Measurements of Soil Water Potential and Conductivity based on a Simple Evaporation Experiment using a Hydraulic Property Analyzer

Soil water retention and hydraulic conductivity within natural and human-altered environments help us understand and observe changes in soil health and functionality. Quantifying hydraulic properties through the soil water retention curve (SWRC) and soil water conductivity curve offers insight into key drivers of soil physical behavior and water movement characterization¹. The relationship between volumetric water content (θ) and matric head (h) is represented within an SWRC, and the ranges within the curve describe the saturation point, field capacity, and permanent wilting point². Soil management practices, amendments, agroecosystem types, and environmental conditions can all have an impact on soil hydraulics^3,4. These factors can in turn influence solute transport⁵ and plant available water⁶, soil respiration and microbial activity⁷, as well as wetting and drying cycles⁸. As an important piece in quantifying healthy and functioning soil, proper analysis of the SWRC is imperative for obtaining an informed understanding of soil hydraulic properties.

A variety of measurement techniques currently exist for developing a reliable SWRC, with the hanging-water column and pressure plate methods being common traditional approaches for determining the pore size distribution of soil². Traditional methods can be time-consuming, usually taking weeks or months to analyze a small set of samples⁹. Moreover, once analysis is complete, these methods result in only a few data points that inform the SWRC⁹. Additionally, the accuracy of producing representative data using traditional methods such as pressure plates can become a concern at lower matric potentials, in particular, with fine-textured soils^10,11. More modern techniques, which involve the simple evaporation experiment approach using tensiometers and the chilled-mirror dew point method, tend to deliver more reproducible data across a wide range of soil textures². Initially developed by Wind in 1968, the simple evaporation experiment involved measuring water mass changes and tension changes through tensiometers in the soil sample over time¹². As evaporation occurs, soil sample mass measurements are taken at specific time intervals to create an SWRC. Later refined by Schindler (1980), the method involved only two tensiometers placed at different pressure heads within the soil sample. The modified method was then tested and validated as capable of being used in scientific analysis^13,14. A key benefit of the simple evaporation experiment is the potential to easily produce data across a large portion of the soil moisture curve (0 to -300 kPa), with more data points than with traditional methods.

These modern methods involve automated instruments that take numerous data points throughout the sample analysis period and produce data using a software interface. The hydraulic property instrument is a contemporary instrument that creates water retention curves and conductivity curves from sample data¹⁵. By employing a simple evaporation experiment using the hydraulic property instrument, the relationship between water content and water potential in the soil can be evaluated¹. In this experiment, water present within the tensiometer shaft exists in an equilibrium with water in the soil solution. As evaporation of soil water occurs and the soil sample dries, cavitation takes place in the tensiometer, and the experiment ends. There is a limitation of the hydraulic property instrument in the dry range of the SWRC, as the instrument is only capable of operating within matric potentials of 0 to -100 kPa. This can be remedied with the inclusion of data generated with a chilled-mirror dew point experiment using a soil water potential instrument¹⁶, which can extend the data range to -300,000 kPa or the permanent wilting point. All these data are brought together in the modeling software post processing to cohesively inform the SWRC from null tensions to higher tensions even beyond wilting point. The SWRC and hydraulic conductivity curves are then generated based on matric potential data points taken throughout the measurement period, allowing a complete curve projected from saturation to permanent wilting point to be generated.

The method described here presents a succinct operating procedure for soil analysis with a hydraulic property instrument. This method has been conducted in a number of scientific settings, including quantification of soil health in a broad range of agroecosystems^3,17,18,19, and efforts have been made to understand best practices beyond the instrument user manual²⁰. Here, a standardized protocol is outlined for all steps of the procedure, including field sampling, sample preparation, software function, and data processing. Following this method will ensure a successful campaign that results in reliable data. Critical steps for ensuring quality data, common challenges, and best practices are presented to ensure proper implementation.