Soil is a complex mixture of mineral particles, organic matter, water, and air. Geologists categorize this material based on its origin: whether it formed in its current location or was moved there from elsewhere. This classification divides the Earth’s surface covering into two primary categories: residual soil and transported soil. The differences in their formation and movement processes result in distinct characteristics that influence agricultural productivity and engineering stability.
Residual Soil: Formation and Connection to Bedrock
Residual soil, also known as sedentary or in situ soil, is material that develops directly in place from the underlying parent rock, or bedrock. The process begins with the physical and chemical breakdown of the solid rock through weathering. Agents like water, temperature fluctuations, and organic acids decompose the minerals. Over thousands of years, this decomposition creates a mantle of soil that remains physically connected to its source material.
Because the soil is derived directly from the rock beneath it, residual soil exhibits a strong mineralogical correlation with that parent material. A soil formed over granite, for example, will be rich in quartz and feldspar remnants, retaining many of the chemical and physical properties of the original rock. The specific characteristics of the soil, such as its thickness and composition, are controlled by the climate and the rock’s resistance to weathering.
The profile of a residual soil shows a gradual transition from the surface horizons down into the unaltered bedrock. The uppermost layers are often finer due to intense weathering and the accumulation of organic material, defining the A and B horizons. Below these layers, the C horizon consists of recognizable, partially broken fragments of the parent rock, slowly grading into the solid, unweathered bedrock below. This continuous and predictable change in the soil profile demonstrates that the entire formation process occurred without significant external movement.
Transported Soil: Mechanisms of Movement and Deposition
In contrast to residual soil, transported soil, or erratic soil, is material that formed elsewhere and was subsequently relocated and deposited by an external natural force. The original weathering of the rock occurred at a distant source, and the resulting particles were carried away by powerful agents of transport. Consequently, the composition of transported soil often has no direct chemical or mineralogical link to the bedrock it currently rests upon.
One of the most common transporting agents is water, which creates fluvial or alluvial deposits in river valleys and floodplains. The energy of the flowing water determines the size of the particles carried. Faster currents move larger gravel and cobbles, and slower currents deposit fine silts and clays. As the water slows down, this material settles out, often creating distinct layers based on the flow rate at the time of deposition.
Wind serves as another significant agent, creating aeolian deposits like loess, which are typically composed of fine silt and clay particles. These wind-blown soils can be carried vast distances before settling when the wind energy dissipates. Glaciers are also powerful movers, transporting and depositing unsorted mixtures of material, known as glacial till, which can range from massive boulders to pulverized rock flour.
Finally, gravity acts as a transporting force, especially on steep slopes, leading to the formation of colluvial deposits. Processes like landslides, debris flows, and slow soil creep move material downslope, where it accumulates at the base of hills. The specific mechanism of transport—water, wind, ice, or gravity—imparts unique physical properties to the soil upon deposition.
Distinguishing Characteristics of Soil Structure and Composition
The most apparent difference between the two soil types lies in their internal structure and particle characteristics. Residual soils are typically poorly sorted, meaning they contain a wide mixture of particle sizes, from fine clay to coarse rock fragments, all mixed together. This poor sorting occurs because the particles remain where they were created, without the natural sifting that happens during movement.
Transported soils, especially those moved by water and wind, are often well-sorted, exhibiting particles that are relatively uniform in size. The sustained energy of the transporting medium acts as a sifter, separating fine particles from coarse ones before depositing them in different locations. This sorting process can result in layers that are composed almost entirely of sand or entirely of silt.
The shape of the individual soil particles also provides a strong clue to the soil’s origin. Particles in residual soil remain angular and irregularly shaped, as they have only undergone initial weathering without significant abrasion. In contrast, particles found in transported soil are frequently more rounded due to the continuous friction and tumbling they experienced during their journey by wind or water.
Furthermore, the layering, or horizon development, is fundamentally different between the two types. While residual soils exhibit the gradual, continuous horizons linked to the underlying bedrock, transported soils often lack clear, continuous horizons or feature distinct layers from different source materials. A soil profile showing an abrupt change in composition or containing material distinct from the local geology is a strong indicator of a deposition event.