CLAY SOILS: CHARACTERISTICS, COMPOSITION, STRUCTURE - ENVIRONMENT - 2025

The clay soils are those whose particle composition (texture) predominantly particles smaller than a diameter of 0.002 mm (called clays). When the dominance of clays is very high, they are considered heavy soils, due to their high density.

Clays are very important in soil fertility. They retain mineral salts by forming aggregates with humus (colloidal fraction of decomposed organic matter) and are good at moisture retention.

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On the other hand, extremely clay soils represent a problem for agriculture due to their poor infiltration. The most characteristic clayey soils are those of the order Vertisols (expandable clays).

This type of soils are distributed throughout the planet. Among the species that are most cultivated in these, rice stands out. Others like pineapple and rubber also have good production.

Characteristics of clay soils

Clays properties

The large active surface and the high exchange capacity of the clays are their most relevant properties from the edaphological point of view. These properties are given by its small size, its negative electrical charge and its electrical conductivity.

Clays give the soil low permeability, high water retention and nutrient storage capacity. This makes your potential fertility high.

On the other hand, they confer poor aeration and have a low to medium susceptibility to erosion.

The physicochemical properties of clay soil depend on its mineralogical composition, particularly on the predominant type of clay. Thus, for example, allophane promotes cation exchange capacity, porosity, moisture retention, and structure.

While kaolinite has a low cation exchange capacity, low element retention and a regular structuring.

Texture

The key category for a soil to be defined as clayey is texture. This refers to the proportion of sand, silt and clay in the soil. Each of these items are categories of particle size.

If the clay particles represent 25% to 45% of the total particles present in the soil, it can be considered sandy-clay, coarse clay clay or silty clay. If the clays exceed 45% of the total composition, we are in the presence of a fine clay clay soil.

Porosity: permeability and aeration

To the extent that the clay content determines the texture and structure of the soil, it affects its porosity.

Due to their small diameter, the clay particles leave very small pores. This hinders the circulation of water and air in the soil matrix. These conditions generate soil saturation, which can lead to surface water stagnation as infiltration does not occur.

If the pores of the soil are saturated with water, the rhizosphere becomes anoxic (with a lack of oxygen). Under these conditions, most of the cultivated plants are in serious difficulties to develop.

In the presence of humus, clay expresses its positive dimension. The clay-humic complex is formed and the aggregates are larger. Therefore the pores are also larger and improves permeability and aeration

Cation exchange capacity

If the clays and organic matter do not retain the cations, these will be dragged by the water towards lower horizons (leaching) affecting the fertility of the soil. The cation exchange capacity is due to the negative electrical charges that both humus and soil clays have.

Soil pH can affect cation exchange capacity. This depends on the type of clay present in the soil.

When kaolinitic and allophane are present, the negative electrical charge varies depending on the pH. Whereas when expanding clays with a 2: 1 ratio are present, the load is constant at any pH value.

Effect on the soil microbiota

Soil microorganisms establish a close adhesion and separation relationship with the clay particles. Ion exchange processes occur on this surface and are captured or released by microorganisms.

Water tank

Due to their low permeability, clay soils are ideal as natural or artificial water reservoirs. Some aquifers are established by the presence of a clay horizon at a certain depth.

Composition

Most clays belong to the group of phyllosilicates (silicates in flaky form). There are different types depending on the number of sheets that make up its structure. Among the most abundant are muscovite, kaolinite, biotite, chlorite, vermiculite, and montmorillonite.

Other groups of clay of medium abundance are quartz oxides. Among the less frequent we find feldspars, hematite, goethite, calcite, gypsum and halite.

Cristobalite and amorphous materials are found in clay soils of pyroclastic origin (volcanic ash).

Due to the colloidal nature of its particles, clay soils retain a large amount of minerals. Clay soils have a tendency to retain iron (Fe) and to a lesser extent aluminum (Al).

Because clay soils retain a lot of moisture, oxidation processes occur. Hydrated iron oxides give these soils yellow or reddish colors

Structure

Clay-humic complex

Clays, in combination with organic matter, contribute to the stability of the soil structure. In most cases, it is the clay-humic complex that facilitates the formation of soil aggregates. In contrast, sodium makes clay unstable.

If a substrate is made exclusively of clay, it would have no structure and would not allow water infiltration. This would end up compacting and hardening.

Expandable clays

A soil with expandable clays in a seasonal tropical climate undergoes drastic structural changes depending on the humidity conditions.

In the rainy season, the clays expand and the soil tends to flood, it is soft, sticky and plastic. In the dry season the clays contract, revealing the dry, hard soil with large cracks.

Location

In the profile

In a complete soil profile, clays are mostly located in the B horizon or the accumulation or precipitation horizon. This is due to their small size, which makes them wash from the surface.

Physiography

In a settling landscape on plains with large rivers, overflows distribute the particles according to weight. In this sense, the clays, being the smallest, end up settling far from the shore in the lower areas.

Also in a landscape of mountains and valleys, the clays will tend to settle in the latter.

Geography

In geographical terms, its distribution is highly variable. Clay soils exist on all continents.

Vertisols are present in various latitudes and cover approximately 335 million hectares worldwide. A potential 150 million hectares in farmland is estimated. In the tropics they cover about 200 million hectares; a quarter of this is considered agricultural utility.

Crops

The main elements to take into account when using clay soils for agriculture are drainage and acidity.

The crop par excellence for clay soils is rice. Cotton, sugar cane, and sorghum can also be grown with proper management.

Some acid-tolerant and undemanding crops such as pineapple, rubber, or African palm, can be grown on some types of clay soils.

Within permanent crops, some fruit trees are adapted to clay soils. Among the fruit trees of temperate climate: the apple tree, the pear tree, the quince, the hazelnut and the walnut. Forest plantations are equally viable.

For grazing, Brachiaria (eg B. humidicola) and Paspalum (eg P. fasciculatum) species tolerate excess water.

References

1. Douglas JT, MJ Goss and D Hill (1980) Measurements of pore characteristics in a clay soil under plowing and direct drilling, including use of a radioactive tracer (144Ce) technique. Soil and Tillage Research, 1: 11–18.
2. Filip Z (1973) Clay minerals as a factor influencing the biochemical activity of soil microorganisms. Folia Microbiologica 18: 56–74.
3. Hassink J (1992) Effects of soil texture and structure on carbon and nitrogen mineralization in grassland soils. Biology and Fertility of Soils 14: 126–134.
4. Pinzon, A and E Amezquita (1991) Soil compaction by trampling grazing animals in the Amazon foothills of Colombia. Tropical Pastures. 13: 21-26.
5. Porta J, M López-Acevedo and C Roquero (2003) Edaphology for agriculture and the environment. 3 Ed. Ediciones Mundi Prensa, SA 917 p.