What is a soil horizon: what is the difference, names and layout

Soil consists of different vertical layers called horizons. They form the soil profile. Consider what soil horizons are, what they are, in what sequence they are located in the profile, how they are interconnected, what thickness they have. What is a soil index, its meaning, classification of horizons.

What is a soil horizon

Horizons are called soil layers, which are formed as a result of the influence of soil-forming processes. They are homogeneous, but differ in morphological features, properties and composition.However, a profile is a combination of interconnected layers that act on each other. They are located vertically, alternately, the successive change of horizons and their types are typical for different types of soils.

The structure of a typical profile is as follows: the upper fertile layer, followed by a transitional layer, which adjoins the parent rock. But in reality, the profile may consist of more horizons or their subtypes.

What are they?

The succession of layers in undisturbed soils and the usually clear boundaries make it possible to visually see their structure. The relationship between the profile structure allows you to bring the structure of typical soils to a single formula, no matter what geographical zone they are in.

In soil science, various genetic horizons are distinguished in soils of different types and denoted by certain symbols. They are arranged in parallel from the top to the deepest and each go deep into a certain number of centimeters.Consider the main genetic horizons that are currently being distinguished.

A0

The first top layer is represented by a litter of the remains of leaves, small pieces of bark, twigs, herbaceous vegetation. The organic matter is in an incompletely decomposed state. The litter is loose, up to 20 centimeters thick. It partially contains mineral substances that are not associated with organic matter, but mechanically mixed.

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Sod layer, which is braided with plant roots by about 50%. When you try to pull out the plants, the sod is separated in a lump together with the root system.

A1

A fertile layer containing a large amount of humus, which accumulates here when plant residues rot, therefore it is also called humus. It has a dark color, slightly lighter in the lower part.Contains 15-35% organic matter, weakly structured, saturated with water.

A2

Eluvial horizon or layer of removal of mineral elements. It is under humus. It differs from it in light color. In podzolic soils, the eluvial horizon is whitish in color, the humus layer is small or absent at all. Land in which this layer is well developed is usually not very fertile. In A2, there are usually few plant nutrients, leaving only sparingly soluble compounds that are not suitable for plant consumption.

B

From the eluvial layer, mineral elements are washed into the underlying illuvial layer. Because of this, it is called the inflow horizon. It is dense in structure, has a different color, due to the admixture of humus it can be brown-black, from the ingress of aluminum and iron compounds into it - brown.When calcium compounds are included, it acquires a white color and is found in forest-steppe and steppe soils. The content of mineral elements is much richer than the previous one.

C

The bottom layer, or parent rock, from which the soil was formed. Its particles mixed with the products of processing of organic residues, gradually forming horizons. Under it, there may be another, the deepest layer - the underlying rock.

In primitive soils, the profile consists of only 2 horizons - the upper and the parent rock, it is thin, the average thickness is 0.5 m.

Index value

The designation of horizons is in capital and small Latin letters, Arabic and Roman numerals are also used. The designation is important for determining the profile formula, the presence and location of certain layers.

When writing, letters are separated by a dash, when one layer is replaced by another, next to the designation of the main horizon, the designation of the replacing horizon is put in parentheses. In the same brackets, but through a dash, they write the layer index, the presence of which is not necessary. Transitional horizons, with signs of the upper and underlying layers, are indicated by indices written side by side.

Additional characteristics, such as alkalinity, the presence of carbonates, are written in lowercase letters after the main designation. If an additional characteristic is not always present, it is enclosed in brackets. The layered structure of the rock and horizons, if they are moved from top to bottom, is indicated by Roman numerals.

Other classifications

The thickness of the profile increases when moving from north to south while maintaining the structure. The thickness of genetic horizons, both basic and transitional, can be different.The depth of the profile of thin soils does not exceed 50 cm, medium-thick - 50-100 cm, powerful - 100-150 cm, heavy-duty - 150-200 cm and more. The thickness of the humus layer depends on the type of soil, it is most pronounced in chernozems, can reach a depth of more than 0.5 m, the least - in the northern tundra and desert.

There are 2 main types of soil horizons - automorphic and hydromorphic. The former are formed in interfluve spaces, where soil-forming rocks are washed by sediments that filter them, and where groundwater occurs relatively deep. Under the influence of washing, the movement of chemical compounds and elements occurs. Hydromorphic ones are formed in conditions of close occurrence of groundwater in floodplains and at the bottom of gullies. The formation of such soils proceeds under the influence of rain, melt and ground moisture. Ground water brings with it mineral elements that are deposited in the soil.

Borders between layers can be vertically straight, but there are also wavy, broken or blurry ones.The stony soil formula also includes clastic material that is visible on the surface or lies close to it. Stony soil can be found in outwash areas, moraine areas, where there is a close occurrence or outcrop of semi-rock or hard rocks.

If less than 5% of clastic material is considered conditionally non-stony, 5-10% - slightly stony, 10-20% - medium stony, 20-40% - strongly stony and over 40% - very strongly stony .

Soil horizons form the soil profile. They are arranged vertically, from the most fertile at the top to the parent rock at the bottom, which is completely barren. By the type, composition, location of soil layers, one can determine what properties it has, its origin and natural fertility, whether it is suitable for use in agricultural production.

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