according to manufacturer recommendations based on the effectiveness and suitability for large turf areas such as golf course fairways. Except for one wetting agent treatment that was sprayed at half label rate, the remainder of the treatments were applied at label rates. Wetting agent treatments were applied at four-week intervals using a CO2 backpack sprayer equipped with a four-nozzle boom. Wetting agent treatments were initiated from May 23, 2018, and May 24, 2019, and ended on Oct. 10, 2018, and Oct. 11, 2019, respectively, and are detailed in Table 1. Following monthly product applications, all plots were irrigated with approximately 0.33 inches (0.84 centimeters) of water to move products into the soil.
Data were recorded every two weeks for turfgrass quality, normalized difference vegetation index (NDVI), soil volumetric water content (VWC) and soil moisture variability (4), percent green cover (PGC), and dark green color index (DGCI).
Wetting agent treatments exhibited higher turf quality than the untreated control, but no differences were observed among the wetting agent treatments. Besides the untreated control, all other plots maintained acceptable turf quality averaged across the study period (Table 2).
Regardless of wetting agent treatments, ETo replacement had no effect on turf quality in the first two months after initiating the treatments. Plots irrigated at 55% and 65% ETo did not show reduction in quality until September. Subsequently, acceptable turf quality was achieved only by plots irrigated at 65% ETo throughout the study (data not shown).
Editor’s note: Read more research from authors Marco Schiavon, Ph.D., and James Baird, Ph.D., in Products for alleviating irrigation salinity stress on turfgrass.
Except for Forté + CounterAct Retain (Simplot) or AquiMax Turf Lateral (Exacto) alone, treated plots significantly increased soil volumetric water content compared with the untreated control (Table 2). Treatments including Revolution (Aquatrols) and Passage (Numerator) greatly increased soil volumetric water content, but no statistical differences were observed between those treatments and TriCure AD (Mitchell) at the higher rate, Forté + Brilliance (Simplot) or Hydro-90 + Symphony (Harrell’s). Highest soil moisture variability was found in control plots in 2019. Turfgrass treated with Revolution, AquiMax Turf Lateral or Passage had greatly decreased soil moisture variability in 2019 compared with turfgrass treated with Forté + Brilliance (Table 2).
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