Alluvial Soils: Characteristics and Classification

What is alluvial soil? The characteristics and classification of these soils will be given by us in this article. The name of the soil is derived from the Latin word alluvio, which means "wash-in", "deposition". This etymology also explains the origin of the soil. They are created by the reclamation of rivers, that is, they are composed of particles of rocks that the rivers carry from the upper stream to the lower one and are left on their shores during the high water. Such a material is called alluvium. It is very fertile, because the rivers lay not only minerals, but also biological remains of plants and animals. Classification of alluvial soils is ramified. After all, the rivers have their own hydrological regime. From the fact, in what area they flow, how often they spill and other similar factors, depends on which soils they form. Let's look at these types of soils in turn.

What are floodplains and terraces

Each water artery with the passage of centuries slowly but steadily changes the relief of the adjacent land. And the larger the river, the more intensive this process. She washes the shore. From this the channel becomes wider. But apart from coastal erosion, the process is also deep. The river cuts into the bottom of its bed. This process can be compared with cutting a wound. The deeper the knife digs, the wider the edges of the skin. But this comparison is very conditional. If you look at the river and its banks in a horizontal section, then you can identify the channel, floodplains and terraces. With the first everything is clear - this is the place where water flows. There, silt and other deposits accumulate on the bottom. The floodplain is a section of the river valley, which is flooded during high water. And each time the flow leaves deposits on it. As a result of this accumulative process, alluvial soils are formed. The terraces were once also understandable. But the river washed the banks, and they parted, forming smooth slopes. Terraces and floodplains are not in all rivers. For example, in canyons, water flows flow among hard rocks and can not blur them.

Characteristics of alluvial soils

This type of soil occupies only three percent of the land area. But it is considered the most fertile. After all, alluvial soils are, in fact, river silt enriched with minerals. Therefore, such soils are valued in agriculture. Recall that all the first human civilizations originated and developed in the riverbeds: the Nile, Yang Zi and the Yellow River, the Tigris and the Euphrates. These waterways gave fertile soil to people , on which it was possible to grow a rich crop even with a primitive degree of cultivation of the land. Even in modern Egypt, all the country's agriculture is concentrated only along the banks of the Nile. In the floodplain on alluvial soils, flood meadows are located, which are the best pastures, and meadows provide livestock with fodder for the winter. Viticulture develops on river terraces. With the help of reclamation on forest plots, rice cultivation is practiced. Floodplains are of great importance in fisheries. After all, during the floods, spawning takes place there and the young is bred.

Classification of alluvial soils

A characteristic feature of these soils is that they grow rapidly upwards. This is especially true of the flood plains. Some rivers flood in early spring, when snow melts, others - in winter (in the Mediterranean climate), and in others in summer, during monsoon rains. But the hydrological regime provides for the yearly highest and lowest (low) levels of flow. Where the river in high water leaves its deposits, the most intensive accumulative process is taking place. But the alluvial soils of floodplains are also heterogeneous in composition. When there is a flood, the current of the river is very fast near the riverbed. Therefore, large particles are deposited in the coastal part - pebbles, sand. When the water leaves, beaches and shafts form on this place. A little further from the riverbed, the flow is slower. Small particles are deposited there - silt, clay. There are parts of the floodplain that are not flooded every year, but only in strong floods. Such soils are layered. Finally, on the terraces there are sod, forest and meadow soils, combined with the addition of alluvium.

Classification of Dobrovolsky

The well-known academician of the Russian Academy of Sciences distinguishes such basic types of soils formed by the activity of rivers. G. V. Dobrovolsky distinguishes the bedrock soils, composed of alluvium and turf. A little further from the river, in the central floodplain, which near the flat rivers can reach a width of several kilometers, there are meadow soils. In marsh alluvial soils, located at the foot of the lower terrace, there is much humus and glue. But the classification of Academician Dobrovolsky applies only to the rivers of Russia, which flow in the plains with a temperate continental climate. In other natural areas, the bogging process of priterrasnyh areas may not pass.

Impact of climate and groundwater

The river plays a fundamental role in the formation of alluvial soils. After all, it's its deposits settle on the banks of the floodplain. But also the alluvial soils are affected by the climate, primarily the amount of precipitation. In wet areas soils have acid reaction. With decreasing precipitation, soils become more neutral. In the arid regions, alkaline soils are formed. Underground waters also affect the soil. True, the impermanent. In the period of intergeneration and drought, groundwater flows deep into the earth. But in the rainy season and in high water they make themselves felt. The aquifer layer can lead to bogging of the soil, gives them some mineralization. This is particularly intense in the central and near-terrain parts of the floodplain.

Soils from the source to the mouth of the river

Usually, water flows are born in the mountains. A small streamlet does not yet have the strength to wash its banks. Yes, and it flows among solid rock. But the water is already diluting the salt, carries in it silica and organic substances, manganese and iron oxides, gypsum and chalk, chloride and sodium sulfate. In the upper course of the mountain rivers, alluvium is coarse, composed of pebbles and coarse sand. Water flows of the plains of Russia have a different hydrography. They are born in swamps. Therefore, floodplain-alluvial soils, even in the headwaters of the rivers, carry in themselves a considerable part of humus. In the middle course, flat streams meanders and often change their beds. The river slows down, causing the water to stagnate in it, becomes mineralized, and in the humid climate it also becomes oxidized. This most directly affects the formation of alluvial soils. The deltas of such water giants as the Volga, Yenisei, Don, are very ramified, are divided into sleeves. In the lower reaches, the alluvial process passes most intensively. There, humus, clay, CaCO ₃ , salts, compounds of potassium, sodium, manganese, and iron are deposited there.

Alluvial turf soils

These soils are located in close proximity to the river, on its gentle banks. They are characterized by a very insignificant amount of humus in the composition. And although these parts of the floodplain are flooded every year, the river lays here only a rough alluvium - coarse sand, pebbles. During the flood, ridges are formed, which are then blurred by atmospheric precipitation. In alluvial turf soils there is little gleying, their composition is mechanical. The top layer is a small thickness of loose turf. Below there is a thin humus horizon. Its width, depending on the coastal vegetation, can reach from three to twenty centimeters. Even lower are sediments of light mechanical composition. Such humus-poor soils are of no interest to agriculture.

What is alluvial layered soil

A little farther from the riverbed, beyond the coastal ramparts, there are areas that are flooded not every year, but only in high water (in Russia - after especially snowy winters). Thus, the deposits of the water flow of light mechanical composition (pebbles, sand) here alternate with layers of humus, which is formed from the decay of meadow vegetation. Alluvial-layered soil, unlike turf soils, is more interesting for agriculture. On such leveled areas of the floodplain, farmers graze cattle or use them for haymaking. In the profile, alluvial layered soils have a layer of humus thirty to forty centimeters thick. This allows the development of lush meadow vegetation and shrubs. The sod is also present in the profile, but this layer is thin - about five centimeters. Below there is glued laminated alluvium. The mechanical composition of such a soil is heavier.

Alluvial meadow soils

They occupy mainly the central plains of the flood plains. These soils are composed of loamy or sandy loamy deposits of the river. Shallow groundwater even during a drought feeds a riotous grassy vegetation. Thus, a powerful top layer of perfectly humified fine-grained nailing is formed in the profile. Aquifer layer, lying at a depth of usually less than a meter, capillary feeds meadow vegetation. In the lower part of the soil profile gleying is observed. Humus in alluvial-meadow soils is three percent higher than in layered soils. If groundwater is too mineralized, solodized or solonetzic soil subtypes develop in such areas of the floodplain. Vegetation has a considerable influence on soil formation. Trees and shrubs form a podzolized subtype of alluvial meadow soils.

Marsh Soils

In the drainless depressions of the relief, which are usually observed in the interterranean zone of the river valley, a moisture stagnation process is observed in a moist climate. In addition, the aquifer leaves the slopes to the surface of the floodplain. All these factors (groundwater, humid climate, depression of the relief) lead to the development of alluvial bog soils in such areas. They are characterized by heavy mechanical composition, high peat content, and gleying. Such a soil develops marsh vegetation, sometimes willow. The gleying processes occur here together with the alluvium deposits. In addition, soils increase due to the accumulation of humus. According to the type of reaction such soils can be both acidic and slightly alkaline.

Soil terraces

It should not be forgotten that the high banks of rivers are also composed of alluvial deposits. Only they are more ancient than the soils of the floodplain itself. Over the centuries and even millennia on the terraces formed a thick layer of other soils - forest podzolic, meadow, chernozem. But under this layer all the same alluvial soils lie.