- Equatorial forest lands
We associate our planet's tropical forests with lush, multi-species vegetation. At first glance, this might suggest the soils on which the jungle grows are highly fertile. However, this is a false conclusion, as over 65% of the world's rainforests and 75% of the Amazon rainforest grow on soils that are barely more fertile than desert sand.
Amazonian soils are very old and weathered. There is no volcanic activity in the Amazon basin that could provide new nutrients. However, heavy rainfall, although it does provide nutrients, promotes soil erosion and washes away nutrients such as phosphorus, potassium, calcium, and magnesium. Amazonian forests grow primarily on soils rich in iron and aluminum oxides. These are clayey, lateritic soils with an acidic pH and a color ranging from yellow to dark red, even brown. The high aluminum oxide content and low pH make it unsuitable for growing many crops due to its toxicity. Plants that are resistant to high aluminum content in the soil, such as tea, coffee, and sugarcane, can be grown.
2. Types of Amazonian soils
The Amazon River basin has 14 soil types, with 9 types covering over 95% of the area. The two main soil types occupying the largest areas are Ferralsol (Ferrality) and Acrysol. They constitute over 60% of the Amazonian subsoil:
Ferralsole
Acidic soils, which contain no more than ten percent erosion-prone minerals at all depths and have low cation exchange. They have the ability to strongly bind phosphorus to iron and aluminum oxides. They are always red or yellowish in color due to the high concentration of iron (III) and aluminum oxides and hydroxides. They also contain quartz and kaolin, as well as small amounts of other clay minerals and organic matter; they were formerly classified as laterites.
Acrysol
They contain iron, aluminum, and titanium oxides, giving them a yellow to reddish-brown color. However, unlike Ferralsol, the SiO2/Al2O3 is 2 or lower. The clay fraction also contains well-crystallized kaolinite and some gibbsite. Acrysols are acidic, contain high levels of toxic aluminum, and also have a high phosphate-binding capacity.
Phlintosols
They form under various climatic and topographic conditions. They contain a mixture of clay minerals rich in iron and aluminum. They may contain iron oxides and hydroxyls, such as lepidocrocite, goethite, and hematite, as well as aluminum oxides, such as gibbsite and boehmite. They are acidic in nature, ranging in color from yellow to brown. The iron content in these soils can reach 80%, and the aluminum content up to 40%. They are acidic, have a low cation exchange capacity, and strongly bind phosphorus.
Gleysols
Soils in which the gley process, induced by significant moisture, plays a dominant role. This manifests itself in areas of very high moisture levels by the appearance of bluish-bluish gley discolorations. The gley process involves the biochemical reduction of elements—primarily iron (FeIII → FeII) and manganese (MnIV → MnII)—in conditions of limited oxygen access. Gleysols contain more organic matter, phosphorus, and potassium compounds, and have a much greater cation exchange capacity than the previously mentioned substrates.
Cambisole
They form in medium- to fine-grained materials derived from a wide range of rocks, primarily alluvial, colluvial , and aeolian. Most of these soils provide good agricultural soils. Cambisols are less common in the tropics and subtropics, but are common in areas of active erosion, where they can occur in association with mature tropical soils. Their pH is 5-6 and their organic matter content is up to about 5%.
Leptosols
Very shallow soils on hard substrates, such as rocks with a pebbly-gravel structure. They can be found from the tropics to the cold polar regions and from sea level to the highest peaks. Leptosols are particularly common in mountainous areas. Leptosols can be found on hard rocks or where erosion has kept pace with soil formation or removed the topsoil. They are completely incapable of retaining water.
Arenosols
These soils have a sandy structure and lack significant soil profile development. They exhibit only a partially formed horizon (uppermost layer) with low humus content. They are highly permeable and very low in nutrients.
Fluvisol
Fluvisols occur in areas periodically flooded by surface water or rising groundwater, such as floodplains, river deltas, and coastal lowlands. These soils have a layered profile reflecting their depositional history or irregular layering of humus and mineral sediments, in which the organic carbon content decreases with depth. Significant variation in texture and mineral composition is observed.
Regosol
They occur in erodible areas, especially in arid and semi-arid areas and in mountainous regions.
Luvisole
Clay minerals with high activity and high saturation of the sorption complex with bases. These minerals have not undergone excessive weathering, and therefore these soils have a high cation exchange capacity.
Podosole
Podzols can occur on almost any parent material, but generally originate from quartz-rich sands and sandstones or sedimentary debris from igneous rocks, provided there is high rainfall. Most podzols are poor soils due to their sandy component, resulting in low moisture and nutrient levels. Some are sandy and overly drained. Others have shallow root zones and poor drainage due to cementation of the subsoil. Low pH causes further problems with phosphate deficiencies and aluminum toxicity.
Alisols
Alisols are highly acidic, poorly permeable soils susceptible to aluminum toxicity and water erosion. Alisols are characterized by the presence of a dense subsurface layer of accumulated clays of mixed mineralogy (primarily kaolin) containing significant amounts of dissolved aluminum ions.
Histosols
This is a soil composed primarily of organic materials. Organic soil material has an organic carbon content (by weight) of 12 to 18 percent or more, depending on the clay content of the soil. Typically, histosols have very low bulk density and are poorly drained because organic matter holds water very well. Most are acidic, and many are deficient in key plant nutrients, which are leached in constantly moist soil.
Nitysols
These fine-textured soils are weathered from parent rock, kaolinite, and iron oxides. They are characterized by a high ion exchange capacity compared to other Amazonian soils, a pH of 5-6.5, and low phosphorus content.
Sources:
– https://rainforests.mongabay.com,
– wikipedia.org,
– CA Quesada, J. Lloyd1, LO Anderson, NM Fyllas, M. Schwarz, and CI Czimczik, “Soils of Amazonia with particular reference to the RAINFOR sites” Biogeosciences, 8, 1415–1440, 2011,
– WG Sombroek “Amazon Soils” PUDOC, 1966.
– IUSS Working Group WRB: World Reference Base for Soil Resources 2014, Update 2015. World Soil Resources Reports 106, FAO, Rome 2015