Acessibilidade / Reportar erro

Sombric-like horizon and xanthization in polychrome subtropical soils from Southern Brazil: implications for soil classification

ABSTRACT

The occurrence of dark subsurface horizons rich in organic matter (OM) associated with polychrome in the B horizon (yellowish over reddish hue) is common in soils from Southern Brazil. The formation of these horizons and the combination with such morphological attributes has not been properly documented, and neither has the cause effect relationship. Four soil profiles with such sombric-like horizons with a yellowish color at the upper part of the B horizon over red subsoil were studied in Southern Brazil. Results from micromorphology, extractable sesquioxide minerals, clay mineralogy and isomorphic substitution of Fe by Al in iron minerals showed that melanization, xanthization, bioturbation, moderate shrinking/swelling and moderate ferralitization were the most evident pedogenetic processes in role. Xanthization is closely related to the sombric-like horizon formation. In the studied area the findings demonstrated that no clay and OM illuviation had taken place. Therefore, the classification of these soils was revisited, so as to take into account the processes that underlie their genesis with emphasis on xanthization, clay illuviation and soil aggregation. The results suggest that the sombric horizon may need redefinition, unless profiles can be found in which illuviation of clay and/or OM can be proven.

Argissolos; goethite; dark subsurface horizon; shiny peds; matte aggregate faces

Introduction

Sombric and sombric-like horizons are found in well-drained landscape positions with high soil moisture, on high plateaus and/or mountains in tropical and subtropical regions, and several theories exist as to their formation (Almeida et al., 2009Almeida, J.A.; Cararo, D.C.; Uberti, A.A.A. 2009. Genesis of the sombric horizon in Ultisols (Red Argisols) in southern Santa Catarina, Brazil. Revista Brasileira de Ciência do Solo 33: 405–416.; Bockheim, 2012Bockheim, J.G. 2012. Revisiting the definitions of the sombric horizon in soil taxonomy and word reference base for soil resources. Geoderma 170: 127–135.; Almeida et al., 2015Almeida, J.A.; Lunardi Neto, A.; Vidal-Torrado, P. 2015. Sombric horizon: five decades without evolution: a review. Scientia Agricola 72: 87–95.). Except for Faivre (1990)Faivre, P. 1990. The sombric horizon: an ‘incipient’ organic-argillic horizon; the example of soils of the intra-Andean region of Colombia (South America). Pédologie 40: 273–297 (in French, with abstract in English)., sombric horizons described in the literature lack evidence of OM illuviation, and are therefore frequently referred to as sombric-like horizons (De Craene and Laruelle, 1955De Craene, A.; Laruelle, J. 1955. Genesis and alteration of equatorial and tropic latosols. Bulletin Agricole du Congo Belge 46: 1113–1243 (in French, with abstract in English).; Sys et al., 1961Sys, C.; Van Wambecke, A.; Frankart, R.; Gilson, P.; Jongen, P.; Pécrot, A.; Berce, J.M.; Jamagne, M. 1961. Soil Cartography in Congo: Principles and Methods = La Cartographie des Sols au Congo, ses Principes et ses Méthodes. National Institute for Agronomy in Belgian Congo, Brussels, Belgium. (Series Science, 66) (in French).; Gouveia et al., 2002Gouveia, S.E.M.; Pessenda, L.C.R.; Aravena, R.; Boulet, R.; Scheel-Ybert, R.; Bendassoli, J.A.; Ribeiro, A.S.; Freitas, H.A. 2002. Carbon isotopes in charcoal and soils in studies of paleovegetation and climate changes during the late Pleistocene and the Holocene in the southeast and center west regions of Brazil. Global and Planetary Change 33: 95–106.; Caner et al., 2003Caner, L.; Toutain, F.; Bourgeon, G.; Herbillon, A.J. 2003. Occurrence of sombric-like subsurface A horizons in some andic soils of the Nilgiri Hills (southern India) and their palaeoecological significance. Geoderma 117: 251–265.; Almeida et al., 2009Almeida, J.A.; Cararo, D.C.; Uberti, A.A.A. 2009. Genesis of the sombric horizon in Ultisols (Red Argisols) in southern Santa Catarina, Brazil. Revista Brasileira de Ciência do Solo 33: 405–416.; Velasco-Molina et al., 2010Velasco-Molina, M.; Almeida, J.A.; Vidal-Torrado, P.; Macías, F. 2010. Chemical fractionation of carbon on Acrisols with sombric horizon from south Brazil. Revista de Ciências Agrárias 33: 277–286 (in Spanish, with abstract in English).). Recently, Chiapini et al. (2018)Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244. studied the sombric-like horizons in Southern Brazil from a site that is representative of such conditions. For this area the authors found that the sombric-like horizon is a remnant of an earlier phase of soil formation in which grass vegetation and frequent natural fires caused deep accumulation of black carbon (BC). During subsequent wetter conditions, from the Late Holocene until the present, forest vegetation replaced grass vegetation and fire incidence declined. A similar mechanism may explain the formation of sombric-like horizons in other areas, and was also proposed by Caner et al. (2003)Caner, L.; Toutain, F.; Bourgeon, G.; Herbillon, A.J. 2003. Occurrence of sombric-like subsurface A horizons in some andic soils of the Nilgiri Hills (southern India) and their palaeoecological significance. Geoderma 117: 251–265..

Taxonomic soil classification systems are based on morphological properties and recognition of key processes responsible for soil genesis (Lebedeva et al, 1999; Bockheim, 2012Bockheim, J.G. 2012. Revisiting the definitions of the sombric horizon in soil taxonomy and word reference base for soil resources. Geoderma 170: 127–135.). Our improved understanding of the formation of the sombric-like horizons from Southern Brazil (Caner et al., 2003Caner, L.; Toutain, F.; Bourgeon, G.; Herbillon, A.J. 2003. Occurrence of sombric-like subsurface A horizons in some andic soils of the Nilgiri Hills (southern India) and their palaeoecological significance. Geoderma 117: 251–265.; Chiapini et al., 2018Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244.), inhibits the correct classification of these soils, in particular, in the Brazilian Soil Classification System (Embrapa, 2018). Associated with sombric-like horizons in subtropical soils the yellowish color at the upper part of the B horizon over red subsoil can be observed. This process is called xanthization (from the Greek xanthos, yellow) (IUSS Working Group WRB, 2015IUSS Working Group WRB. 2015. World Reference Base for Soil Resources 2014: Update 2015. International Soil Classification System for Naming Soils and Creating Legends for Soil Maps. FAO, Rome, Italy. (World Soil Resources Reports, 106).). Xanthization is the conversion of hematite into goethite influenced by constantly moist climatic conditions. In subtropical soils moist conditions and the presence of organic matter inhibit hematite formation or microbial oxidation and are responsible for the preferential reductive dissolution of originally formed hematite turning red soils yellow (Cornell and Schwertmann, 2003). In the present study, the classification of these soils is revisited, taking into account the processes that underlie their formation. The relationship with the formation of the sombric-like horizon and xanthization is highlighted. With this aim in mind, new data of micromorphological, physicochemical, mineralogical and iron and aluminum dissolution analysis were interpreted.

Materials and Methods

Study site

The study area is located in Tijucas do Sul (in the state of Paraná, Brazil; 25°55’41” S, 49°11’56” W) (Figure 1). The parent material was described as a colluvium derived from migmatites with local influence of other metamorphic rocks (Santos et al., 2006Santos, L.C.; Oka-Fiori, C.; Canali, N.E.; Fiori, A.P.; Silveira, C.T.; Silva, J.M.F.; Ross, J.L.S. 2006. Geomorphological mapping of the state of Paraná. Revista Brasileira de Geomorfologia 2: 3–12 (in Portuguese, with abstract in English).). The native vegetation was classified as a mixed ombrophylous forest with grassland. The climate is temperate and humid subtropical (Behling et al., 2001Behling, H.; Bauermann, S.G.; Neves, P.C. 2001. Holocene environmental changes from the São Francisco de Paula region, southern Brazil. Journal of South American Earth Science 14: 631–639.). The soils found in the region are classified as Argissolos, Nitossolos, Cambissolos mainly with some local occurrence of Latossolos (Embrapa, 2018). The pedologic system is made up of four soil profiles with a sombric-like horizon, previously studied by Chiapini et al. (2018)Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244. to assess environmental changes. In this study we used this pedologic system to examine the main soil forming processes in the context of their classification within the three soil classification systems. These included three profiles from a toposequence (P1, P2 and P3) and a profile on the summit of a nearby hill (P4). Figure 2 illustrates a representative profile. Pedogenic horizons were described in the field. Morphological description was based on the field guide elaborated by the FAO (2006). The soil samples were collected according to pedogenic horizon (Table 1). The samples were dried and sieved and the fraction < 2 mm was used for physical (granulometry; Table 2) and chemical analysis ( pHH2O, pHKCl; Al3, H+Al; Ca2, Mg2, K+, total C; base saturation (BS), clay activity and C/N ratio; Table 3) that were presented in Chiapini et al. (2018)Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244..

Figure 1
– Location of the study area.

Figure 2
– Representative soil profile with sombric-like horizon (P1; Chiapini et al., 2018Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244.).

Table 1
– Morphology of the studied soil profiles.
Table 2
– Granulometry of the studied soil profiles.
Table 3
– Chemical properties of the studied soil profiles.

Fe and Al contents

Fe and Al contents were determined from samples of each profile. First, the OM of the soil samples was eliminated by the addition of 30 % H2O2 in a water bath. Free Fe and Al oxyhydroxides were extracted by sodium citrate-bicarbonate-dithionite (CBD) treatment (four times at 80 ºC for 30 min in a water bath) (Mehra and Jackson, 1960Mehra, J.P.; Jackson, M.L. 1960. Iron oxides removal from soils and clays by a dithionite-citrate-bicarbonate system buffered with bicarbonate sodium. Clays and Clay Minerals 7: 317–327.; Jackson, 1979Jackson, M.L. 1979. Soil Chemical Analysis: Advanced Course; A manual of methods useful for instruction and research in soil chemistry, physical chemistry of soils, soil fertility, and soil genesis. The author, Madison, WI, USA.). Amorphous Fe (Feo) and Al (Alo) oxyhydroxides were determined by extraction with 0.2 mol L1 ammonium oxalate in the dark at 3.0 pH (McKeague and Day, 1966McKeague, J.A.; Day, J.H. 1966. Dithionite and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. Canadian Journal of Soil Science 46: 13–22.). Fe and Al contents were determined by atomic absorption spectrometry. Sodium pyrophosphate (0.1 mol L1) was extracted at pH 10 according to USDA (1996) providing the Fe and Al that is bound to OM (Fep and Alp) (Table 4).

Table 4
– Iron oxides dissolution of the studied soils.

Mineralogical analysis in the clay fraction

After dispersion with 0.2 mol L1 NaOH, the sand fraction was separated from the silt + clay fractions by sieving (0.053–mm sieve opening), the soil samples (P1: B2; P2: B3; P3: B2 e P4: B3), after which the clay was separated from the silt by decantation, following Stokes’ law (Jackson, 1979Jackson, M.L. 1979. Soil Chemical Analysis: Advanced Course; A manual of methods useful for instruction and research in soil chemistry, physical chemistry of soils, soil fertility, and soil genesis. The author, Madison, WI, USA.). Clay samples were saturated with Mg2(Mg 25 ºC) and solvated with ethylene glycol (Mg + EG). In another aliquot, the samples were saturated with K and then heated to 500 ºC. Slides of oriented clay were prepared for X-ray diffraction (XRD) using a Rigaku Miniflex II device, with Cu (CuKα) radiation, a graphite monochromator, operated at 10 mA and 15 kV, at a rate of 0.02 º 2θ and a speed of 1 sec/step, in the range of 3 to 45 ° 2θ.

Furthermore, in samples from the A and B horizons from all profiles (P1: A6, B1, B2; P2: A4, B1, B3; P3: A5, AB, B2; P4: A3, B1 and B3) the Fe-oxyhydroxides in the clay fraction were concentrated with 5 mol L1NaOH, removing clay minerals and Al-hydroxides (Norrish and Taylor, 1961Norrish, K.; Taylor, M. 1961. The isomorphous replacement of iron by aluminium in soil goethites. European Journal of Soil Science 12: 294–306.). Sodium metasilicate was added to reach 0.2 mol L1 Si concentration in solution to avoid the formation of iron oxides with high Al isomorphic substitution (Kämpf and Schwertmann, 1982Kämpf, N.; Schwertmann, U. 1982. The NaOH concentration method for iron oxides in soils. Clays and Clay Minerals 30: 401–408.). Sodalite [Na4Al3Si3O12(OH)] formed during extraction was removed by washing twice with 50 mL of 0.5 mol L1 HCl solution and washing once with 50 mL of deionized water (Norrish and Taylor, 1961Norrish, K.; Taylor, M. 1961. The isomorphous replacement of iron by aluminium in soil goethites. European Journal of Soil Science 12: 294–306.; Singh and Gilkes, 1991Singh, B.; Gilkes, R.J. 1991. Concentration of iron oxides from soils clays by 5 M NaOH treatment: the complete removal of sodalite and kaolin. Clay Minerals 26: 463–472.). The mineral components of the concentrated Fe residue were identified by XRD, carried out on un-oriented powdered samples on glass slides, using a scanning range of 3 to 45 ° 2θ. An internal standard was added to the samples (5 % NaCl) for correction of distortions and mounted on glass slides (non-oriented).

The goethite/hematite ratio [Gt/(Gt + Hm)] was estimated using the main diffraction peak areas (Torrent and Cabedo, 1986Torrent, J.; Cabedo, A. 1986. Sources of iron oxides in reddish brown soil profiles from calcarenites in southern Spain. Geoderma 37: 57–66.). Fe isomorphic substitution by Al (IS) in Gt was calculated according to Schulze (1984)Schulze, D.G. 1984. The influence of aluminium on iron oxides. VIII. Unit-cell dimen- sions of Al-substituted goethites and estimation of Al from them. Clays and Clay Minerals 32: 36–44. and in Hm according to Schwertmann et al. (1979)Schwertmann, U.; Fitzpatrick, R.W.; Taylor, R.M.; Lewis, D.G. 1979. The influence of aluminium on iron oxides. Part II. Preparation and properties of Al-substituted hematites. Clays and Clay Minerals 29: 269–276.. The Gt and Hm contents were estimated based on the crystalline Fe content (FeCBD - Feo), considering the Gt/(Gt+Hm) ratio, the IS level, and the least mineral formulas (Melo et al., 2001Melo, V.F.; Fontes, M.P.F.; Novais, R.F.; Singh, B.; Schaefer, C.E.G.R. 2001. Iron and aluminum oxides of different Brazilian soils. Revista Brasileira de Ciência do Solo 25: 19–32 (in Portuguese, with abstract in English).) (Table 5).

Table 5
– Iron oxides quantification of some studied horizons.

Micromorphological analysis

Thin sections (5 × 9 cm) were obtained from Chiapini et al. (2018)Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244., and additional data were presented in relation to pedogenetic processes to assist in soil classification (Table 6 and Figures 3, 4 4 Wageningen University and Research Earth System Science Group The Netherlands Wageningen University and Research/Earth System Science Group, P.O. Box 47 – 6700 AA Wageningen – The Netherlands. and 5); these processes included clay illuviation, soil aggregation and, in particular, xanthization.

Table 6
– Micromorphological characteristics of the studied soils.

Figure 3
– Photomicrographs. A), D) (polarized light), B) and D) (cross polarized light) Porostriated b fabric (shiny peds) due to moderate shrinking and swelling of aggregates in B horizon of P1; E) and F) Soil matrix in B horizon (variegated color) and microscopic fragments of charcoal of P2, representing the xanthization process below the ‘sombric’ horizon (sombric-like horizon). Q: quartz; P: pore and M: soil matrix.

Figure 5
– Increase in structure density with depth. A) and B) Subsurface horizon with subangular block and granular structure (P3 transition A3/A4; 57–72 cm); C), D) and E) Subsurface horizon with subangular block structure (P3 transition A5/A6; 77–90 cm); F) Subsurface horizon with subangular block structure (P3 horizon B2; 170–183 cm).

Results and Discussion

Soil morphology and micromorphology

Surface horizons have a yellowish color, with 7.5 YR hues, which suggests the predominance of goethite rather than hematite (Tables 1 and 5). The darker color observed in sombric-like horizons compared to overlying horizons is related to a larger contribution from BC in the sombric-like horizons (Chiapini et al., 2018Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244.), which has a strong pigmentation effect on the soil matrix (Silva and Vidal-Torrado, 1999Silva, A.C.; Vidal-Torrado, P. 1999. Genesis of humic Oxisols and its relationships with the evolution of the landscape of a cratonic area in the south of Minas Gerais state, Brazil. Revista Brasileira de Ciência do Solo 23: 329–341(in Portuguese, with abstract in English).; Macedo et al., 2017Macedo, R.S.; Teixeira, W.G.; Corrêa, M.M.; Martins, G.C.; Vidal-Torrado, P. 2017. Pedogenetic processes in anthrosols with pretic horizon (Amazonian Dark Earth) in Central Amazon, Brazil. PloS ONE 12: e0178038.). Thus, the concentration of both BC and OM influence the melanization process in the upper part of the soil profiles (Macedo et al., 2017Macedo, R.S.; Teixeira, W.G.; Corrêa, M.M.; Martins, G.C.; Vidal-Torrado, P. 2017. Pedogenetic processes in anthrosols with pretic horizon (Amazonian Dark Earth) in Central Amazon, Brazil. PloS ONE 12: e0178038.).

Field (Table 1) and micromorphological (Table 6) descriptions showed that the soil structure of the surface horizons and sombric-like ones are characterized by granular and subangular blocks with a moderate to strong degree of pedality. This is a result of the high OM content (Table 3; Ct) and an intense bioturbation process (Figure 4 and 5) (De Craene and Laruelle, 1955De Craene, A.; Laruelle, J. 1955. Genesis and alteration of equatorial and tropic latosols. Bulletin Agricole du Congo Belge 46: 1113–1243 (in French, with abstract in English).; Bennema et al., 1970Bennema, J.; Jongerius, A.; Lemos, R. 1970. Micromorphology of some oxic and argillic horizons in south Brazil in relation to weathering sequences. Geoderma 4: 333–355.; Macedo et al., 2017Macedo, R.S.; Teixeira, W.G.; Corrêa, M.M.; Martins, G.C.; Vidal-Torrado, P. 2017. Pedogenetic processes in anthrosols with pretic horizon (Amazonian Dark Earth) in Central Amazon, Brazil. PloS ONE 12: e0178038.). On the other hand, the structure in the subsurface horizons was characterized by subangular blocks with strong pedality indicating a much denser structure (Figure 5). This higher density was related to lower OM content, lower contribution from roots, lower faunal activity, and higher clay content at depth. Shiny and matte aggregate faces were observed in the field (Table 1). On the micro scale, a porostriated b-fabric’s features were described and are the result of swell-shrinking of soil in alternating dry and humid periods (Figure 3A, 3B, 3C and 3D). Thus, both the shiny and matte surfaces described in the field are actually in fact compression features (shrinking and swelling process) and are not due to clay illuviation.

Figure 4
– The bioturbation process. A) and B) Loose discontinuous infillings in the A4 horizon of P2 and the granular structure; C) Loose discontinuous infillings in the A5 horizon of P3; D) Detail of C).

Thin sections showed a variegated color (red/yellow) in the matrix of B horizons, which were not observed on the macro scale (Figure 3E e 3F). This variegated color is due to xanthization in the A, AB and upper B horizons (Cornell and Schwertmann, 2003). The Gt/(Gt+Hm) ratio would reflect this process, its values varying between 0.423 and 0.754 and indeed indicate predominance of Gt over Hm in the surface horizons of all profiles (Table 5). In fact, the Gt/(Gt+Hm) ratio decreased with depth, indicating xanthization (yellow colors; hues 5 YR and more yellow) in the upper part of the soils (Table 5). The high organic matter content and/or microbial oxidation in the sombric-like horizon inhibits hematite formation at the point of contact with the B horizon, causing preferential reductive dissolution of originally formed hematite turning red soils yellow in the lower subdivisions of the A horizon (Cornell and Schwertmann, 2003). Furthermore, the formation of Gt in soils is favored by wetter conditions (Schwertmann and Taylor, 1989Schwertmann, U.; Taylor, R.M. 1989. Iron oxides. p. 379–438. In: Dixon, J.B.; Weed, S.B., eds. Minerals in soil environments. 2ed. Soil Science Society of America, Madison, WI, USA.), high OM content, low temperature and low contents of Fe in solution (Camêlo et al., 2018Camêlo, D.L; Ker, J.C.; Fontes, M.P.F.; Costa, A.C.S.; Corrêa, M.M.; Leopold, M. 2018. Mineralogy, magnetic susceptibility and geochemistry of Fe-rich Oxisols developed from several parent materials. Scientia Agricola 75: 410–419.), all of which are evident in the soils from the study area.

Chemical and physical properties

The pH in water of the studied soil profiles ranged from 3.8 to 5.4 and increased with depth (Table 3). Extractable aluminum contents (Al3) in the surface horizons of profiles P1, P2 and P3 were high enough (> 4 cmolc kg1) for an aluminic qualifier (Embrapa, 2018); however, in the subsurface horizons this was not observed. Bases (K+, Na+, Ca2, and Mg2 data not shown) and base saturation (BS) were low in all horizons, due to a significant contribution of H+Al ions, as well as low clay activity, which are typical for such highly weathered soils. Similar to Dalmolin et al. (2006)Dalmolin, R.S.D.; Gonçalves, C.N.; Dick, D.P.; Knicker, H.; Klamt, E.; Kögel-Knabner, I. 2006. Organic matter characteristics and distribution in Ferralsol profiles of a climosequence in southern Brazil. European Journal of Soil Science 57: 644–654., we observed that high Cation Exchange Capacity (CEC) values in the A horizons were a result of the high OM content (Ct; Table 3).

The contents of Fe and Al forms are given in Table 4. The Fed contents showed a clear gradual increase with depth, while the opposite was observed for Feo (r2 = 0.93, 0.44, 0.36 and 0.84 for profiles P1–P4, respectively). This reflects that low OM content favors pedogenic Fe oxyhydroxides formation at depth (mainly hematite) (Curi and Franzmeier, 1987Curi, N.; Franzmeier D.P. 1987. Effect of parent rocks on chemical and mineralogical properties of some Oxisols in Brazil. Soil Science Society of America Journal 51:153–158.). The Feo/Fed ratio showed generally higher values in the A than in the B horizons because of the high influence of OM in the A horizons (Table 4). The Feo/Fed ratio indicated that the Fe in the A horizons was in poorly ordered form (values > 0.05), while in the B horizons the major part of Fe oxides were present in crystals with greater ordered form (values < 0.05; Inda Junior and Kämpf, 2003). The Fep was substantially higher than the Alp, and both showed positive correlation (r2 = 0.76). The high values of Fep and of the Fep/Fed ratio indicate that Fe is bound to soil OM and confirms that part is in the non-crystalline form.

High Ald values were observed in surficial horizons, which can be related to several methodological factors during the sodium citrate-bicarbonate-dithionite (CBD) extractions. First, successive extractions with CBD (80 °C) may dissolve a certain amount of kaolinite and gibbsite thereby releasing Al3 and causing an increase in Ald (Inda Junior and Kämpf, 2003). Second, the high Ald values can be due to complexation to sodium citrate catalyzed by the high temperature during the extraction procedure (Zhang et al., 1985Zhang, Y.; Kallay, N.; Matijevic, E. 1985. Interactions of metals hydrous with chelating agentes. VII. Hematite-oxalic and citric acid systems. Langmuir 1: 201–206.). Third, the high values of Ald can be related to the extraction of the Al3 resulting from the isomorphic substitution of Fe3 in the iron oxides (Tables 4 and 5).

All four profiles presented a high clay content that was regularly distributed with depth (Table 2). The highest clay contents were observed in B horizons but this was not sufficient to characterize a textural difference (clay increase in depth) (IUSS Working Group WRB, 2015IUSS Working Group WRB. 2015. World Reference Base for Soil Resources 2014: Update 2015. International Soil Classification System for Naming Soils and Creating Legends for Soil Maps. FAO, Rome, Italy. (World Soil Resources Reports, 106).; Soil Survey Staff, 2014Soil Survey Staff. 2014. Keys to Soil Taxonomy. 12ed. USDA-NRCS, Washington, DC, USA.; Embrapa, 2018). The B horizons presented a low silt/clay ratio (~ 0.3), which is typical of highly weathered soils (Fox, 1982Fox, R.L. 1982. Some highly weathered soils of Puerto Rico. 3. Chemical properties. Geoderma 27: 139–176.) where the clay content increases due to weathering of minerals in the silt fraction. Thus, properties such as low activity clays, silt/clay and Feo/Fed ratios characterizes the occurrence of a moderate ferralitization process in our studied soils (Kämpf and Curi, 2012Kämpf, N.; Curi, N. 2012. Formation and evolution of soil: pedogenesis = Formação e evolução do solo: pedogênese. p. 207–302. In: Ker, J.C.; Curi, N.; Schaefer, C.E.G.R.; Vidal-Torrado, P, eds. Pedology: elements = Pedologia: fundamentos. Sociedade Brasileira de Ciência do Solo, Viçosa, MG, Brazil (in Portuguese).).

Clay mineralogy

Clay components identified with XRD in the iron-free clay fraction of B horizons of all profiles were kaolinite (Kt), gibbsite (Gb), hydroxy-interlayered 2:1 minerals (2:1 HI) and quartz (Qz) (Figure 6). The intensities of Kt001 and Gb002 decreased from the profile at the summit (P1) towards the profile at the footslope (P3). 2:1 HI showed similar intensities in profiles P1 and P2, but was very weak in profiles P3 and P4. The absence of significant asymmetry of Kt001 in profiles P1 and P2 indicates no or little interstratification with smectite, which is in agreement with observations on similar soils in the region by Oliveira Junior et al. (2014).

Figure 6
– Diffractograms of the clay content of the B horizons, saturated with Mg (Mg 25 °C), solvated with ethylene glycol (Mg + EG) and K heated until 500 ºC (500 °C). All profiles (P1, P2, P3 and P4), there is the presence of kaolinite (Kt), gibbsite (Gb), 2:1 hydroxy-interlayered (HI) and quartz (Qz).

Goethite and hematite were identified in the concentrated oxide fraction (Figure 7). The contents of goethite varied from 5 % to 11 % and that of hematite from 2 % to 7 %. Goethite predominated in all studied soil profiles, with the exception of the B3 and B2 horizons from P2 and P3, respectively (Table 5). The data showed the highest goethite content in soil profiles was in the first B horizon (B1) (Table 5). The yellow color was observed reflecting the morphology of xanthization (Figures 3E3F) which occurs in the interface of the sombric-like horizon that is rich in OM (Ct) and the B horizon; and the action of soil fauna in this interface aid which is responsible for the distribution of OM in depth (melanization), that consequently expands xanthization. According to Chiapini et al. (2018)Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244., the sombric-like horizon of these soils is a remnant of an earlier phase of soil formation under grass vegetation and frequent natural fires resulting in considerable accumulation of black carbon (BC) at depth. During wetter conditions, from the Late Holocene period to the present, xanthization was influenced by this accumulation of OM and contributed to the development of polychrome of soil profiles.

Figure 7
– Diffractograms after concentration of iron oxides in some soil horizons.

The Hm content showed a clear increase with depth, which was the reverse for Gt (Almeida et al., 2000Almeida, J.A.; Maçaneiro, K.C.; Klamt, E. 2000. Clay mineralogy of high altitude red soils with brown or yellowish brown surface horizons. Revista Brasileira de Ciência do Solo 24: 815-828 (in Portuguese, with abstract in English).). The values for isomorphic substitution (IS) varied from 9 % to 32 % for Gt and from 1 % to 11 % for Hm. The IS for Gt presented similar values in both the A and upper B horizons for all profiles, with the exception of profile P2 where the A horizon presented a higher IS value (Table 5). This high Al goethite substitution is related to elevated, dessicated non-hydromorphic conditions that were observed in our soil profiles similar to other highly weathered Brazilian soils (Schwertamenn and Kämpf, 1985; Motta and Kämpf, 1992Motta, P.E.F.; Kämpf, N. 1992. Iron oxide properties as support to soil morphological features for prediction of moisture regimes in Oxisols of central Brazil. Journal of Plant Nutrition and Soil 155: 385–390.; Almeida et al., 2000Almeida, J.A.; Maçaneiro, K.C.; Klamt, E. 2000. Clay mineralogy of high altitude red soils with brown or yellowish brown surface horizons. Revista Brasileira de Ciência do Solo 24: 815-828 (in Portuguese, with abstract in English).). The deepest horizon showed lower IS values for both Gt and Hm (Table 5). In P2 and certain horizons of P3 (A5) and P4 (A3 and B1) it was not possible to obtain IS values for Hm.

The mean crystal diameter (MCD) of Gt showed a similar growth tendency for both directions (110 and 111), with the exception of the B3 horizon from profile P2. In Hm, the crystals showed a tendency to grow in a 104 rather than a 110 direction, again with exception of the B3 horizon from profile P2, in which similar dimensions for both directions were observed. The MCD for Gt and Hm in the studied profiles was smaller than that observed by Melo et al. (2001)Melo, V.F.; Fontes, M.P.F.; Novais, R.F.; Singh, B.; Schaefer, C.E.G.R. 2001. Iron and aluminum oxides of different Brazilian soils. Revista Brasileira de Ciência do Solo 25: 19–32 (in Portuguese, with abstract in English). in tropical conditions (hot and humid). Furthermore, the size of Hm and Gt crystals was smaller (< 5 nm; Table 5) than that observed in Oxisols (Melo et al., 2001Melo, V.F.; Fontes, M.P.F.; Novais, R.F.; Singh, B.; Schaefer, C.E.G.R. 2001. Iron and aluminum oxides of different Brazilian soils. Revista Brasileira de Ciência do Solo 25: 19–32 (in Portuguese, with abstract in English).; Lima et al., 2017)Lima, D.C.; Ker J.C.; Fontes M.P.F.; Corrêa, M.M.; Costa A.C.S.; Melo V.F. 2017. Pedogenic iron oxides in iron-rich oxisols developed from mafic rocks. Revista Brasileira de Ciência do Solo 41: 1–16.. Thus, high Kt content favors the development of a block structure rather than a granular microstructure in the B horizons (De Wispelaere et al., 2015)De Wispelaere, L.; Marcelino, V.; Regassa, A.; De Grave, E.; Dumon, M.; Mees, F.; Van Ranst, E. 2015. Revisiting nitic horizon properties of Nitisols in SW Ethiopia. Geoderma 243–244: 69–79.. No evidence of coalescence of microstructures nor transformation from small granular to blocky or prismatic aggregates were observed, as was proposed by Cooper et al. (2010)Cooper, M.; Vidal-Torrado, P.; Grimaldi M. 2010. Soil structure transformations from ferralic to nitic horizons on a toposequence in southeastern Brazil. Revista Brasileira de Ciência do Solo 34: 1685–1699..

Soil Classification

Important items for classification at the Order level are the morphological features related to the B horizon. These include particle rearrangement on the surfaces of structural units, shiny peds, low textural difference, clay-rich subsoil and structure from moderate to strong subangular blocks with waxiness characterizing a Nítico and Textural horizon at the same time as according to the Brazilian Soil Classification System (Embrapa, 2018). At WRB-FAO (IUSS Working Group, 2015) the B horizon is classified as a Nitic horizon, presenting the same morphological features and chemical characteristics such as ≥ 4 % Fed and ≥ 0.2 % Feo. In relation to Soil Taxonomy (Soil Survey Staff, 2014Soil Survey Staff. 2014. Keys to Soil Taxonomy. 12ed. USDA-NRCS, Washington, DC, USA.) the B horizon is classified as a Kandic horizon.

Another important classification item is the polychrome character of the soil profiles. As per the Brazilian Soil Classification System (Embrapa, 2018), in the Nitossolos Order (Nítico B horizon), the polychrome indication is only allowed in specific situations, none of which fit in our soils. The soil profiles showed colors of more than one hue page, and variation in value and chroma in A and B horizons (Table 1). Similarly, in the suborder, Nitossolos Brunos, this variation in color is also not allowed. Because of this, the profiles must be classified as Argissolos in the first Order.

Classification of the four soil profiles (P1–P4) under the Brazilian Soil Classification System (Embrapa, 2018) would result in the following reasoning:

Due to the presence of the Nítico and Textural B horizon with shiny peds that cannot be attributed to clay illuviation, nor polychrome characteristic, the first category level (Order) is classified as Argissolos. Because the soils presented a red-yellow color (P1), dominant yellow color (P2 and P4) and dominant red color (P3) in the BA and B horizons, they were classified as Vermelho-Amarelo (Red-Yellow) in the second level in P1, Amarelo (Yellow) in the second level of P2 and P4, and Vermelho (Red) in the second level in P3. The soil profiles showed low base saturation (BS < 50 %). Thus the third category level they were classified as Distrófico (Dystrophic). At the fourth category level the soil profiles P1 and P3 were classified as nitossólico, because of the soil morphology similar to Nitossolos and soil profiles P2 and P4 such as típico (typical) (Table 7). Additional problems with the Brazilian Soil Classification System (Embrapa, 2018) were observed at the subgroup level, where the sombric feature (caráter sômbrico) is not allowed because i) no OM illuviation was confirmed in the micromorphological analysis (organic or clay-organic coatings) and ii) no increase in carbon content in the sombric-like horizons was observed. The impossibility of classifying it as a sombric horizon thereby neglects an important process related to its formation, i.e., decomposition of the upper part of a humic horizon (Caner et al., 2003Caner, L.; Toutain, F.; Bourgeon, G.; Herbillon, A.J. 2003. Occurrence of sombric-like subsurface A horizons in some andic soils of the Nilgiri Hills (southern India) and their palaeoecological significance. Geoderma 117: 251–265.; Chiapini et al., 2018)Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244.. As such, a great effort dispensed in pedogenic studies that use the soil as a record of past climatic conditions will be missed, as well as one of the main soil forming factors, i.e. time, is neglected.

Table 7
– Soil Classification in the Brazilian Soil Classification System (SiBCS; Embrapa, 2018), WRB (IUSS Working Group WRB, 2015IUSS Working Group WRB. 2015. World Reference Base for Soil Resources 2014: Update 2015. International Soil Classification System for Naming Soils and Creating Legends for Soil Maps. FAO, Rome, Italy. (World Soil Resources Reports, 106).) and Soil Taxonomy (Soil Survey Staff, 2014Soil Survey Staff. 2014. Keys to Soil Taxonomy. 12ed. USDA-NRCS, Washington, DC, USA.).

This situation must be revisited once all of the soil classification systems used in this work have assumed the taxonomic approach, in other words, considered the pedogenic process to fit a soil in a given class.

In the WRB classification (IUSS Working Group WRB, 2015IUSS Working Group WRB. 2015. World Reference Base for Soil Resources 2014: Update 2015. International Soil Classification System for Naming Soils and Creating Legends for Soil Maps. FAO, Rome, Italy. (World Soil Resources Reports, 106).), the polychrome characteristic is not a problem and the sombric horizon can be assigned to this soil order, but it cannot appear in principal qualifiers nor in supplementary ones. Therefore, the presence of a sombric horizon or a sombric qualifier could not be identified in the studied soil profiles and their classification in WRB would result in Umbric Nitisol due to the presence of a nitic and umbric horizon. As regards qualifiers, the soils P1, P2 and P4 were classified as Dystric and Humic and P3 as Rhodic, Dystric and Humic (Table 7). Our classification differs from Bockheim (2012)Bockheim, J.G. 2012. Revisiting the definitions of the sombric horizon in soil taxonomy and word reference base for soil resources. Geoderma 170: 127–135. who found the sombric horizons primarily in Umbric Ferralsols (Sombric).

In Soil Taxonomy (Soil Survey Staff, 2014Soil Survey Staff. 2014. Keys to Soil Taxonomy. 12ed. USDA-NRCS, Washington, DC, USA.) no problems with morphological characteristics were found and the soils were classified as Oxisol in the Order level because of the low base saturation and high mineral weathering (moderate ferralitization). A Udic moisture regime was identified as the Suborder level. At Great Group the soil profiles were classified as Typic Kandiudox (P1), Xanthic Kandiudox (P2 and P4: dominant yellow color) and Rhodic Kandiudox (P4: dominant red color) (Table 7). Similar to those described above, Bockheim (2012)Bockheim, J.G. 2012. Revisiting the definitions of the sombric horizon in soil taxonomy and word reference base for soil resources. Geoderma 170: 127–135. classified soils with a sombric horizon primarily as Sombriudox and Sombrihumult in Soil Taxonomy, which differs somewhat from our soils.

Thus, the sombric-like horizon in the soils from our study area cannot be classified as sombric horizons according to the Soil Taxonomy and WRB classification systems, nor as a sombric qualifier according to the Brazilian Soil Classification System.

The process of OM illuviation (Soil Survey Staff, 2014Soil Survey Staff. 2014. Keys to Soil Taxonomy. 12ed. USDA-NRCS, Washington, DC, USA.; IUSS Working Group WRB, 2015IUSS Working Group WRB. 2015. World Reference Base for Soil Resources 2014: Update 2015. International Soil Classification System for Naming Soils and Creating Legends for Soil Maps. FAO, Rome, Italy. (World Soil Resources Reports, 106).) in the formation of the sombric horizon was not observed in any of our four soil profiles (Figure 3A and 3B). Nevertheless, the other characteristics belonging to the sombric horizon were present, and included base saturation < 50 %, lateral tracing that distinguished them from some buried epipedons, and a lower color value and chroma than the overlying horizon.

Conclusions

The distinctive properties of subtropical soils with sombric-like horizon from the studied soil profiles included: the presence of shiny peds, a base saturation < 50 %, a lower color value and chroma than the overlying horizon, clear and sharp lateral tracing, the yellowish color in depth, and the polychrome characteristic. Melanization, xanthization, bioturbation, moderate shrinking/swelling and moderate ferralitization are the most evident pedogenetic processes. Xanthization is closely related to the formation of the sombric-like horizon during the past change from drier to wetter conditions.

The current definition of the sombric horizon in the soil classification systems (Soil Taxonomy, WRB-FAO and Brazilian Soil Classification) can be improved in terms of soil forming processes and polychrome characteristic. We propose developing classification criteria for the sombric-like horizon, or alternatively accept them as a special type of sombric horizon.

Acknowledgments

The authors thank the São Paulo Research Foundation (FAPESP) for financial support (Projects 2015/03577-2, 2014/23969-0 and 2013/03953-9) and the Coordination for the Improvement of Higher Education Personnel agency (CAPES). The Brazilian National Council for Scientific and Technological Development (CNPq) also supported this work through a grant 301818/2017-7 given to the sixth author. We thank Nivanda Maria de Moura Ruiz and Luiz Antonio Silva Junior for analytical activities, José Luiz Vicente and Sonia Aparecida de Moraes for thin section confection, and three anonymous reviewers, in particular reviewer #2, for their critical comments.

References

  • Almeida, J.A.; Maçaneiro, K.C.; Klamt, E. 2000. Clay mineralogy of high altitude red soils with brown or yellowish brown surface horizons. Revista Brasileira de Ciência do Solo 24: 815-828 (in Portuguese, with abstract in English).
  • Almeida, J.A.; Cararo, D.C.; Uberti, A.A.A. 2009. Genesis of the sombric horizon in Ultisols (Red Argisols) in southern Santa Catarina, Brazil. Revista Brasileira de Ciência do Solo 33: 405–416.
  • Almeida, J.A.; Lunardi Neto, A.; Vidal-Torrado, P. 2015. Sombric horizon: five decades without evolution: a review. Scientia Agricola 72: 87–95.
  • Behling, H.; Bauermann, S.G.; Neves, P.C. 2001. Holocene environmental changes from the São Francisco de Paula region, southern Brazil. Journal of South American Earth Science 14: 631–639.
  • Bennema, J.; Jongerius, A.; Lemos, R. 1970. Micromorphology of some oxic and argillic horizons in south Brazil in relation to weathering sequences. Geoderma 4: 333–355.
  • Bockheim, J.G. 2012. Revisiting the definitions of the sombric horizon in soil taxonomy and word reference base for soil resources. Geoderma 170: 127–135.
  • Camêlo, D.L; Ker, J.C.; Fontes, M.P.F.; Costa, A.C.S.; Corrêa, M.M.; Leopold, M. 2018. Mineralogy, magnetic susceptibility and geochemistry of Fe-rich Oxisols developed from several parent materials. Scientia Agricola 75: 410–419.
  • Caner, L.; Toutain, F.; Bourgeon, G.; Herbillon, A.J. 2003. Occurrence of sombric-like subsurface A horizons in some andic soils of the Nilgiri Hills (southern India) and their palaeoecological significance. Geoderma 117: 251–265.
  • Chiapini, M.; Schellekens, J.; Calegari, M.R.; Almeida, J.A.; Buurman, P.; Camargo, P.B.; Vidal-Torrado, P. 2018. Formation of black carbon rich ‘sombric’ horizons in the subsoil: a case study from subtropical Brazil. Geoderma 314: 232–244.
  • Cooper, M.; Vidal-Torrado, P.; Grimaldi M. 2010. Soil structure transformations from ferralic to nitic horizons on a toposequence in southeastern Brazil. Revista Brasileira de Ciência do Solo 34: 1685–1699.
  • Cornell, R.M.; Shwertmann, U. 2003. The Iron Oxides: Structure, Properties, Reactions and Uses. Wiley, New York, NY, USA.
  • Curi, N.; Franzmeier D.P. 1987. Effect of parent rocks on chemical and mineralogical properties of some Oxisols in Brazil. Soil Science Society of America Journal 51:153–158.
  • Dalmolin, R.S.D.; Gonçalves, C.N.; Dick, D.P.; Knicker, H.; Klamt, E.; Kögel-Knabner, I. 2006. Organic matter characteristics and distribution in Ferralsol profiles of a climosequence in southern Brazil. European Journal of Soil Science 57: 644–654.
  • De Craene, A.; Laruelle, J. 1955. Genesis and alteration of equatorial and tropic latosols. Bulletin Agricole du Congo Belge 46: 1113–1243 (in French, with abstract in English).
  • De Wispelaere, L.; Marcelino, V.; Regassa, A.; De Grave, E.; Dumon, M.; Mees, F.; Van Ranst, E. 2015. Revisiting nitic horizon properties of Nitisols in SW Ethiopia. Geoderma 243–244: 69–79.
  • Empresa Brasileira de Pesquisa Agropecuária [EMBRAPA]. 2018. Brazilian soil classification system. Available at: https://www.embrapa.br/busca-de-publicacoes/-/publicacao/1094001/brazilian-soil-classification-system [Accessed Feb 9, 2019]
    » https://www.embrapa.br/busca-de-publicacoes/-/publicacao/1094001/brazilian-soil-classification-system
  • Food and Agriculture Organization of the United Nations [FAO]. 2006. Guidelines for Soil Description. 4ed. FAO, Rome, Italy.
  • Faivre, P. 1990. The sombric horizon: an ‘incipient’ organic-argillic horizon; the example of soils of the intra-Andean region of Colombia (South America). Pédologie 40: 273–297 (in French, with abstract in English).
  • Fox, R.L. 1982. Some highly weathered soils of Puerto Rico. 3. Chemical properties. Geoderma 27: 139–176.
  • Gouveia, S.E.M.; Pessenda, L.C.R.; Aravena, R.; Boulet, R.; Scheel-Ybert, R.; Bendassoli, J.A.; Ribeiro, A.S.; Freitas, H.A. 2002. Carbon isotopes in charcoal and soils in studies of paleovegetation and climate changes during the late Pleistocene and the Holocene in the southeast and center west regions of Brazil. Global and Planetary Change 33: 95–106.
  • Inda Junior, A.V.; Kämpf, N. 2003. Evaluation of pedogenic iron oxide extraction procedures with sodium dithionite-citrate-bicarbonate. Revista Brasileira de Ciência do Solo 27:1139–1147 (in Portuguese, with abstract in English).
  • IUSS Working Group WRB. 2015. World Reference Base for Soil Resources 2014: Update 2015. International Soil Classification System for Naming Soils and Creating Legends for Soil Maps. FAO, Rome, Italy. (World Soil Resources Reports, 106).
  • Jackson, M.L. 1979. Soil Chemical Analysis: Advanced Course; A manual of methods useful for instruction and research in soil chemistry, physical chemistry of soils, soil fertility, and soil genesis. The author, Madison, WI, USA.
  • Kämpf, N.; Curi, N. 2012. Formation and evolution of soil: pedogenesis = Formação e evolução do solo: pedogênese. p. 207–302. In: Ker, J.C.; Curi, N.; Schaefer, C.E.G.R.; Vidal-Torrado, P, eds. Pedology: elements = Pedologia: fundamentos. Sociedade Brasileira de Ciência do Solo, Viçosa, MG, Brazil (in Portuguese).
  • Kämpf, N.; Schwertmann, U. 1982. The NaOH concentration method for iron oxides in soils. Clays and Clay Minerals 30: 401–408.
  • Lebedeva, I.I.; Tonkonogov, V.D.; Gerasimova, M.I. 1999. Diagnostic horizons in substantive-genetic soil classification systems. Eurasian Soil Science 32: 959–965
  • Lima, D.C.; Ker J.C.; Fontes M.P.F.; Corrêa, M.M.; Costa A.C.S.; Melo V.F. 2017. Pedogenic iron oxides in iron-rich oxisols developed from mafic rocks. Revista Brasileira de Ciência do Solo 41: 1–16.
  • Macedo, R.S.; Teixeira, W.G.; Corrêa, M.M.; Martins, G.C.; Vidal-Torrado, P. 2017. Pedogenetic processes in anthrosols with pretic horizon (Amazonian Dark Earth) in Central Amazon, Brazil. PloS ONE 12: e0178038.
  • McKeague, J.A.; Day, J.H. 1966. Dithionite and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. Canadian Journal of Soil Science 46: 13–22.
  • Mehra, J.P.; Jackson, M.L. 1960. Iron oxides removal from soils and clays by a dithionite-citrate-bicarbonate system buffered with bicarbonate sodium. Clays and Clay Minerals 7: 317–327.
  • Melo, V.F.; Fontes, M.P.F.; Novais, R.F.; Singh, B.; Schaefer, C.E.G.R. 2001. Iron and aluminum oxides of different Brazilian soils. Revista Brasileira de Ciência do Solo 25: 19–32 (in Portuguese, with abstract in English).
  • Motta, P.E.F.; Kämpf, N. 1992. Iron oxide properties as support to soil morphological features for prediction of moisture regimes in Oxisols of central Brazil. Journal of Plant Nutrition and Soil 155: 385–390.
  • Norrish, K.; Taylor, M. 1961. The isomorphous replacement of iron by aluminium in soil goethites. European Journal of Soil Science 12: 294–306.
  • Oliveira Junior, J.C.; Melo, V.F.; Souza L.C.P.; Rocha, H.O. 2014. Terrain attributes and spatial distribution of soil mineralogical attributes. Geoderma 213: 214–225.
  • Santos, L.C.; Oka-Fiori, C.; Canali, N.E.; Fiori, A.P.; Silveira, C.T.; Silva, J.M.F.; Ross, J.L.S. 2006. Geomorphological mapping of the state of Paraná. Revista Brasileira de Geomorfologia 2: 3–12 (in Portuguese, with abstract in English).
  • Silva, A.C.; Vidal-Torrado, P. 1999. Genesis of humic Oxisols and its relationships with the evolution of the landscape of a cratonic area in the south of Minas Gerais state, Brazil. Revista Brasileira de Ciência do Solo 23: 329–341(in Portuguese, with abstract in English).
  • Schulze, D.G. 1984. The influence of aluminium on iron oxides. VIII. Unit-cell dimen- sions of Al-substituted goethites and estimation of Al from them. Clays and Clay Minerals 32: 36–44.
  • Schwertmann, U.; Taylor, R.M. 1989. Iron oxides. p. 379–438. In: Dixon, J.B.; Weed, S.B., eds. Minerals in soil environments. 2ed. Soil Science Society of America, Madison, WI, USA.
  • Schwertmann, U.; Kämpf, N. 1985. Properties of goethite and hematite in kaolinitic soils of southern and central Brazil. Soil Science 139: 344–350.
  • Schwertmann, U.; Fitzpatrick, R.W.; Taylor, R.M.; Lewis, D.G. 1979. The influence of aluminium on iron oxides. Part II. Preparation and properties of Al-substituted hematites. Clays and Clay Minerals 29: 269–276.
  • Singh, B.; Gilkes, R.J. 1991. Concentration of iron oxides from soils clays by 5 M NaOH treatment: the complete removal of sodalite and kaolin. Clay Minerals 26: 463–472.
  • Soil Survey Staff. 2014. Keys to Soil Taxonomy. 12ed. USDA-NRCS, Washington, DC, USA.
  • Sys, C.; Van Wambecke, A.; Frankart, R.; Gilson, P.; Jongen, P.; Pécrot, A.; Berce, J.M.; Jamagne, M. 1961. Soil Cartography in Congo: Principles and Methods = La Cartographie des Sols au Congo, ses Principes et ses Méthodes. National Institute for Agronomy in Belgian Congo, Brussels, Belgium. (Series Science, 66) (in French).
  • Torrent, J.; Cabedo, A. 1986. Sources of iron oxides in reddish brown soil profiles from calcarenites in southern Spain. Geoderma 37: 57–66.
  • United States Department of Agriculture [USDA]. 1996. Soil Survey Laboratory Methods Manual. USDA, Washington, DC, USA.
  • Velasco-Molina, M.; Almeida, J.A.; Vidal-Torrado, P.; Macías, F. 2010. Chemical fractionation of carbon on Acrisols with sombric horizon from south Brazil. Revista de Ciências Agrárias 33: 277–286 (in Spanish, with abstract in English).
  • Zhang, Y.; Kallay, N.; Matijevic, E. 1985. Interactions of metals hydrous with chelating agentes. VII. Hematite-oxalic and citric acid systems. Langmuir 1: 201–206.

Edited by

Edited by: Paulo Cesar Sentelhas

Publication Dates

  • Publication in this collection
    08 July 2020
  • Date of issue
    2021

History

  • Received
    06 May 2019
  • Accepted
    08 Feb 2020
Escola Superior de Agricultura "Luiz de Queiroz" USP/ESALQ - Scientia Agricola, Av. Pádua Dias, 11, 13418-900 Piracicaba SP Brazil, Phone: +55 19 3429-4401 / 3429-4486 - Piracicaba - SP - Brazil
E-mail: scientia@usp.br