Probable causes of hardening of redoximorphic features in Plinthosols of the Araguaia River floodplain, Central region of Brazil

Abstract

This study aimed to investigate the possible causes or conditions for the greater hardening or more hardened consistency of the plinthic feature, particularly in relation to forms and quantities of iron both in wet and dry conditions, taking into account that variations in the degree of consistency of these features are a phenomenon of great importance in the behavior of the soils of the Araguaia River floodplain, Brazil, due to its wide agricultural use. Soil features were collected at five positions in the profiles, where initial, central, and base plinthic horizons were detected. The separated samples of matrix, mottle, and plinthite were broken by the Instron apparatus and ground into powder. Then, chemical determinations were carried out by X-ray fluorescence (XRF), sulfuric acid attack (H2SO4), citrate-bicarbonate-dithionite (CBD), acid ammonium oxalate, and X-ray diffraction. The greater hardness of the plinthic features of the soils is directly related to the iron content in the oxidized form contained in its fine fraction and iron oxides are the elements responsible for promoting and maintaining the hardening of the features. The presence of the plinthic feature with the adequate hardness requirements was observed in the cases that presented a combination of the following factors: seasonality of the water regime; absolute Fed contents of at least 40.0 g kg−1 associated with a Fed/Fes ratio of at least 0.35; and Fes values equal to or >100 g kg−1 associated with a Fes/Fet ratio of at least 0.70. High Feo/Fed ratios, above 0.07, are indicative of more unstable features and, at the same time, with greater potential for hardening after drying.