Minerals in the clay fraction of Brazilian Latosols (Oxisols): a review

Abstract

This review focuses on the clay mineralogy of the most important Brazilian soils: the Latosols, which cover >60% of the country by area, and occur in association with other soils. They are typically deep, highly-weathered soils, dominated by low-activity 1:1 clay minerals and Fe and Al oxyhydroxides, with varying proportions of these minerals, depending on parent material and weathering intensity. They are usually of low fertility, although eutric types also occur. Latosols are generally correlated with Oxisols (American soil taxonomy) and Ferralsols (WRB system). Clay mineralogy is typically monotonous: kaolinite, gibbsite, hematite, goethite, maghemite and Ti minerals (mainly ilmenite and anatase) are the prominent mineral phases in the clay fraction. Some Latosols developing on basalt from southern Brazil contain significant amounts of hydroxyl-interlayed vermiculite. Among the pedogenic oxides the most frequent are goethite (α-FeOOH), indicated by yellowish colours (2.5Y–10YR; in the absence of hematite), and hematite (α-Fe2O3), which imbues reddish colors (2.5YR–5R), even when present in very minor amounts. Maghemite (γ-Fe2O3) is less frequent; it imparts a reddish-brown colour (5YR–2.5YR) and magnetic properties. Both goethite and hematite show Al-substitution, with a greater relative proportion in soil goethites. Hence, in similar drainage conditions, goethite is less prone to dissolution than hematite. Most reddish Latosols also contain maghemite, due to partial or complete oxidation of magnetite, which generally occurs naturally or is fire-induced. Magnetite and/or maghemite are associated with trace elements which are important in plant nutrition, such as Cu, Zn and Co. The contents of gibbsite in Latosols are extremely variable, from a complete absence in brown Latosols, to 54% in red Latosols from mafic rocks. Relatively large amounts of gibbsite are found in the clay fraction of these soils and this mineral is important in P sorption in deeply weathered Latosols in association with goethite and hematite. Even though most Latosols are dystrophic, some are eutrophic, revealing an unusually large base saturation in areas under ustic regimes where the parent material is particularly rich in bases, such as basalts. This eutrophic nature is attributed to the protecting role of micro-aggregates in ferric red Latosols, which retard baseleaching from the inner aggregate. At the other extreme, some Brazilian Latosols are acric and positively-charged in sub-surface horizons, as revealed by the relationship pH KCl > pH H2O. These acric Latosols are the result of long-term weathering and intensive leaching, during which pH tends to increase to values close to the zero point charge of Fe and Al oxides (between 6 and 7), greatly increasing P adsorption, which is mainly attributed to gibbsite, goethite and hematite. Soil kaolinites in Brazilian Latosols are mostly of low crystallinity, with Hughes and Brown indexes of between 6 and 15. In this review we have discussed the role of these clay-fraction minerals in soil genesis and fertility, highlighting the marked role of inheritance from deeply-weathered parent material. Latosols typically retain large amounts of Fe oxides, some of which are magnetic, with spontaneous magnetization >1 J T–1 kg–1. In this regard, reddish Latosols developed from mafic rocks are the most representative magnetic soils, and cover as much as 3.9% of Brazil. An overview of magnetic soils on four representative examples of mafic lithologies is presented, together with some aspects of their Fe-oxide mineralogy and related field and laboratory technqiues.

Topolitossequências de solos do Alto Paranaíba: atributos físicos, químicos e mineralógicos

“…Com o aumento da profundidade, os horizontes subsuperficiais dos Latossolos tornam-se eletropositivos ou com balanço de cargas próximo à zero, indicando uma mineralogia predominantemente gibbsítica/oxídica (Camargo et al, 1988;Schaefer et al, 2008 De acordo com as classes de interpretação propostas por Alvarez V. et al (1999), o P extraível por Mehlich-1 (Quadro 2) apresenta-se com valores variando de baixo a muito baixo na maioria dos horizontes dos solos estudados. As exceções são o perfil P7 de Cambissolo e o P3 de Neossolo Litólico, bem como os horizontes A do P5 (Latossolo Vermelho) e BC do P10 de Cambissolo, em que os valores situam-se nas classes boa e muito boa daqueles autores.…”

Relações pedomorfogeológicas nas chapadas elevadas do Distrito Federal

“…A composição mineralógica de solos formados em tais condições é, geralmente, constituída por assembleia de minerais cauliníticos e sesquióxidos de Fe e Al, conforme citado por Mugller et al (2007). Os Latossolos brasileiros, de modo geral, apresentam mineralogia da fração argila composta por caulinita, gibbsita, goethita e hematita, variando apenas nas suas proporções, em função do grau elevado de intemperização dos minerais primários (Curi & Franzmeier, 1984;Melo et al, 2001a,b;Resende et al, 2005;Schaeffer et al, 2008).…”

Pedology and plant physiognomies in the cerrado, Brazil

“…The soil is clayey throughout the profile, with clay contents varying from 43 dag/kg in the 0-10 cm layer, to 66 dag/kg in the 50-70 cm layer. The high silt contents do not indicate primary minerals, but rather the presence of pseudo-silt, which represents the clay fraction that could not be dispersed by the texture analysis method used here (Table III) (Schaefer et al 2008).…”