VERTISOL: CHARACTERISTICS AND USES - ENVIRONMENT - 2024

Vertisol is a group of soils in the classification of the World Reference Base for Soil Resources. They are characterized by being soils with a high content of expansive clays in a profile of little differentiated horizons.

Its name derives from the Latin vertere, that is, to invert or mix, referring to the aspect of beaten or mixed soil that it presents.

Profile of a vertisol. Source: Retallack / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

They form in tropical or temperate climates that have a prolonged hot dry season and a rainy season, in flat or slightly undulating areas. They are common in ancient lake beds, river banks, and periodically flooded areas.

The expansive clays they contain are characterized by contracting in the dry season, forming wide and deep cracks. While when the rains enter, these clays expand, closing said cracks and therefore affect the profile and physical properties of the soil.

As the cracks form, they fill with external sediment and then as they close, the pressure raises the interior material. This process causes the constant mixing of the soil horizons and therefore they are not clearly stabilized.

Additionally, the presence of these clays gives the soil poor drainage, which is why water accumulates in the rainy season. Likewise, this clayey character makes it difficult to work, because the soil adheres to work implements such as plows and harrows.

However, with proper agronomic management, they are very productive soils, since in general they have good fertility. They produce corn, sorghum, rice, cotton, sugar cane and other crops, in addition to supporting pastures for animal production.

Characteristics of vertisols

Vertisols are dark colored mineral soils with a high content of expansive clays that form deep and wide vertical cracks in the dry season. Its main component is smectitic clays, which have a laminar structure.

Parenting material and training

The parent material from which the soils of the vertisol group are formed are sedimentary rocks type marl, with a high content of calcite and clays. They are also formed from volcanic igneous rocks such as basalt, materials that formed in abundance in the Tertiary era.

Marl rocks. Source: Wilson44691 / Public domain

For their formation they require water that produces the weathering of the rock in conditions that prevent the loss of bases by washing. On the other hand, the crystallization of expansive clays has as a condition the existence of a long dry period, with high temperatures and basic pH.

Profile

They present a vertical clayey horizon in the first 100 cm of soil, formed by expansive clays. The deep and wide cracks that are formed in these soils when drying, due to the expansive clays, play an important role in their profile.

The cracks are filled with external materials and then, in the rainy season, the clays contract, closing the cracks. This pushes material from the lower parts of the profile upwards, causing constant mixing over time.

Due to this, there is no clear differentiation of soil profiles, although two horizons can be distinguished, even an incipient third.

The types of vertisols are defined based on the presence of variable diagnostic horizons that can be seen between 50 and 100 cm in depth. Among these, there may be a sulphic horizon (content of sulfur derivatives) of acid pH, in this case it is called thionic vertisol.

Likewise, the surface horizon can be salic, with 1% or more salts, or even natric with a sodium content greater than 15%. There are also calcium vertisols (high calcium content), yésicos (gypsum) or the so-called duric with nodules or silicon aggregates.

These latter horizons can form hardened layers in the middle or deep levels of the soil. Additionally, there are another series of subtypes depending on various aggregates that are likely to be present in vertisols.

Environments and regions where they develop

This type of soil develops in seasonal climates with defined rainy and dry periods, covering about 335 million hectares worldwide. They occur both in arid tropical climates, as well as in sub-humid and Mediterranean climates.

They can be found in areas with rainfall of only 500 mm per year and in others of up to 3,000 mm per year. They are especially abundant in America, Australia, India and northeast Africa (Ethiopia and Sudan).

They are generally found in low areas, but in any case in flat areas or with little slope. They are common in lake beds, river banks and periodically flooded areas and the vegetation that develops naturally is grasslands (savannas, grasslands) and forests or jungles.

Applications

They are generally highly fertile soils, but the expansive clay content makes them difficult to work. However, properly worked through the use of irrigation, they are very productive soils for various crops.

With irrigation, cereals and other crops such as cotton and sugar cane grow adequately on these soils. For example, in Mexico vertisols occupy 8.3% of the national territory and are considered the most productive soils.

Limitations and handling

Vertisols have physical limitations that affect their management for agricultural production, especially in relation to mechanization and drainage. In the first case, the sticky consistency conferred by the clays when wet makes the use of machinery difficult.

Therefore, when they dry, they are extremely hard soils and improper mechanization deteriorates their structure. On the other hand, its clayey texture gives it poor drainage, so there are problems of excess water accumulated in the rainy season.

An additional limitation in some vertisols is the formation of a hardened deep horizon (calcium or duric vertisols) that hinders both drainage and root development. In these cases it may be necessary to use a deep plow to break up said hard layer or the use of subsoilers.

The subsoiler is a type of plow that allows working deeper and breaking the aforementioned hard layers.

Corn crop

Corn sowing. Source: Guanape (Venezuela) / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Approximately half of these soils worldwide are dedicated to corn cultivation, thus demonstrating the productivity of well-managed vertisols.

Grasslands

Another important use of vertisols is as a grassland support for extensive cattle (cows), sheep (sheep) and goats (goats). These soils under natural conditions support natural grasslands that can be useful as pastures and it is possible to establish cultivated grasslands.

Forests

Under natural conditions, vertisols can also support forests of varying complexity, including even high-canopy semi-deciduous forests. With an adequate forest management plan, these forests can be used to obtain various resources, such as wood, honey, fibers and others.

Building

Due to the expandable character of their clays, these soils present disadvantages to build on them. This is because during the dry period the shrinkage of the clays causes the material to separate from the structure.

In any case, this characteristic must be taken into account, which requires establishing deep foundations or foundations.

References

1. Coulombe, CE, Wilding, LP, and Dixon, JB (2000). Vertisols. pp. 269-286. In: ME Sumner (ed.). Handbook of Soil Science. CRC Press. New York, NY, USA.
2. Driessen, P. (Edit). (2001). Lecture notes on the major soils of the world. FAO.
3. FAO-Unesco. The FAO-Unesco Soil Classification System. The World Reference Base for soil resources. (Seen on April 11, 2020). Taken from: fao.org
4. Gutiérrez-Rodríguez, F., González Huerta, A., Pérez-López, DJ, Franco-Mora, O., Morales-Rosales, EJ, Saldívar-Iglesias, P. Y Martínez-Rueda, CG (2012). Compaction induced by running-in agricultural tractors in a Vertisol. Terra Latinoamericana.
5. ISRIC (International Soil Reference and Information Center). 2020. Vertisols. Available at: isric.org
6. Torres-Guerrero, CA, Gutiérrez-Castorena, MC, Ortiz-Solorio, CA and Gutiérrez-Castorena, EV (2016). Agronomic management of Vertisols in Mexico: a review. Terra Latinoamericana.