Soil water potential: A historical perspective and recent breakthroughs

Abstract

Soil water potential is a cornerstone in defining the thermodynamic state of soil water required to quantify phenomena such as water phase change, water movement, heat transfer, electric current, chemical transport, and mechanical stress and deformation in the earth's shallow subsurface environment. This potential has historically been conceptualized as free energy stored in a until volume of soil water. Though the concept of soil water potential has been evolving over the past 120 yr, a consensual definition is still lacking, and answers to some fundamental questions remain controversial and elusive. What are the origins and mechanisms for the free energy of soil water? Can the common mathematical expression of soil water potential as superposition of gravitational, osmotic, and matric potentials be used to define water phase transitions in soil? Are these major components of soil water potential independent or coupled? Is pore water pressure always tensile under unsaturated conditions? If so, how can soil water density be as high as 1.7 g cm −3 ? How do adsorptive soilwater interactions originating from the electromagnetic field around and within soil particles transfer to mechanical pore pressure? In this review, the authors (a) provide critical analysis of historical definitions of soil water potential to identify their strengths, limitations, and flaws; (b) synthesize the origins of electromagnetic energies in soil to clarify the fundamental differences between adsorptive and capillary soil water potential mechanisms; (c) introduce a recently emerging concept of soil matric potential that unifies contributions of adsorption and capillarity to soil water potential; and (d) illustrate the generality and promise of the unified definition of soil water potential for answering some of the fundamental questions that remain elusive to the hydrology, geoengineering, and geoscience communities.

Quantifying Coupling Effects Between Soil Matric Potential and Osmotic Potential

“…To sum up, the four components constituting SSP unfailingly depend on the soil water salt concentration c 0 (Luo et al, 2022). Thereby, the SSP and the unitary definition of the matric potential (Equations 3 and 4) should be extended as:…”

“…To sum up, the four components constituting SSP unfailingly depend on the soil water salt concentration c 0 (Luo et al., 2022). Thereby, the SSP and the unitary definition of the matric potential (Equations and ) should be extended as: $$${\psi }_{\text{sorp}}\left(x,{c}_{0}\right)={\psi }_{\text{vdW}}\left(x,{c}_{0}\right)+{\psi }_{\text{hyd}}\left(x,{c}_{0}\right)+{\psi }_{\text{osm}}\left(x,{c}_{0}\right)+{\psi }_{\text{ele}}\left(x,{c}_{0}\right)$$$ $$${\psi }_{\mathrm{m}}\left(w,{c}_{0}\right)={u}_{\mathrm{w}}(x,w)-{u}_{\mathrm{a}}+{\psi }_{\text{sorp}}\left(x,{c}_{0}\right)$$$ …”

“…It is anticipated that this change in relative permittivity can result in a substantial alteration of SSP thereby matric potential. Besides, the osmotic and electric components of SSP stem from EDL, and the EDL thickness is highly dependent on salt concentration (e.g., Luo et al., 2022). Therefore, it can be theorized that all the adsorption components of matric potential are dependent on salt concentration, that is, coupled with osmotic potential.…”

<em>In situ</em> Soil Moisture Sensors in Undisturbed Soils

“…Many ecosystem processes are also regulated by soil water potential, which is less commonly measured in situ 62 . Soil water potential, recently reviewed by S. Luo, N. Lu, C. Zhang, and W. Likos 63 , is the energy state of water; such sensors may be less affected by soil properties and provide quality control for SWC sensors 64 . Furthermore, the bulk field SWC includes gravels, rocks, roots, and void space (e.g., preferential flow paths).…”

Reproducibility of the Wet Part of the Soil Water Retention Curve: A European Interlaboratory Comparison

“…The soil water retention curve (SWRC) is a soil specific hydraulic property that represents the relationship between the matric potential and the water content of the soil (Hopmans, 2019). The matric potential represents the energy state of water in soil, induced by physicochemical interactions between soil particles and water molecules (Luo et al, 2022). These physicochemical interactions are divided into capillary forces dominating at the wet part of the SWRC and adsorptive forces dominating at the dry part (Tuller et al, 1999;Tuller and Or, 2005).…”