ALLUVIAL SOIL: CHARACTERISTICS, GEOLOGY, PROPERTIES, USES - GEOGRAFÍA - 2025

An alluvial soil is a sedimentary soil that has been formed from material that has been transported by streams of water. Although generally alluvial soils are considered of fluvial origin, the water currents that transport the sediment can also come from the rain or from the marshes.

Sediment transport may have occurred a long time ago, forming the old alluvial soils, or be more recent, even current, originating the new alluvial soils. The former are normally found in terrain above the current flood level, while the recent ones are found in alluvial plains.

Alluvial soil profile of the Great Ouse River in the United Kingdom. Taken and edited from: Rodney Burton / Soil profile in river alluvium, Gt Ouse floodplain SE of Bedford.

Ancient alluvial soils are usually poor, while recent ones are rich in minerals and nutrients, which allows the development of agriculture in them. These soils are linked to the history of humanity, as they allowed the settlement and development of great civilizations such as the Egyptian and Mesopotamian.

Characteristics and properties of alluvial soil

The alluvial soils present a poorly structured profile, which is made up of unconsolidated materials to which the water currents transport and which accumulate a certain amount of organic matter on their surface.

They are generally soils with enough clay to make them waterproof and are dark in color. Those of recent origin are rich in nutrients.

Their particular characteristics vary greatly and depend to a great extent on the deposited material. They generally contain quite a few minerals, although the proportion and type of minerals present will vary depending on the location and source of the transported material.

These soils generally have a neutral pH, although they can be slightly acidic or slightly basic, varying in a range from 6.0 to 8.0. The content of phosphoric acid, potash and silt is high.

They form in areas of flat or slightly undulating topography and are normally subject to periodic flooding.

Its texture generally ranges from silty loam to silty clay loam.

According to their composition, they can be divided into several subtypes, such as tonics, molluscs and calcium with higher amounts of sulfur, organic matter or calcium, respectively, in the first centimeters of depth.

geology

All rocks and soils are exposed to the erosive action of winds, waters, biological agents, climate, among others. Erosion wears down and breaks down these elements, producing particles that can be carried by wind or water to form new soils.

When particulate matter is transported by water, it can travel long distances before reaching the place where it will settle. Normally the places of sedimentation of rivers, and of formation of alluvial soils include alluvial plains, river deltas, river estuaries and coastal plains.

The soils that are formed are heterogeneous in terms of the size and distribution of the particles, with the largest ones located closer to the stream, while the finest ones are further away from it, and on their surface they accumulate organic matter..

The fine sand, silt and clay particles are deposited during periodic floods and do not undergo any cementing process and are therefore not consolidated. Ancient alluvial soils originated in the same way, but may have undergone some degree of compaction over time.

These soils, contrary to what happens with recent ones, are poor since they have lost their organic matter content and are normally located at levels above current flood levels.

For its part, recent alluvial soils are constantly being formed and in each flood there may be contributions of different material, which is why a marked stratification is formed. They can also contain standing water in the subsurface part.

Applications

Agricultural

The recent alluvial soils, as already mentioned, are fertile soils due to the continuous supply of materials. In addition, as they are unconsolidated floors, they are light and easy to handle. In addition, they are located in areas where the water supply is not a limitation. All these characteristics make them ideal for agriculture.

However, most of these are also considered risky soils because they are prone to periodic flooding, so they require large infrastructures to stop these floods.

They are formed mainly in alluvial plains and in river deltas. They were the main soils to be cultivated and allowed the development of important ancient civilizations, as happened on the banks of the Tigris and Euphrates rivers, the Nile, the Ganges, among others.

Today, more than 40% of India's agricultural production, for example, comes from crops grown on these soils.

Formation of alluvial soil deltas used as tree planting sites, Newberry County, South Carolina, USA Taken and edited from: National Archives in College Park.

Despite the high productivity of many recent alluvial soils, there is a historical tendency to shift their agricultural uses to urban uses by increasing the size of the populations settled there.

Aquaculture

The alluvial soils are also of interest for the practice of aquaculture activities because their topography is generally almost flat, with only slight undulations, for which the investment in earthworks to level the land and build culture ponds is minimal.

Besides that, the retention of liquids due to the levels of clay it contains is also adequate and generally no more material needs to be added to waterproof bottom and dikes. Lastly, there is usually a nearby water source to supply the farm's requirements.

Water treatment

Alluvial soils also serve for the filtration and purification of wastewater (Riberbank filtration). The procedure consists of passing the water through the alluvial soil banks of a river or a lake. The Germans began using this process from the 1870s.

The passage of water through alluvial soils allows a physical filtration because the interstices between the soil particles prevent the passage of larger particles suspended in the water, remaining retained on the soil surface.

At the same time, a biological filtration occurs thanks to the presence of numerous microorganisms in the soil that degrade and digest both dissolved and particulate organic material and chemical nutrients.

There is also a chemical leaching when the components of the soil react with the different chemical compounds present in the water.

Lastly, the time it takes for the water to pass through the interstices in the soil also helps to inactivate some chemicals, as well as to eliminate microorganisms that could have passed through the three filtration mechanisms described.

The alluvial soils also serve to purify wastewater by percolation in artificial lagoons created for such purposes. In these cases, the lagoons require periodic maintenance to remove the layer of debris that accumulates on the ground and thus avoid anoxic conditions.

References

1. Alluvium. On Wikipedia. Recovered from en.wikipedia.org
2. 1. Generalities. Recovered from fao.org
3. C. Thomson. Porperties of the alluvial soils. Recovered from hunter.com
4. MF Ghazali, MN Adlan, MKN Shamsuddim & MH Roslan (2016). International Journal of Scientific Research in Knowledge.
5. FAO (2009). Guide for the description of soils.
6. JJ Ibañez & FJ Manríquez (2011). Fluvisols in Latin America. Recovered from madrimasd.org