Soil water is a dynamic component of the soil system, cycling through different forms that determine its movement and availability to plant life. This moisture is classified based on the physical forces that hold it within the soil structure. Gravitational water moves most freely and rapidly through the soil profile. This mobility means it is the first water to leave the root zone following a significant rain or irrigation event.
Defining Gravitational Water
Gravitational water is the excess water present when the soil is completely saturated, meaning all pore spaces are filled. This water is located within the large soil voids, often called macro-pores. The soil holds this water only weakly because the forces between the water molecules and soil particles are not strong enough to counteract gravity.
This form of soil moisture is defined by its eventual fate: drainage. It is the volume of water that inevitably moves downward under the influence of gravity. Because of its loose association with the soil matrix, it is frequently called free water. Its presence is temporary, existing only between full saturation and the point when drainage slows significantly.
The Mechanism of Downward Drainage
The primary force driving this movement is gravity, which pulls the water downward through the interconnected macro-pores. This downward movement is called saturated flow, occurring when the soil is waterlogged. Gravitational drainage starts immediately after a heavy watering or rainfall event ceases.
Drainage continues until the soil reaches “field capacity.” Field capacity is the moisture content remaining after gravitational water has largely drained away, typically a day or two after saturation. Gravitational water represents the difference in moisture content between saturation and field capacity. Soil texture significantly influences the drainage rate; coarse-textured soils, like sand, have large pores that allow gravitational water to drain very quickly.
Availability for Plant Life
Gravitational water is generally considered unavailable for long-term plant use because it drains out of the root zone too rapidly. While roots may absorb a small amount immediately following a soaking, the bulk of this water is lost before plants can utilize it efficiently. Its rapid movement beneath the root zone has two negative implications for agriculture.
One concern is the development of anaerobic conditions, or waterlogging, when drainage is slow. If gravitational water remains in the pores for an extended time, it displaces the air roots need for respiration, effectively suffocating the plant. The other major concern is nutrient leaching. As the water percolates downward, it dissolves and carries soluble plant nutrients, such as nitrates, below the depth where roots can access them.
Distinguishing Gravitational Water from Other Soil Moisture
Gravitational water is one of three main classifications of soil moisture, defined by the strength of the forces holding the water in place. The other two forms are capillary water and hygroscopic water. Capillary water is held in smaller soil pores, called micro-pores, by surface tension and is retained against gravity. This is the most important form of water for plant growth, as it is readily available for roots to absorb.
In contrast, hygroscopic water is held tightly as an extremely thin film around soil particles due to strong adhesive forces. This water is bound so tightly that plants cannot extract it, even at the permanent wilting point, making it unavailable for biological use. Gravitational water is unique because it is held by the weakest forces, present only when stronger capillary forces have been overwhelmed by excess moisture.