Plant Available Water in Hydrophobic Soil Affected By Wetting Agents.

Soil hydrophobicity, which causes localized dry spots (LDS), is a common problem for golf courses, especially for sand-based growing media such as USGA putting greens.  Wetting agents, which contain hydrophobic and hydrophilic functional groups, are the primary tool for the treatment of water-repellency.  Superintendents use wetting agents for purposes other than treating LDS, including improving water retention in a hydrophobic soil.  Under field conditions, it is fairly easy to evaluate LDS reductions; however, it is difficult, if not totally impossible, to investigate the effect of wetting agents on plant available water in the soil profile, due to significant variations in soil hydrophobicity and environmental conditions.  The objective of this study was to develop a system in an environmentally-controlled growth chamber for evaluating plant available water as influenced by wetting agent treatments.  Creeping bentgrass (Agrostis stolonifera L.) plants were established from 1.3 cm in diameter plugs and grown in containers with a 21 cm depth and 3.8 cm diameter.  The hydrophobic sand was collected from a putting green where severe to extreme hydrophobicity was observed and homogenized before planting.  Water droplet penetration tests revealed a consistent hydrophobicity at 600 s or greater.  After incubating in the greenhouse under ideal growth conditions, plants were treated with various wetting agents at their labeled rates and moved to a growth chamber with temperature of 30/25 °C (day/night) and a 12 h photoperiod.  Plants were watered daily at either 75, 100, or 125% ET replacement based on weight changes for each container every 24 h for a 3-week period. The experimental design was a randomized complete block with four replications, with treatment combinations of three wetting agents in addition to an untreated control at each of the three water regimes.  At three weeks after treatment, above-ground tissues were collected and samples were analyzed for relative water content (RWC), dry biomass, and N content.  Results indicate that at 100 or 125% ET replacement, all plants maintained RWC at 66% or above, regardless of wetting agent treatment.  However, when only 75% ET replacement was compared, the untreated control plants showed decreased RWC (42%), which was significantly less than wetting agent treated plants.  At 75% ET replacement, all treated plants maintained RWC at 60% or above, which values were not statistically different from the 100% or 125% ET replacement.  Dry biomass and N content data are currently being analyzed and results will be presented.