What are alluvial soils? The characteristics and classification of these soils will be given by us in this article. The name of the soil comes from the Latin word alluvio, which means alluvium, alluvium. This etymology explains the origin of soils. They are created by the alluvium of rivers, that is, they are composed of particles of rocks that the rivers carry from the upper to the lower reaches and leave on their banks during floods. This material is called alluvium. It is very fertile, because the rivers deposit not only minerals, but also the biological remains of plants and animals. The classification of alluvial soils is branched. After all, rivers have their own hydrological regime. The kind of soil they form depends on the area in which they flow, how often they overflow, and similar other factors. Let's look at these types of soil in turn.
What are floodplains and terraces
Each water artery withover the course of centuries, slowly but steadily changes the relief of the adjacent land. And the larger the river, the more intense this process is. She washes away the coast. From this, the channel becomes wider. But in addition to coastal erosion, there is also a deep process. The river crashes into the bottom of its bed. This process can be compared to the application of a cut wound. The deeper the knife digs, the wider the edges of the skin diverge. But this comparison is very conditional. If you look at the river and its banks in a horizontal section, you can distinguish the channel, floodplain and terraces. With the first, everything is clear - this is the place where the water flows. There, silt and other deposits accumulate at the bottom. A floodplain is a section of a river valley that is flooded during floods. And every time the flow leaves deposits on it. As a result of this accumulative process, alluvial soils are formed. The terraces were also once a floodplain. But the river washed away the banks, and they parted, forming smooth slopes. Terraces and floodplains are not found in all rivers. For example, in canyons, water flows through hard rocks and cannot wash them away.
Characterization of alluvial soils
This type of soil occupies only three percent of the land. 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 Tzu and Huang He, Tigris and Euphrates. These waterways gave people fertile soil on which to grow a rich harvest even withprimitive degree of tillage. Even in modern Egypt, all the country's agriculture is concentrated only along the banks of the Nile. In the floodplain, on alluvial soils, water meadows are located, which are the best pastures, and mowing provides livestock with fodder for the winter. Viticulture develops on the river terraces. With the help of land reclamation, rice cultivation is practiced in forest areas. Floodplains are of great importance in fisheries. Indeed, during floods, spawning takes place there and young animals are bred.
Classification of alluvial soils
A characteristic feature of these soils is that they grow rapidly upwards. This is especially true for floodplain areas. Some rivers flood in early spring when the snow melts, others in winter (in a Mediterranean climate), and others in summer during monsoon rains. But the hydrological regime provides for the annual highest and lowest (low) levels of flow. Where the river leaves its deposits during the flood, the most intensive accumulative process takes place. But the alluvial soils of the floodplains are heterogeneous in composition. When the flood comes, the flow of the river is very fast near the channel. Therefore, large particles are deposited in the coastal part - pebbles, sand. When the water leaves, beaches and ramparts form in this place. A little further from the riverbed, the current is slower. Small particles settle there - silt, clay. There are sections of the floodplain that are not flooded every year, but only during strong floods. Such soils are layered. And finally, on the terraces there are sod, forest and meadow soils,folded with the addition of alluvium.
Dobrovolsky classification
A well-known academician of the Russian Academy of Sciences identifies such main types of soils formed by the activity of rivers. G. V. Dobrovolsky distinguishes near-channel soils composed of alluvium and sod. A little further from the river, in the central floodplain, which near lowland rivers can reach a width of several kilometers, meadow soils are located. The swampy alluvial soils located at the foot of the lower terrace contain a lot of humus and gley. But the classification of Academician Dobrovolsky is applicable only to the rivers of Russia, which flow in a flat land with a temperate continental climate. In other natural areas, the process of waterlogging of terraced areas may not take place.
Influence of climate and groundwater
The river plays a fundamental role in the formation of alluvial soils. After all, it is her sediments that settle on the banks in the floodplain. But also alluvial soils are influenced by the climate, primarily by the amount of precipitation. Soils in wet areas are acidic. As rainfall decreases, soils become more neutral. In arid areas, alkaline soils form. Groundwater also has an impact on soils. True, it's not permanent. During the period of low water and drought, groundwater goes deep into the earth. But in the rainy season and in the flood, they make themselves felt. The aquifer can lead to waterlogging of soils, giving them one or another mineralization. This is especially intense in the central and terraced parts of the floodplain.
Soils from the source to the mouth of the river
Usually water flows are born in the mountains. A small stream does not yet have the strength to wash away its banks. Yes, and it flows among solid rocks. But water already erodes s alts, carries silica and organic substances, manganese and iron oxides, gypsum and chalk, sodium chloride and sulfate. In the upper reaches of mountain rivers, the alluvium is rough, composed of pebbles and coarse sand. The water flows of the flat part of Russia have a different hydrography. They are born in swamps. Therefore, floodplain-alluvial soils, even in the upper reaches of the rivers, carry a significant part of the humus. In the middle reaches, the flat streams meander and often change their channels. The river slows down, causing the water in it to stagnate, mineralize, and in a humid climate it also oxidizes. This most directly affects the formation of alluvial soils. The deltas of such water giants as the Volga, Yenisei, Don are very branched, divided into sleeves. In the lower reaches, the alluvial process is most intensive. Humus, clay, CaC03, s alts, compounds of potassium, sodium, manganese, iron are deposited there.
Alluvial soddy soils
These soils are located in close proximity to the river, on its gently sloping banks. They are characterized by a very small amount of humus in the composition. And although these parts of the floodplain are flooded every year, the river deposits here only coarse alluvium - coarse sand, pebbles. During floods, ridges are formed, which are then eroded by atmosphericprecipitation. There is little gleying in alluvial soddy soils, and their composition is mechanical. The top layer is a loose turf of a small thickness. Below lies a thin humus horizon. Its width, depending on coastal vegetation, can reach from three to twenty centimeters. Even lower are deposits of light mechanical composition. Such soils poor in humus are not of interest for agriculture.
What are alluvial layered soils
A little further from the riverbed, behind the coastal ramparts, there are areas that are flooded not every year, but only during strong floods (in Russia - after especially snowy winters). Thus, deposits of a 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 soddy soils, is more interesting for agriculture. On such leveled floodplain areas, farmers graze their livestock or use them for hayfields. In profile, alluvial layered soils have a layer of humus thirty to forty centimeters thick. This allows the development of lush meadow vegetation and shrubs. Sod is also present in the profile, but this layer is thin - about five centimeters. Below is gleyed layered alluvium. The mechanical composition of such soil is heavier.
Alluvial meadow soils
They occupy mainly the central flat parts of the floodplains. These soils are composed of loamy or sandy loamy weakly bedded deposits.rivers. Shallow groundwater, even during a period of drought, nourishes the lush grassy vegetation. Thus, a powerful upper layer of finely humus fine-grained piling is formed in the profile. The aquifer, which usually lies at a depth of less than a meter, feeds the meadow vegetation by capillary action. Gleying is observed in the lower part of the soil profile. There is three percent more humus in alluvial-meadow soils than in layered ones. If the groundwater is too mineralized, solodized or solonetsous subtypes of soils develop in such areas of the floodplain. Vegetation has a significant effect on soil formation. Trees and bushes form a podzolized subtype of alluvial-meadow soils.
Marsh soils
In drainless depressions of the relief, which are usually observed in the terraced zone of the river valley, in a humid climate, the process of moisture stagnation is observed. In addition, the aquifer comes out of the slopes to the surface of the floodplain. All these factors (groundwater, humid climate, relief depression) lead to the development of alluvial marsh soils in such areas. They are characterized by heavy mechanical composition, high content of peat, and gleying. Swamp vegetation, sometimes willows, develops on such soil. Gleying processes here occur together with 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.
Terrace soils
It should not be forgotten that the high banks of the rivers are also composed of alluvial deposits. Onlythey are older than the soils of the floodplain itself. Over centuries and even millennia, a thick layer of other soils has formed on the terraces - forest podzolic, meadow, chernozem. But under this layer all the same alluvial soils lie.
