Ferralsol

Yellowish-red colored Ferralsol.

The Ferralsol is a reference soil group of the international soil classification World Reference Base for Soil Resources (WRB), which is assigned to the climatic region of the always humid tropics . The term is an artificial word made up of the words Ferrum ( iron ), Alumen ( aluminum ) and Solum ( soil ).

Soils of this type are deeply weathered, acidic and usually intensely yellow, orange or red in color. They are characterized by the dominance of kaolinite in the clay fraction and the accumulation of iron and aluminum ( oxides ). Furthermore, they show extreme nutrient depletion , which poses great challenges for agricultural use.

In the German soil systematics, there is no soil type that adequately corresponds to the Ferralsolen, as they are not widely used in Central Europe . An outdated name is laterite . In the US classification ( Soil Taxonomy ) they are called Oxisol .

distribution

Soil regions in which ferralsoles occur.

For the formation of the deeply and intensely weathered ferralsole, two criteria must be met: On the one hand, the soils must be subject to strong and continuous weathering , and on the other hand, this weathering must have taken place over an extremely long period of time. Therefore, the distribution of ferralsole is closely tied to certain regions.

The weathering criterion is only fulfilled in the always humid tropics , i.e. in the areas of the tropical rainforest and in parts of the adjacent humid savannas . Because of this connection to a certain climatic zone, ferralsoles belong to the zonal soils .

Due to the time criterion, however, they do not occur uniformly in every humid tropical region of the world. Their main distribution areas are rather the old continental shields of the wet tropics, which are located in South America (especially Brazil ) and Africa (especially Central Africa , Madagascar and West Africa ). Ferralsoles are the absolutely dominant soils on these surfaces, which have been undisturbed for millions of years. Worldwide they occupy about 750 million hectares .

In contrast, ferrasols are hardly widespread in tropical Asia , because there is repeated surface renewal due to mountain folds ( Himalayan foothills in Indochina ) and volcanism ( Indonesia , Malaysia, etc.).

Outside the humid tropics, they can only be found as small-scale relics of past climatic conditions. In Germany there are small relic deposits from the Tertiary , which are referred to here as ferrallite or fersiallite .

Emergence

If a surface in the humid tropics weather undisturbed over a period of several million years, soils several tens of meters deep are formed. (In Germany, soils are rarely more than 2 m deep). The soil is very acidic and due to the protracted leaching it is almost free of nutrients. Almost only sparingly soluble substances such as quartz and sesquioxide (iron and aluminum (hydr) oxides, in smaller quantities also manganese and titanium (hydr) oxides) remain as constituents.

The strong chemical weathering is due to the continuous water supply and the constantly high temperatures of the humid tropics. The primary minerals originally present in the soil disintegrate , so that sesquioxides, silica (H ₄ SiO ₄ ) and "bases" ( nutrients such as K ⁺ , Ca ²⁺ or Mg ²⁺ ) are released. Only the weather-resistant quartz remain.

When silica reacts with aluminum hydroxide ( gibbsite , γ-Al (OH) ₃ ), more stable secondary minerals can form. In Ferralsolen this is primarily the kaolinite . However, these are very poor in nutrients and can hardly retain nutrients.

After the minerals have been destroyed, the bases are largely subject to leaching and leave the site. Only very small amounts can be retained by the soil. Over time, the silica is largely dissolved and washed out ( desilification ). Leaving only kaolinite and quartz and the most stubborn iron and aluminum (hydr) oxides ( Ferrallitisierung ). Manganese and titanium compounds are rarer and only of regional importance. At the end of the soil development , the accumulation of weathering residues ( residual accumulation ) results in a state that almost has the character of an ore .

properties

Soil physics

The physical soil properties of the ferralsole can be classified as moderate to advantageous.

The clay content is because of the weathering of unstable minerals very low (below 10% by mass, usually close to 0% by weight). The clay fraction is almost exclusively dominated by low-quality clay minerals (especially kaolinite).

Rather coarse textures ranging from coarse silt (PDO) to fine sand (fS) are typical . The actual texture is only of secondary importance, because a stable aggregate structure , which is created by combining the kaolinite with the sesquioxides, ensures the formation of " pseudosand ". Although the actual soil type is not necessarily sandy, the soil components form stable grains that behave like such. As a result, the soil has excellent water conductivity, thermal conductivity, workability, root penetration and ventilation. The risk of erosion is also very low, as the aggregates do not silt up and precipitation seeps away very quickly . However, just like real sand, pseudosand can hardly store any water , so that the plants can quickly become water-stressed during dry periods.

Soil chemistry

Ferralsoles have high aluminum and iron contents and are depleted in nutrients and silica. The soil chemical properties must generally be classified as disadvantageous.

Because the soils are enriched with large amounts of aluminum and iron oxides, they have a low pH . As a result, the aluminum (Al ³⁺ ), which is toxic to plants, can dissolve in places and damage the vegetation ( aluminum toxicity ). This makes the soil downright toxic for cultures that are sensitive to aluminum. Manganese toxicity can also occur.

As in Germany, the iron compounds play a key role in the color of the soil. Depending on the humidity, they form yellow ( goethite , α-FeO ​​(OH)) to red ( hematite , Fe ₂ O ₃ ) compounds, which give the ferralsoles their intensely bright colors ( rubefication ).

In addition, the high aluminum and iron contents lead to a strong fixation of phosphorus (P fixation). Since the element phosphorus is a main nutrient for plants, P fixation can lead to phosphorus deficiency and yield depressions despite intensive fertilization .

The nutrient retention capacity of Ferralsole behaves completely differently than with us. In Central European soils, positively charged nutrients ( above all K ⁺ , Ca ²⁺ , Mg ²⁺ and NH ₄ ⁺ ) are stored because the soils have a more or less high cation exchange capacity (CEC). Negatively charged nutrients such as sulfate (SO ₄ ²⁻ ) or nitrate (NO ₃ ^- ), on the other hand, can hardly be held because the anion exchange capacity (AAC) is almost missing here.

In Ferralsolen, the nutrient retention capacity - and thus the ability to supply plants with them - is generally extremely low. Because of the intensive weathering, ferralsoles have almost no high-quality clay minerals, which are the basis for the KAK. The two-layer clay mineral kaolinite still occurring there has hardly any exchange capabilities. Since all weatherable primary minerals have already disappeared, there is no need for nutrient replenishment from the soil. The soils therefore contain few nutrients by themselves and are not able to retain them, for example after fertilization measures. Almost all nutrients can be irretrievably washed out after just one or two downpours. The effective cation exchange capacity of ferralsoles often tends to zero and never exceeds the (very low) value of 12 cmol ₊ / kg clay.

However, the acidic pH values ​​of the soil can certainly result in positive charges, so that a sometimes high AAC is built up. Ferralsoles therefore rarely have problems with nitrate leaching , which is ubiquitous in Germany.

A special feature of Ferralsole is the property called geric , that the pH value in the calcium chloride extract is higher than in distilled water . In Central European soils, this is always the opposite.

Soil biology

Because of the high temperatures and the always moist conditions, the soil life in Ferralsolen is very intense.

In the topsoil there is a relatively high humus content of at least 1.4 mass%. The conversion of all organic waste takes place so rapidly that litter is completely converted within a few days to weeks, so that even with a high volume of litter there is almost no organic deposit. In the subsoil there is low humus content and hardly any nutrients. The horizon border is diffuse because of the lively soil life ( termites , millipedes , ants, etc.).

The material cycle of the rainforest is very tight and fast. All released nutrients are immediately absorbed or washed out by the vegetation. Much of the nutrients are also stored in the plants and not in the soil. If an area is cleared, the nutrients will also disappear in a very short time.

Leveling

According to the WRB, a Ferralsol is present if

• the first 13 reference soil groups do not apply and
• no argic horizon ( clay shift ) is present (unless there are geric properties or at least 1.4 mass% organic carbon)
• no vitric or andic properties are present and
• the bottom has a ferralic horizon starting within 150 cm.

The ferralic horizon has the following essential criteria:

• the soil type is sandy loam or finer and
• the KAK is low with values ​​below 16 cmol _c / kg clay (potential) and below 12 cmol _c / kg clay (effective) and
• the content of water-soluble clay is less than 10 mass% (higher values ​​must have geric properties or at least 1.4 mass% organic carbon) and
• the proportion of primary minerals that are unstable in the humid tropics is less than 10% (particle number) in the fraction 0.05 to 0.2 mm and
• the thickness is at least 30 cm (often many meters).

use

A tropical rainforest with a closed nutrient cycle grows on Ferralsolen under natural conditions. As soon as the forest is cleared for agriculture , there is an interruption in the cycle and major problems, especially with the nutrient balance.

Without constant and intensive fertilization (high input), long-term use with crops is impossible, as the soil is immediately leached. The traditional, and optimal use is made via the shifting cultivation ( shifting cultivation ). After only one cultivation period, fallow phases of 10–30 years must be planned in order to ensure that the areas can recover. This makes shifting cultivation a very land-intensive use. Due to the population development, the farmers in many affected states can no longer keep these times. One consequence is a significant drop in yields.

Intensive use requires intensive liming and fertilization several times a year. A constant supply of mulch also helps to maintain fertility. However, as a rule, smallholders cannot finance this type of management. In addition, there is an enormous leaching of nutrients.

On the other hand, sustainable agroforestry can be operated without great financial expense , in which cultivated plants are integrated into the natural system of the forest without complete clearing. This form of management is z. B. traditionally operated in parts of Papua New Guinea .

The closest to agroforestry are long-term plantations which, even if they can influence nature to a large extent, represent a management system that is somewhat adapted to Ferralsole. Above all, the permanent covering of the ground is a great advantage of this system.

Web links

• Profile photos (with classification) WRB homepage
• Profile photos (with classification) IUSS World of Soils

literature

• IUSS Working Group WRB: World Reference Base for Soil Resources 2014, Update 2015. World Soil Resources Reports 106, FAO, Rome 2015. ISBN 978-92-5-108369-7 ( PDF 2.3 MB).
• W. Zech, P. Schad, G. Hintermaier-Erhard: Soils of the world. 2nd Edition. Springer Spectrum, Heidelberg 2014. ISBN 978-3-642-36574-4 .
• W. Amelung, H.-P. Blume , H. Fleige, R. Horn, E. Kandeler , I. Kögel-Knabner , R. Kretschmar, K. Stahr , B.-M. Wilke: Scheffer / Schachtschabel textbook of soil science. 17th edition. Heidelberg 2018. ISBN 978-3-662-55870-6 .
• WEH Blum , P. Schad, S. Nortcliff: Essentials of Soil Science. Soil formation, functions, use and classification (World Reference Base, WRB). Borntraeger Science Publishers, Stuttgart 2018. ISBN 978-3-443-01090-4 .
• B. Eitel, D. Faust: Soil geography. 4th edition. Westermann Schulbuchverlag, Braunschweig 2013. ISBN 978-3-14-160368-2 .