Soils can process and contain considerable amounts of water. They can take in water, and will keep doing so until they are full, or the rate at which they can transmit water into, and through, the pores is exceeded. Some of this water will steadily drain through the soil (via gravity) and end up in the waterways and streams. But much of it will be retained, away from the influence of gravity, for use of plants and other organisms to contribute to land productivity and soil health.
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The spaces that exist between soil particles, called pores, provide for the passage and/or retention of gasses and moisture within the soil profile. The soil’s ability to retain water is strongly related to particle size; water molecules hold more tightly to the fine particles of a clay soil than to coarser particles of a sandy soil, so clays generally retain more water (Leeper and Uren, 1993). Conversely, sands provide easier passage or transmission of water through the profile. Clay type, organic content and soil structure also influence soil water retention (Charman & Murphy 1977).
The maximum amount of water that a given soil can retain is called field capacity, whereas a soil so dry that plants cannot liberate the remaining moisture from the soil particles is said to be at wilting point (Leeper & Uren 1993). Available water is that which the plants can utilise from the soil within the range of field capacity and wilting point.
The role of soil water retention is profound - its effects are far reaching and relationships are invariably complex. This section focuses on a few key roles and recognises that it is beyond the scope of this discussion to encompass all roles that can be found in the literature.
Soil water retention is essential to life. It provides an ongoing supply of water to plants between periods of replenishment (infiltration) so as to allow their continued growth and survival. Over much of temperate Victoria, Australia, for example, this effect is seasonal and even inter-annual; the retained soil water that has accumulated in preceding wet winters permits survival of most perennial plants over typically dry summers when monthly evaporation exceeds rainfall.
Soil moisture has an effect on the thermal properties of a soil profile, including conductance and heat capacity, explains Oke (1987); the association of soil moisture and soil thermal properties has a significant effect on temperature-related biological triggers, including seed germination, flowering and faunal activity.
Recent climate modelling by Timbal et al. (2002) suggests a strong linkage between soil moisture and the persistence and variability of surface temperature and precipitation; further, that soil moisture is a significant consideration for the accuracy of “inter-annular” predications regarding the Australian climate.
The role of soil in retaining water is significant in terms of the hydrological cycle; including the relative ability of soil to hold moisture and changes in soil moisture over time:
Soil water retention is widely studied and reported on in the literature due to its extensive and profound role; it is a key part of the hydrological cycle, provides support to organisms, interacts with climate, and is a major consideration in ground and surface water supply, environmental and geo-technical aspects.