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TURFGRASS SCIENCE

An Alternative Method for Measuring Plant Available Water in Inorganic Amendments

University of California, Davis, LAWR, 1110 PES BLDG, One Shields Ave., Davis, CA 95616-8627

^* Corresponding author (mjcurtis{at}ucdavis.edu).

Inorganic amendments (IA) added to the rootzone of turfgrass are used to increase plant available water (PAW), however conflicting amounts of PAW contents have been reported for some of these amendments. The concept of PAW and the common technique used to estimate PAW in IAs have been reviewed. We hypothesized that hydraulic nonequilibrium in pressure-plate measurements results in an underestimation of PAW. An alternative standardized method of measuring PAW in coarse-textured inorganic amendments is proposed using a combination of pressure plates, saturated salt solutions and a dew-point potentiometer. Water release curves were measured for four IAs (calcined clay, calcined diatomaceous earth, calcined volcanic ash mixed with diatomaceous earth, and zeolite) and their PAW contents determined. Results show that the proposed method provides a more accurate –1500 J kg^–1 water content measurement than the pressure-plate method. Pressure-plate hydraulic nonequilibrium at –1500 J kg^–1 resulted in a 0.002 to 0.012 m³ m^–3 underestimation of PAW. The IAs examined in this study did not release appreciable amounts of internally held water until they were exposed to matric potentials more negative than usual in turfgrass management (<–300 J kg^–1). The use of a standardized methodology for estimating PAW in IAs, such as the one described in this paper, will help to provide needed consistency and accuracy in information to practitioners.

Abbreviations: IA, inorganic amendment • PAW, plant available water • CVAD, calcined volcanic ash and diatoms • CC, calcined clay • CDE, calcined diatomaceous earth • Zeo, zeolite • FC, field capacity • PWP, permanent wilting point