This study presents the results of the water retention characteristics of six groups (0.25 mm to 8.35 mm) of expanded perlite (EP) produced from two crushed raw perlite rocks of Turkey (A) and China (B). The particle size proportions of EPs showed that the particle size less than 1.68 mm was higher in B than that of A and the particle size distribution curves (PSDC) of three samples of EP A and B were similar, while the percent finer value (D_N) of B was higher than that of A in all corresponding particle sizes. Volumetric water content (θ_v) for EPs of A and B were obtained at 11 levels of matric potentials between 0 to −50 kPa (sandbox with kaolin-plate) and −50 to −1500 kPa (pressure chamber), respectively, after fully saturating the uniformly packed EP in open-ended metal round rings. The results showed that the shapes of measured WRCs and particle size distribution curves of EP A and B were similar, while the percent values of PSDC were higher than those values of θ_v corresponding to each water potential greater than 0.1 kPa. The WRCs of respective EP showed that a substantial decline started at about −1 kPa and approached the residual water content (θ_r) observed at −500 kPa for all measurements. θ_v for all particle sizes of the EP A and B was not only decreased with the increasing matric potential but also distinctive with increasing particle size greater than 2.36 mm. θ_r was increased with decreasing particle size. For these results, we assumed that θ_v of the different particle sizes of EP A and B could be determined by actual pore volume excluding occluded pore volume depending on size and surface area in the EP particles.

This investigation was to evaluate the applicability and prediction accuracy of pedotransfer function (PTF) to estimate the water retention curves of Ap horizon soils for five soil series which cannot be directly used to cultivate upland crops other than rice because of their high salinity. Soil water retention curves (SWRCs) were obtained from 150 undisturbed soil samples collected from the Ap horizons of which soil textures were grouped into sandy loam with high contents of sand (>60%) and low clay contents (60%) and low sand content (<10%). Soil-water retention characteristics between 0 to −50 kPa and −50 to −1500 kPa were measured using the sandbox, kaolin-plate, and pressure chamber methods, respectively. The SWRCs were also estimated by Rawls and Brakensiek PTF (RB-PTF). The measured SWRCs of sandy loam and silt loam were compared against those obtained from RB-PTF. The SWRCs estimated with the modeled data closely fit the measured data of sandy loam and silt loam although saturated volume water content (θs) and residual volume water content (θr) of RB-PTF were higher than those of the measured mean for both soil textures. The air entry point (α) and the steepness of the water-retention curve (n) were higher in sandy loam than those of silt loam. SWRCS estimated by the RB-PTF yielded the best fit of all soil samples from the five series, whereas the MBE values less than zero indicate that θv measured at the water potentials is under-predicted. The r² values greater than 0.9 for sandy loam and silt loam represent RB-PTF and are suitable for predicting SWRCs in RTFS because the measured data points do not vary around the estimated regression line.