Mineralogy of the clay fraction of Alfisols in two slope curvatures: III - spatial variability

Abstract

SUMMARYA good knowledge of the spatial distribution of clay minerals in the landscape facilitates the understanding of the influence of relief on the content and crystallographic attributes of soil minerals such as goethite, hematite, kaolinite and gibbsite. This study aimed at describing the relationships between the mineral properties of the clay fraction and landscape shapes by determining the mineral properties of goethite, hematite, kaolinite and gibbsite, and assessing their dependence and spatial variability, in two slope curvatures. To this end, two 100 × 100 m grids were used to establish a total of 121 regularly spaced georeferenced sampling nodes 10 m apart. Samples were collected from the layer 0.0-0.2 m and analysed for iron oxides, and kaolinite and gibbsite in the clay fraction. Minerals in the clay fraction were characterized from their X-ray diffraction (XRD) patterns, which were interpreted and used to calculate the width at half height (WHH) and mean crystallite dimension (MCD) of iron oxides, kaolinite, and gibbsite, as well as aluminium substitution and specific surface area (SSA) in hematite and goethite. Additional calculations included the goethite and hematite contents, and the goethite/(goethite+hematite) [Gt/(Gt+Hm)] and kaolinite/ (kaolinite+gibbsite) [Kt/(Kt+Gb)] ratios. Mineral properties were established by statistical analysis of the XRD data, and spatial dependence was assessed geostatistically. Mineralogical properties differed significantly between the convex area and concave area. The geostatistical analysis showed a greater number of mineralogical properties with spatial dependence and a higher range in the convex than in the concave area.Index terms: iron oxides, kaolinite, gibbsite, goethite, hematite, relief.

Modelagem geoestatística das incertezas da distribuição espacial do fósforo disponível no solo, em área de cana-de-açúcar

“…Essa alta variabilidade indica que se a aplicação de fertilizantes for feita considerando apenas a média dos teores de P lábil , a dose recomendada será adequada em alguns locais, porém em outros será sub ou superdimensionada. Essa grande amplitude de variação dos teores de P lábil está relacionada à forma de aplicação do fertilizante, que, como no caso em estudo, é geralmente realizada no sulco de plantio e na linha por meio do uso da torta de filtro (resíduo da indústria do açúcar), que apresenta teores variáveis de P; e à pouca mobilidade do P, o que pode causar acúmulo desse elemento, justificando os valores mais altos, bem como o fenômeno de adsorção desse nutriente aos óxidos de Fe e Al, uma vez que esses óxidos também apresentaram variabilidade nessa área (Silva Júnior et al, 2012;Camargo et al, 2013a), justificando os valores mais baixos (Barbieri et al, 2009;Camargo et al, 2013b). Dessa forma, esses são os principais aspectos que contribuem para uma distribuição irregular no solo.…”

Mineralogy of the clay fraction of alfisols in two slope curvatures: IV - spatial correlation with physical properties

“…The models fitting the cross-semivariograms best were gaussian or spherical. The nugget effects of the crosssemivariograms for the physical properties were smaller than of the simple semivariograms for the same properties (Camargo et al, 2010(Camargo et al, , 2013a …”

“…Although some physical and mineralogical properties in the concave area were spatially dependent (Camargo et al, 2010;2013a), there was no spatial correlation between them. This was a result of most of the physical properties in the concave area without spatial dependence.…”

“…X-ray diffraction patterns were obtained, using an equipment with a cobalt anode and an iron filter for hematite (Hm) and goethite (Gt) Tables 2 and 3 show the mean values of the clay mineralogical and physical properties in the concave and convex areas. A more detailed description of the method used to characterize the physical and mineralogical properties was given by Camargo et al (2010) and Camargo et al (2013a), respectively. Separated materials and stable aggregates were determined by the method of Kemper & Chepil (1965).…”

“…The correlation between soil properties was studied based on Pearson correlation coefficients and spatial correlation by cross-semivariograms for the specific variables with spatial dependence (Camargo et al, 2010(Camargo et al, , 2013a. Correlations were positive when the increase in one property caused an increase in the others and negative if an increase in one led to a decrease in the others (Bhatti et al, 1991;Ceddia et al, 2009;Camargo et al, 2013b).…”