Comparative evaluation of pressure plate and evaporation–dewpoint methods for improved irrigation decision-making

Accurate determination of soil water retention parameters is essential for optimizing irrigation decisions, but inconsistencies between laboratory methods can lead to different recommendations. This study aims to evaluate and compare the Pressure Plate (PP) method and the Evaporation–Dewpoint method (Hyprop™ and WP4C™, referred to as HW) to see how they affect irrigation threshold estimates in different soil types. A total of 120 soil samples representing 10 soil series, ranging from sandy loam to clay, were collected from eastern Arkansas. Soil water content (SWC) was measured continuously with HW and at discrete intervals with the PP. The HW tended to overestimate SWC for each soil series, with regression slopes (HW vs. PP) greater than 1, but showed good agreement and a strong goodness of fit with PP RMSE: 0.02–0.1 cm³ cm⁻³; R²: 0.79–0.97). Differences in available water (∆AW) showed that the HW estimated significantly greater AW than the PP (p < 0.001) in most soil series. HW gave more conservative thresholds for loamy soils (-70 to -195 kPa), while PP was more conservative for clayey soils (-45 to -90 kPa). This reflects significant differences in field capacity (ƟFC), permanent wilting point (ƟPWP), and available water (ƟAW) (p < 0.001) between methods, leading to different matric potential estimates at the same allowable depletion (AD). Based on the findings, the PP method may be more suitable for fine-textured soils when a conservative threshold is desired, while the HW method may be more appropriate for intermediate soils.