Soil transport is the physical movement of soil particles and unconsolidated sediments from one location to another. This movement is a natural component of the Earth’s surface dynamics, often referred to as erosion. The primary driving force is the energy supplied by natural agents, which overcome the soil’s resistance. Understanding this process requires examining how water, wind, gravity, and ice mobilize and carry material across the landscape. The result is the redistribution of material, sculpting the planet’s surface and creating new landforms.
Soil Movement Driven by Water (Fluvial and Hydraulic Transport)
Water is the most dominant agent of soil transport on Earth, moving material across land surfaces, through rivers, and along coasts. Movement begins with rain splash, where the impact of raindrops dislodges individual particles. This progresses into sheet erosion, where a thin, uniform layer of soil is carried away by surface runoff. As the flow concentrates, it forms small channels, leading to rill erosion, which can be smoothed out by normal farming operations.
The transport of soil by water, known as fluvial transport, occurs through three main mechanisms related to particle size.
Suspended Load
The smallest particles, such as fine silts and clays, are carried within the flow as the suspended load, remaining aloft due to the water’s turbulence.
Saltation
Larger particles, typically sand-sized grains, move by saltation, bouncing or skipping along the riverbed. These particles are briefly lifted by hydrodynamic forces but quickly return to the bed under gravity.
Bedload (Traction)
The coarsest material, including gravel, cobbles, and boulders, moves along the bottom as the bedload, primarily through rolling and sliding. High-energy flood events can transport significantly larger material than normal flow.
Soil Movement Driven by Wind (Aeolian Transport)
Wind transport, or aeolian transport, is effective in arid, semi-arid, or newly disturbed environments where vegetation cover is sparse and the soil is dry. Wind mobilizes soil through distinct mechanisms based on particle size.
Suspended Load
The smallest particles, typically less than 0.1 millimeters in diameter, are carried high into the atmosphere as the suspended load, sometimes traveling hundreds or even thousands of miles.
Saltation
The most significant mechanism by mass is saltation, accounting for 50 to 90% of total soil movement. These medium-sized grains (0.1 to 0.5 millimeters) bounce along the surface, typically remaining within 30 centimeters of the ground. The impact of a saltating grain dislodges other particles, kicking finer material into suspension or pushing larger material forward.
Surface Creep (Traction)
The largest particles (greater than about 0.5 millimeters) are moved by surface creep or traction. These grains are too heavy to be lifted by the wind alone, but the kinetic energy transferred from the impacts of saltating particles causes them to roll or slide along the ground. This mechanism accounts for 5 to 25% of the total soil moved by wind.
Soil Movement Driven by Gravity and Ice (Mass Wasting and Glacial Processes)
Soil can be transported without fluid agents through the direct action of gravity, a process broadly termed mass wasting. This movement occurs on slopes and ranges from sudden, rapid displacement (landslides and rockfalls) to the extremely slow, continuous downslope movement called soil creep. Soil creep involves the gradual expansion and contraction of soil due to freezing, thawing, or wetting and drying cycles, causing a net downhill shift over time.
Ice, in the form of glaciers, is a powerful, large-scale agent of transport, capable of moving immense quantities of soil and rock. Glaciers scrape and grind the underlying bedrock, creating finely pulverized material known as rock flour. This material, along with larger boulders, is incorporated into the ice mass and carried as the glacier flows. The sediment deposited directly by a melting glacier is called till, which is unsorted and unstratified.
The Final Destination: Sedimentation and Deposition
The transport cycle ends when the agent’s energy diminishes to the point where it can no longer keep the material in motion. This settling process is called sedimentation, and the resulting accumulation is deposition. Deposition often leads to the sorting of sediments, as finer particles are carried farther than coarser ones before settling.
Water-borne deposition creates landforms such as alluvial fans, which form where a stream leaves a steep valley and spreads onto a flatter plain. River deltas are formed when a river slows down as it enters a standing body of water, dropping its suspended load to create new land. These fluvial deposition zones result in highly fertile floodplains due to the accumulation of fine, nutrient-rich silts.
Wind-deposited sediments create distinct landforms, the most notable being loess, an accumulation of wind-blown silt. Loess deposits can be tens of meters thick and often originate from glacial outwash plains where winds pick up fine rock flour during dry periods. These deep deposits are highly porous and yield some of the world’s most productive agricultural soils.