Iron oxides in plinthic soils on sedimentary deposits in northeastern Brazil

Heck, R. J.; Mermut, A. R.; Santos, M. C.

doi:10.1590/S0100-06831999000300019

Abstracts

This study was conducted to examine the distribution and nature of Fe oxides in plinthic soils on the sediments of Barreiras Group (in the state of Piauí) and Itapecuru Formation (in the state of Maranhão) in Northeastern Brazil. Four pedons were selected: a "plinthic, dystrophic, epieutrophic Gray Podzolic with low activity clay" and a "dystrophic Plinthosol with low activity clay" (both Plinthic Kandiustalfs) on the Barreiras sediments, as well as an "eutrophic Plinthosol with low activity clay" and an "allic Plinthosol with low activity clay" (both Plinthustalfs) on the Itapecuru sediments. Soil samples were fractionated into > 2 mm (pisoliths), water-stable aggregates (plinthite) and matrices; the aggregates and matrices were further fractionated into sand, silt and clay sizes. Dithionite extractable iron (Fe d) and aluminum (Al d), as well as oxalate extractable iron (Fe o), were determined for all fractions, and X-ray diffraction analyses were performed on the pisoliths. It was observed that the Plinthustalfs contain more iron oxides, exhibit more extensive plinthite development and have a greater potential for further plinthite development than the Kandiustalfs. The distribution of values for the Fe d indicates that plinthite formation and induration in all soils were accompanied by an enrichment of Fe oxides in all particle size fractions. This Fe segregation was accompanied by aggregation of particles leading to a greater degree of crystallinity, as indicated by analysis of the ratios of Al d:Fe d. Larger ratios of goethite to hematite, and relatively smaller amounts of silicates in the more mature pisoliths were revealed by X-ray diffraction analysis. Ratios of Al d:Fe d were larger in the Kandiustalfs than in the Plinthustalfs, and also larger than expected for Al-substituted Fe oxides. According to ratios of Al d:Fe d, Fe mobilization in all soils has likely occurred under reducing conditions, facilitated by organic matter on the soil surface.

plinthosol; Ustalf; Barreiras Group; Itapecuru Formation; citrate; dithionite; oxalate; X-ray diffraction; aluminum; goethite; hematite

O presente estudo foi conduzido com a finalidade de examinar a natureza e a distribuição de óxidos de ferro em solos plínticos desenvolvidos de sedimentos atribuídos ao Grupo Barreiras (no estado do Piauí) e à Formação Itapecuru (no estado do Maranhão), no nordeste do Brasil. Quatro perfis foram selecionados: um Podzólico Acinzentado argila de atividade baixa distrófico epieutrófico plíntico e um Plintossolo argila de atividade baixa distrófico (ambos "Plinthic Kandiustalfs"), desenvolvidos de sedimentos do Grupo Barreiras, e dois Plintossolos argila de atividade baixa álicos (ambos "Plinthustalfs"), desenvolvidos de sedimentos da Formação Itapecuru. As amostras de solos foram separadas em frações > 2 mm (pisolitos), agregados estáveis em água (plintita) e matriz; os agregados e a matriz foram posteriormente fracionados em areia, silte e argila. Foram efetuadas extrações de ferro (Fed) e alumínio (Ald), usando o extrator ditionito, e do ferro (Feo), usando oxalato, de todas as frações. Análise de difração de raios-X foi executada nos pisolitos. Observou-se que os plintossolos ("Plinthustalfs") da Formação Itapecuru contêm mais óxidos de ferro, exibem extensivo desenvolvimento de plintita e têm maior potencial para um grau ainda maior de desenvolvimento desta plintita, quando comparados ao Podzólico Acinzentado e ao Plintossolo do Grupo Barreiras ("Plinthic Kandiustalfs"). A distribuição dos valores de Fed indica que a formação de plintita e o endurecimento em todos os solos foram acompanhados por enriquecimento de ferro em todas as frações. A segregação de ferro foi acompanhada pela agregação de partículas, levando a um maior grau de cristalinidade, como indicado pela análise das razões Feo/Fed. Análise da difração de raios-X revelou maiores razões de goethita para hematita, bem como menores quantidades de silicatos nos pisolitos mais maturos. Razões Ald/Fed foram maiores no Podzólico que nos Plintossolos e, maiores que as esperadas para os óxidos de ferro com substituição por alumínio. De acordo com as razões Ald/Fed, é provável que a mobilização de ferro em todos os solos tenha ocorrido sob condições de redução, facilitada por matéria orgânica na superfície.

Plintossolo; Ustalf; Grupo Barreiras; Formação Itapecuru; citrato ditionito; oxalato; difração de raios-X; alumínio; goethita; hematita

SEÇÃO V - GÊNESE, MORFOLOGIA E CLASSIFICAÇÃO DO SOLO

R. J. Heck^I; A. R. Mermut^II; M. C. Santos^III

^IProfessor Visitante, Universidade Federal Rural de Pernambuco. Rua Dom Manoel de Medeiros, s/n, Dois Irmãos, CEP 52171-900. Recife (PE)

^IIAdjunct Professor, University of Saskatchewan, 51 Campus Drive, Saskatoon (SK), S7N 5A8, Canada

^IIIProfessor Adjunto , Universidade Federal Rural de Pernambuco, Rua Dom Manoel de Medeiros, s/n, Dois Irmãos, CEP 52171-900 Recife (PE). Bolsista do CNPq

SUMMARY

This study was conducted to examine the distribution and nature of Fe oxides in plinthic soils on the sediments of Barreiras Group (in the state of Piauí) and Itapecuru Formation (in the state of Maranhão) in Northeastern Brazil. Four pedons were selected: a "plinthic, dystrophic, epieutrophic Gray Podzolic with low activity clay" and a "dystrophic Plinthosol with low activity clay" (both Plinthic Kandiustalfs) on the Barreiras sediments, as well as an "eutrophic Plinthosol with low activity clay" and an "allic Plinthosol with low activity clay" (both Plinthustalfs) on the Itapecuru sediments. Soil samples were fractionated into > 2 mm (pisoliths), water-stable aggregates (plinthite) and matrices; the aggregates and matrices were further fractionated into sand, silt and clay sizes. Dithionite extractable iron (Fe_d) and aluminum (Al_d), as well as oxalate extractable iron (Fe_o), were determined for all fractions, and X-ray diffraction analyses were performed on the pisoliths. It was observed that the Plinthustalfs contain more iron oxides, exhibit more extensive plinthite development and have a greater potential for further plinthite development than the Kandiustalfs. The distribution of values for the Fe_d indicates that plinthite formation and induration in all soils were accompanied by an enrichment of Fe oxides in all particle size fractions. This Fe segregation was accompanied by aggregation of particles leading to a greater degree of crystallinity, as indicated by analysis of the ratios of Al_d:Fe_d. Larger ratios of goethite to hematite, and relatively smaller amounts of silicates in the more mature pisoliths were revealed by X-ray diffraction analysis. Ratios of Al_d:Fe_d were larger in the Kandiustalfs than in the Plinthustalfs, and also larger than expected for Al-substituted Fe oxides. According to ratios of Al_d:Fe_d, Fe mobilization in all soils has likely occurred under reducing conditions, facilitated by organic matter on the soil surface.

Index Terms: plinthosol, Ustalf, Barreiras Group, Itapecuru Formation, citrate, dithionite, oxalate, X-ray diffraction, aluminum, goethite, hematite.

RESUMO

O presente estudo foi conduzido com a finalidade de examinar a natureza e a distribuição de óxidos de ferro em solos plínticos desenvolvidos de sedimentos atribuídos ao Grupo Barreiras (no estado do Piauí) e à Formação Itapecuru (no estado do Maranhão), no nordeste do Brasil. Quatro perfis foram selecionados: um Podzólico Acinzentado argila de atividade baixa distrófico epieutrófico plíntico e um Plintossolo argila de atividade baixa distrófico (ambos "Plinthic Kandiustalfs"), desenvolvidos de sedimentos do Grupo Barreiras, e dois Plintossolos argila de atividade baixa álicos (ambos "Plinthustalfs"), desenvolvidos de sedimentos da Formação Itapecuru. As amostras de solos foram separadas em frações > 2 mm (pisolitos), agregados estáveis em água (plintita) e matriz; os agregados e a matriz foram posteriormente fracionados em areia, silte e argila. Foram efetuadas extrações de ferro (Fed) e alumínio (Ald), usando o extrator ditionito, e do ferro (Feo), usando oxalato, de todas as frações. Análise de difração de raios-X foi executada nos pisolitos. Observou-se que os plintossolos ("Plinthustalfs") da Formação Itapecuru contêm mais óxidos de ferro, exibem extensivo desenvolvimento de plintita e têm maior potencial para um grau ainda maior de desenvolvimento desta plintita, quando comparados ao Podzólico Acinzentado e ao Plintossolo do Grupo Barreiras ("Plinthic Kandiustalfs"). A distribuição dos valores de Fed indica que a formação de plintita e o endurecimento em todos os solos foram acompanhados por enriquecimento de ferro em todas as frações. A segregação de ferro foi acompanhada pela agregação de partículas, levando a um maior grau de cristalinidade, como indicado pela análise das razões Feo/Fed. Análise da difração de raios-X revelou maiores razões de goethita para hematita, bem como menores quantidades de silicatos nos pisolitos mais maturos. Razões Ald/Fed foram maiores no Podzólico que nos Plintossolos e, maiores que as esperadas para os óxidos de ferro com substituição por alumínio. De acordo com as razões Ald/Fed, é provável que a mobilização de ferro em todos os solos tenha ocorrido sob condições de redução, facilitada por matéria orgânica na superfície.

Termos de indexação: Plintossolo, Ustalf, Grupo Barreiras, Formação Itapecuru, citrato ditionito, oxalato, difração de raios-X, alumínio, goethita, hematita.

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Full text available only in PDF format.

ACKNOWLEDGMENT

The authors acknowledge the financial support of the Canadian International Development Agency, as well as of the Natural Sciences and Engineering Research Council of Canada and the University of Saskatchewan, Saskatoon, Canada, for the graduate scholarship award to the first author. The participation of representatives of the Serviço Nacional de Levantamento e Conservação de Solos of the Empresa Brasileira de Pesquisa Agropecuária, in Recife (pedologists Dr. Antônio Cabral Cavalcanti and Sergio Costa Pinto) and their provision of routine analytic services are also appreciated. We also thank the participation of Dr. Paulo Klinger Tito Jacomine in the trip to collect the samples.

LITERATURE CITED

Recebido para publicação em setembro de 1997

Aprovado em janeiro de 1999

AHMED, N. & JONES, R.L. A plinthaquult of the Aripo Savannas, North Trinidad II. Mineralogy and genesis. Soil Sci. Soc. Am. Proc., 33:765-768, 1969.
AMBROSI, J.P.; NAHON, D. & HERBILLON, A.J. The epigenetic replacement of kaolinite by hematite in laterite-petrographic evidence and the mechanism involved. Geoderma, 37:283-294, 1986.
AMOURIC, M.; BARONNET, A.; NAHON, D. & DIDIER, P. Electron microscopic investigations of iron oxyhydroxides and accompanying phases in lateritic iron-crust pisolites. Clays Clay Miner., 34:45-52, 1986.
ARDUINO, E.; BARBERIS, E. & BOERO, V. Iron oxides and particle aggregation in B horizons of some Italian soils. Geoderma, 45:319-329, 1989.
BARBERIS, E.; AJOME MARSAN, F.; BOERO, V. & ARDUINO, E. Aggregation of soil particles by iron oxides on various size fractions of soil B horizons. J. Soil Sci., 42:535-542, 1991.
BATISTA, M.A. & SANTOS, M.C.D. Morfologia e gênese de dois solos com plintita da região Meio-Norte do Brasil. R. Bras. Ci. Solo, 19:287-296,1995.
BLUME, L.J.; PERKINS, H.F. & HUBBARD, R.K. Subsurface water movement in an upland coastal plain soil as influenced by plinthite. Soil Sci. Soc. Am. J., 51:774-779, 1987.
BOAGGARD, O.K. Kinetics and mechanisms of soil oxide dissolution in EDTA, oxalate and dithionite. Sci. Geol. Mem., 85:139-148, 1990.
BOSSI, G.E.; ROLIM, J.L. & ANDREIS, R.R. El grupo Barreiras en el noreste Brasileño. In: CONGRESSO LATINOAMERICANO DE GEOLOGIA, 5., Buenos Aires, 1982. Actas. Buenos Aires, 1982. v.1. p.173-190.
BRINKMAN, R. Ferrolysis, a hydromorphic soil forming process. Geoderma, 3:199-206, 1970.
CARLAN, W.L.; PERKINS, H.F. & LEONARD, R.A. Movement of water in a Plinthic Paleudult using a bromide tracer. Soil Sci., 139:62-66, 1985.
COMERMA, J.A.; ESWARAN, H. & SCHWARTMANN, U. A study of plinthite and ironstone from Venezuela. In: INTERNATIONAL CONFERENCE ON CLASSIFICATION AND MANAGEMENT OF TROPICAL SOIL, Kuala Lumpur, 1977. Proceedings. Kuala Lampur, 1977. p.18-26.
DANIELS, R.B.; PERKINS, H.F.; HAJEK, B.F. & GAMBLE, E.E. Morphology of discontinuous phase plinthite and criteria for its field identification in the Southeastern United States. Soil Sci. Soc. Am. J., 42:944-949, 1978.
DOS ANJOS, L.H.C.; FRANZMEIER, D.P. & SCHULZE, D.G. Formation of soils with plinthite on a toposequence in Maranhão State, Brazil. Geoderma, 64:257-279, 1995.
EMPRESA BRASILEIRA DE PESQUISA AGROPECUÁRIA - EMBRAPA. Sistema brasileiro de classificação de solos : 4^a Aproximação. Rio de Janeiro, 1997. 169p.
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EMPRESA BRASILEIRA DE PESQUISA AGROPECUÁRIA - EMBRAPA. Manual de métodos de análise do solo. Rio de Janeiro, 1979. 227p.
ESWARAN, H.; COMERMA, J. & SOORYANARYANAN, V. Scanning electron microscopic observations on the nature and distribution of iron minerals in plinthite and petroplinthite. In: LATERITIZATION PROCESSES, INTERNATIONAL SEMINAR ON LATERIZATION PROCESSES, New Delhi, 1980. Proceedings. Oxford, IBH Publ., 1980. p.335-341.
ESWARAN, H.; DECONINCK, F. & VARGHESE, T. Role of plinthite and related forms in soil degradation. In: LAL, R.R. & STEWART, B.A., eds. Soil degradation. Adv. Soil Sci., 11:109-127, 1990.
FITZPATRICK, R.W. & SCHWERTMANN, U. Al-substituted goethite- an indicator of pedogenic and other weathering environments in South Africa. Geoderma, 27:335-347, 1982.
HECK, R.J. The nature, distribution and genetic interpretations of iron oxides in Brazilian Plinthic soils. Saskatoon, University of Saskatchewan, 1994. 275p. (Tese de Doutorado)
JACKSON, M.L. Soil chemical analysis-advanced course. 2.ed. Madison, 1979. 895p.
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JEANROY, E.; RAJOT, J.L.; PILLON, P. & HERBILLON, A.J. Differential dissolution of hematite and goethite in dithionite and its implication on soil yellowing. Geoderma, 50:79-94, 1991.
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SCHWERTMANN, U. Inhibitory effect of soil organic matter on the crystallization of amorphous ferric hydroxide. Nature, 212:645-646, 1966.
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SCHWERTMANN, U. & TAYLOR, R.M. Iron Oxides. In: DIXON, J.B. & WEED, S.B., eds. Minerals in soil environments. 2.ed. Madison, Soil Science Socety of American, 1989. p.379-438 (SSSA Book Series, 1)
SHELDRICK, B.H. Analytical methods manual. 1984. Ottawa, Research Branch Agriculture Canada, 1984. 188p. (Land Resource Research Institute, 84-30)
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SOIL SURVEY STAFF. Keys to soil taxonomy, 6.ed. Washington, SMSS Technical Monograph USDA-SCS, 1994. 306p.
STUMM, W. & MORGAN, J.J. Aquatic chemistry. An introduction emphasizing chemical equilibria in natural waters. New York, John Wiley & Sons, 1981. 780p.
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van BREEMEN, N. Long-term chemical, mineralogical, and morphologal effects of iron-redox processes in periodically flooded soils. In: STUCKI, J.W., GOODMAN, B.A. & SCHWERTMANN, U., eds. Iron in soils and clay minerals. Dordrecht, D. Reidel Publishing, 1988. p.811-823. (NATO ASI Series)
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Iron oxides in plinthic soils on sedimentary deposits in northeastern Brazil

Óxidos de ferro em solos plínticos de depósitos sedimentares no nordeste do Brasil

Publication Dates

Publication in this collection
07 Oct 2014
Date of issue
Sept 1999

History

Received
Sept 1997
Accepted
Jan 1999

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

[1] AHMED, N. & JONES, R.L. A plinthaquult of the Aripo Savannas, North Trinidad II. Mineralogy and genesis. Soil Sci. Soc. Am. Proc., 33:765-768, 1969.

[2] AMBROSI, J.P.; NAHON, D. & HERBILLON, A.J. The epigenetic replacement of kaolinite by hematite in laterite-petrographic evidence and the mechanism involved. Geoderma, 37:283-294, 1986.

[3] AMOURIC, M.; BARONNET, A.; NAHON, D. & DIDIER, P. Electron microscopic investigations of iron oxyhydroxides and accompanying phases in lateritic iron-crust pisolites. Clays Clay Miner., 34:45-52, 1986.

[4] ARDUINO, E.; BARBERIS, E. & BOERO, V. Iron oxides and particle aggregation in B horizons of some Italian soils. Geoderma, 45:319-329, 1989.

[5] BARBERIS, E.; AJOME MARSAN, F.; BOERO, V. & ARDUINO, E. Aggregation of soil particles by iron oxides on various size fractions of soil B horizons. J. Soil Sci., 42:535-542, 1991.

[6] BATISTA, M.A. & SANTOS, M.C.D. Morfologia e gênese de dois solos com plintita da região Meio-Norte do Brasil. R. Bras. Ci. Solo, 19:287-296,1995.

[7] BLUME, L.J.; PERKINS, H.F. & HUBBARD, R.K. Subsurface water movement in an upland coastal plain soil as influenced by plinthite. Soil Sci. Soc. Am. J., 51:774-779, 1987.

[8] BOAGGARD, O.K. Kinetics and mechanisms of soil oxide dissolution in EDTA, oxalate and dithionite. Sci. Geol. Mem., 85:139-148, 1990.

[9] BOSSI, G.E.; ROLIM, J.L. & ANDREIS, R.R. El grupo Barreiras en el noreste Brasileño. In: CONGRESSO LATINOAMERICANO DE GEOLOGIA, 5., Buenos Aires, 1982. Actas. Buenos Aires, 1982. v.1. p.173-190.

[10] BRINKMAN, R. Ferrolysis, a hydromorphic soil forming process. Geoderma, 3:199-206, 1970.

[11] CARLAN, W.L.; PERKINS, H.F. & LEONARD, R.A. Movement of water in a Plinthic Paleudult using a bromide tracer. Soil Sci., 139:62-66, 1985.

[12] COMERMA, J.A.; ESWARAN, H. & SCHWARTMANN, U. A study of plinthite and ironstone from Venezuela. In: INTERNATIONAL CONFERENCE ON CLASSIFICATION AND MANAGEMENT OF TROPICAL SOIL, Kuala Lumpur, 1977. Proceedings. Kuala Lampur, 1977. p.18-26.

[13] DANIELS, R.B.; PERKINS, H.F.; HAJEK, B.F. & GAMBLE, E.E. Morphology of discontinuous phase plinthite and criteria for its field identification in the Southeastern United States. Soil Sci. Soc. Am. J., 42:944-949, 1978.

[14] DOS ANJOS, L.H.C.; FRANZMEIER, D.P. & SCHULZE, D.G. Formation of soils with plinthite on a toposequence in Maranhão State, Brazil. Geoderma, 64:257-279, 1995.

[15] EMPRESA BRASILEIRA DE PESQUISA AGROPECUÁRIA - EMBRAPA. Sistema brasileiro de classificação de solos : 4^a Aproximação. Rio de Janeiro, 1997. 169p.

[16] EMPRESA BRASILEIRA DE PESQUISA AGROPECUÁRIA - EMBRAPA. Sistema brasileiro de classificação de solos. 3^a Aproximação. Rio de Janeiro, 1988. 122p.

[17] EMPRESA BRASILEIRA DE PESQUISA AGROPECUÁRIA - EMBRAPA. Manual de métodos de análise do solo. Rio de Janeiro, 1979. 227p.

[18] ESWARAN, H.; COMERMA, J. & SOORYANARYANAN, V. Scanning electron microscopic observations on the nature and distribution of iron minerals in plinthite and petroplinthite. In: LATERITIZATION PROCESSES, INTERNATIONAL SEMINAR ON LATERIZATION PROCESSES, New Delhi, 1980. Proceedings. Oxford, IBH Publ., 1980. p.335-341.

[19] ESWARAN, H.; DECONINCK, F. & VARGHESE, T. Role of plinthite and related forms in soil degradation. In: LAL, R.R. & STEWART, B.A., eds. Soil degradation. Adv. Soil Sci., 11:109-127, 1990.

[20] FITZPATRICK, R.W. & SCHWERTMANN, U. Al-substituted goethite- an indicator of pedogenic and other weathering environments in South Africa. Geoderma, 27:335-347, 1982.

[21] HECK, R.J. The nature, distribution and genetic interpretations of iron oxides in Brazilian Plinthic soils. Saskatoon, University of Saskatchewan, 1994. 275p. (Tese de Doutorado)

[22] JACKSON, M.L. Soil chemical analysis-advanced course. 2.ed. Madison, 1979. 895p.

[23] JACOMINE, P.K.T. Levantamento exploratório-reconhecimento de solos do estado do Piauí. Rio de Janeiro, EMBRAPA/SNLCS. 1986a. 782p. (Boletim de Pesquisa - EMBRAPA/SNLCS, 36)

[24] JACOMINE, P.K.T. Levantamento exploratório-reconhecimento de solos do Estado do Maranhão. Rio de Janeiro, 1986b. 964p. (Boletim de Pesquisa - EMBRAPA/SNLCS, 35)

[25] JEANROY, E.; RAJOT, J.L.; PILLON, P. & HERBILLON, A.J. Differential dissolution of hematite and goethite in dithionite and its implication on soil yellowing. Geoderma, 50:79-94, 1991.

[26] KRISHNASWAMY, V.S., ed. LATERIZATION PROCESSES, INTERNATIONAL SEMINAR ON LATERIZATION, Trivandrum, 1979. Proceedings. Rotterdam, A.A. Balkema, 1981. 450p.

[27] McFARLANE, M.J. Laterite and landscape. London, Academic Press, 1976. 151p.

[28] McFARLANE, M.J. Laterites. Some aspects of current research. In: LATERITE WORKSHOP, INTERNATIONAL GEOMORPHOLOGICAL CONFERENCE, 1., Manchester, 1985. Proceedings. Manchester, Zeitschrift für Geomorphologie, 1987. 180p. (Suppl.-Bd., 64)

[29] MELFI, A.J. & CARVALHO, A., eds. Laterization processes. In: INTERNATIONAL SEMINAR ON LATERIZATION PROCESSES, 2., 1982. São Paulo, Proceedings. São Paulo, 1983. 590p.

[30] OGURA, Y., ed. INTERNATIONAL SEMINAR ON LATERITE, 1985. Proceedings. Tokyo, Chem. Geol., 60, 1987.

[31] SANCHEZ, P.A. & BUOL, S.W. Soils of the tropics and the world food crisis. Sci., 188:598-603, 1975.

[32] SCHWERTMANN, U. & KÄMPF, N. Properties of goethite and hematite in kaolinitic soils of southern and central Brazil. Soil Sci., 139:344-350, 1985.

[33] SCHWERTMANN, U. Inhibitory effect of soil organic matter on the crystallization of amorphous ferric hydroxide. Nature, 212:645-646, 1966.

[34] SCHWERTMANN, U. & FANNING, D.S. Iron-manganese concretions in hydrosequences of soils in loess in Bavaria. Soil Sci. Soc. Am. J., 40:731-738, 1976.

[35] SCHWERTMANN, U. & TAYLOR, R.M. Iron Oxides. In: DIXON, J.B. & WEED, S.B., eds. Minerals in soil environments. 2.ed. Madison, Soil Science Socety of American, 1989. p.379-438 (SSSA Book Series, 1)

[36] SHELDRICK, B.H. Analytical methods manual. 1984. Ottawa, Research Branch Agriculture Canada, 1984. 188p. (Land Resource Research Institute, 84-30)

[37] SOIL SURVEY STAFF. Soil taxonomy: A basic system of soil classification for making and interpreting soil surveys. Washington, U.S. Govt. Printing Office, 1975. 754p. (U.S. Dept. Agric. Handbook, 436)

[38] SOIL SURVEY STAFF. Keys to soil taxonomy, 6.ed. Washington, SMSS Technical Monograph USDA-SCS, 1994. 306p.

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[40] TORRENT, J.; SCHWERTMANN, U. & BARRON, V. The reductive dissolution of synthetic goethite and hematite in dithionite. Clay Miner., 22:329-337, 1987.

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