Turfgrass root-growth response to subsoil acidity and amelioration of acid subsoils with gypsum in Ultisols
Kruse, John Sayler
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Highly weathered Ultisols typically have subsoils with little exchangeable calcium and high exchangeable acidity, often restricting root growth into depths that offer a potential reservoir of water. Research has not quantified the extent to which turfgrass root growth will respond to acid subsoils ameliorated by gypsum. Hydroponic, column, and field studies were utilized to determine gypsum efficacy. Tall fescue was grown in solutions containing levels of Al and/or CaSO4·2H2O at pH 4.5 for 7 d. Root growth was significantly reduced in low Al concentrations absent Ca. The addition of CaSO4·2H2O significantly increased root growth in the presence of Al. A simple logistic model adequately explained the relationship between Al and CaSO4·2H2O and relative root growth. Mechanistically, a computer-aided predictor of Al species (VMINTEQ) demonstrated that the addition of CaSO4·2H2O in Al solution reduced Al activity and the quantity of Al3+ as a percentage of total Al, and increased the percentage of relatively non-toxic AlSO4+ and solution ionic strength. A previously published calcium-aluminum balance equation did not adequately predict root growth or root-aluminum concentrations. Tall fescue was grown 85 d in columns filled with acid subsoils treated with lime or gypsum. After an initial period, the tall fescue was physiologically stressed by a column dry-down procedure. At experiment termination, soil columns were sectioned for chemical analysis and root density determination. Lime and gypsum ameliorated soils significantly increased exchangeable Ca and reduced exchangeable acidity. Lime and gypsum treated soils significantly increased root density over control treatments, with lime-treated soil producing a higher root-growth response than gypsum. Column evapotranspiration did not differ significantly between treatments. A field study utilizing lime, gypsum, and combinations of the two was conducted growing bermudagrass and zoysiagrass on an acid Cecil soil. Soil moisture was measured by time domain reflectometry. After 15 months, soils were analyzed by depth for chemical properties and root density. Differences in root density due to the addition of lime and gypsum were significant (a=0.10) in zoysiagrass but not bermudagrass. Despite a wet growing season, slight differences in volumetric water content were detected for zoysiagrass between 20-58 cm, but were nonexistent for bermudagrass.