Abstract
Large areas of Plinthosols with ferruginous materials such as plinthite and/or petroplinthite are fairly common in the Brazilian Amazon basin. This work was carried out to investigate the chemical behavior, mineralogical composition and weathering stage of four representative soil profiles with plinthite and petroplinthite, in Iranduba, AM (Central Amazon). Three well-drained soil profiles at high elevations were studied (P1, Plinthic Vetic Ferralsol; P2 and P3, Vetic Endopetric Plinthosol) and a contrasting poorly drained soil (P4 Haplic Plinthosol), located at low elevation. After profile descriptions, soil samples were collected from each horizon, air-dried, sieved (2 mm), and analyzed for particle-size distribution, pH, exchangeable cations (Al3+, Ca2+, Mg2+, K+, and Na+), as well as available P and total organic carbon (TOC) content. The minerals present in the clay and sand fractions, as well as in the ferruginous materials were identified by X-ray Diffraction (XRD). The weathering stage of these soils was assessed by means of Ki and Kr indexes, and the amounts of free and amorphous Fe and Al oxides by using dithionite citrate bicarbonate (DBC) and ammonium oxalate dissolution procedures, respectively. The results showed that all soils were extremely unfertile, with pH levels ranging between strong and moderate acidity, very low sum of bases and organic matter content, and of available P. The mineralogy of the soil profiles was very similar, mainly of the well-drained soils, with predominance of kaolinite and quartz in the clay and sand fractions, respectively. In the poorly-drained P4, 2:1 clay particles were also observed. These profiles can be considered highly developed according to the Ki index, however, the Ki value of P4 was higher, indicating that this soil was less developed than the others. In summary, these profiles with plinthite and petroplinthite can be characterized as highly developed and infertile soils and are, with exception of P4, well-drained.
Highlights
Large areas of Plinthosols with ferruginous materials such as plinthite and/or petroplinthite are fairly common in the Brazilian Amazon basin
In the Central Amazon, these soils represent around 7.63 % (Schaefer et al, 2000), while the major areas with strong presence of plinthite and/or petroplinthite are found in the upper Solimões (Embrapa, 1986; Teixeira et al, 2010)
Morphological and physical properties Based on the Brazilian Soil Classification System (Embrapa, 2006), the four soil profiles were classified as: P1, petroplinthic dystrophic Yellow Latosol (LAd); P2 and P3, concretionary Petric Plinthosol (FFc); and P4, Haplic Plinthosol (FXal), and according to the World Reference Base of Soil Resources (IUSS Working Group, 2006) as follows: P1, Plinthic Vetic Ferralsol; P2 and P3, Vetic Endopetric Plinthosol; and P4, Haplic Plinthosol
Summary
Large areas of Plinthosols with ferruginous materials such as plinthite and/or petroplinthite are fairly common in the Brazilian Amazon basin. These ferruginous constituents are used as diagnostic properties to characterize the plinthic horizon of these soils (Soil Survey Staff, 1999; Embrapa, 2006; IUSS Working Group WRB, 2006). In the Central Amazon, these soils represent around 7.63 % (Schaefer et al, 2000), while the major areas with strong presence of plinthite and/or petroplinthite are found in the upper Solimões (Embrapa, 1986; Teixeira et al, 2010). Petroplinthite is developed from plinthite by oxidation and irreversible hardening of these Fe-rich materials, after being exposed to repeated wetting and drying cycles in soils (Driessen et al, 2001; IUSS Working Group WRB, 2006). In our study area in the Central Amazon, the occurrence of such soils is quite frequent; there is still little research on their chemical and mineralogical properties
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