Water is essential for soil, transporting nutrients and supporting plant roots and microorganisms. Soil holds water differently based on the physical forces determining retention and escape. Understanding the distinction between retained and draining water is fundamental for managing irrigation and assessing contaminant movement. The quantity of water that drains relates directly to the soil’s structure and the forces governing water movement through the spaces between soil particles.
Understanding How Soil Holds Water
The soil acts like a complex sponge, holding water based on the strength of the forces involved. Scientists classify soil water into three main categories, defined by how tightly they are bound to the soil matrix. The strongest force is adhesion, which binds a thin layer of water, known as hygroscopic water, directly to the surface of soil particles. This water is held so tightly that plants cannot extract it for use, even when the soil is completely dry.
A less intense force, capillary action, holds water within the small pores (micropores) of the soil. Capillary water is retained against gravity through the combined forces of adhesion and cohesion. This is the primary form of water available for plant uptake and remains in the soil after initial drainage, a condition known as Field Capacity. Neither hygroscopic nor capillary water drains readily because the forces holding them in the small pore spaces are stronger than gravity.
Gravitational Water: The Draining Kind
The water that drains from the soil is called gravitational water, also known as free water. This water occupies the larger pore spaces (macropores) within the soil structure. Unlike capillary water, gravitational water is held loosely and is primarily subject to the downward pull of gravity.
Gravitational water exists when the soil is saturated, typically after heavy rainfall or excessive irrigation. Because it is held loosely, it moves quickly through the soil profile, a process called saturated flow, until the soil moisture level drops to Field Capacity. This rapid drainage means plants can only access a small amount of gravitational water before it moves out of the root zone. The speed of drainage is heavily influenced by soil texture; coarse, sandy soils allow gravitational water to drain much faster than fine-textured clay soils.
The Journey and Quality of Drained Water
Once gravitational water exits the root zone, it continues its journey, carrying dissolved substances. This transport of water-soluble components downward through the soil profile is called leaching. The drained water often contributes directly to local water reserves, moving toward groundwater, aquifers, streams, or engineered drainage systems.
As the water percolates, it dissolves and picks up mobile substances, including nutrients like nitrate, excess salts, and agricultural chemicals such as pesticides and herbicides. Nitrate is susceptible to leaching because its negatively charged form is not retained by the negatively charged soil particles. This movement of dissolved components impacts the environment, as high concentrations of leached nitrate can contaminate drinking water and contribute to eutrophication of aquatic ecosystems. Understanding the volume and composition of this drained water is important for managing soil fertility and water quality.