Negative water potentials arise as soil dries resulting in suction or tension on water allowing the soil to retain water like a sponge. When at or near saturation, soils typically display water potentials near 0 MPa. A potential energy gradient dictates soil moisture redistribution and losses, where water moves from areas of high- to low-potential energy (Hillel 1982).
Water is stored and redistributed within soil in response to differences in potential energy. These processes are governed by potential energy. However, total water holding capacity does not describe how much water is available to plants, or how freely water drains in soil. Generally speaking, clay-rich soils have the largest pore space, hence the greatest total water holding capacity. Water storage and redistribution are a function of soil pore space and pore-size distribution, which are governed by texture and structure (Childs 1940). These concepts are integrated with a case study that describes the interplay between hydrologic processes, water dynamics in soil, and soil genesis. The learning objectives discussed here include: 1) an introduction to the soil water balance equation 2) the factors and soil properties that govern water potential and plant available water holding capacity and 3) soil morphologic features and classification methods to describe the fate of water in soil. This article focuses on soil water dynamics and introduces concepts of soil moisture storage, water flow and the soil properties that influence these processes. Water dynamics in soil are governed by many factors that change vertically with depth, laterally across landforms and temporally in response to climate (Swarowsky, et al., 2011). Thus, most aspects of terrestrial- and freshwater aquatic-life depend on hydrologic processes in soil (O'Geen et al. As such, the sustainability of water resources (considering both quantity and quality) is directly influenced by soil.
Water percolating through soil is filtered, stored for plant utilization, and redistributed across flow paths to groundwater and surface water bodies. The capacity of soil to regulate the terrestrial freshwater supply is a fundamental ecosystem service.
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