Estimation of readily available waterholding capacity using field hand texture

The determination of readily available waterholding capacity (RAW) in the field provides essential information when conducting soil survey’s for irrigation design and scheduling. This paper outlines a method for estimation of RAW in the field based on the accepted descriptive method of “field hand textures” as outlined in the “yellow book”, McDonald et al. (1990). Relationships of field texture to soil carbonate and bulk density were also investigated. Intact core samples (triplicate samples of 329 discernable soil layers from 94 profiles) from the Riverland, Barossa and Adelaide Hills regions in South Australia and Sunraysia district in Victoria were equilibrated on ceramic plates in a pressure chamber to determine the water retention characteristics and readily available waterholding capacity deficit ranges from field capacity (-8 kPa) to -40, -60 and –1500 kPa. A significant linear relationship (r 2 values of 0.91, 0.95, 0.95 and 0.92 respectively) exists between volumetric water content and average clay percent from field texture at matric potentials in the plant available range. Soil carbonate significantly increases readily available waterholding capacity in the deficit ranges of field capacity to -40 kPa, -60 kPa and 1500 kPa for light sandy clay loam and sandy clay loam field textures. A bulk density greater than 1.6 g/cm 3 decreases readily available waterholding capacity for sandy loam (SL) and light sandy clay loam