Cohesive character in Alfisols, Ultisol and Oxisols in Northeast of Brazil: Relationship of tensile strength with silicon, aluminum and iron from poorly crystalline compounds

Abstract

The expression cohesive character in soil horizons, regard to their consistency, which ranges from very hard to extremely hard when dry, changing to friable or firm when wet, has been attributed to several processes, many of which have not yet been fully clarified, hence the need for further studies in search of its elucidation. This study aimed to evaluate the contribution of silicon, aluminum and iron associated with poorly crystalline compounds to the cementation of cohesive horizons, as well as to know the variability of tensile strength within the soil horizon. In six soils, Alfisols, Ultisol and Oxisols, the horizons with and without cohesive character were sampled at the top, middle and base positions, collecting blocks with dimensions of 15 × 20 × 10 cm in width, length and height, respectively. The collected material was used to obtain the aggregates/clods, which were subjected to tensile strength tests, and, in addition, the contents of silicon, iron and aluminum constituting oxides of low crystallinity. Data analysis was carried out considering a completely randomized design, in a 6 × 2 × 3 factorial scheme (six soils, two degrees of cohesion, three positions in the horizon) with five replicates. F test was applied for the analysis of variance and Tukey test was used to compare the means (p ≤ 0.05). Regression analyses were also performed between the mean values of tensile strength and silicon, aluminum and iron extracted by ammonium oxalate. Tensile strength was higher in the horizons with cohesive character, reaching mean values of 49.9 kPa, followed by the values found in soils without this attribute (average 34.1 kPa). Silicon and aluminum contents were higher in the horizons with cohesive character, but iron did not follow this trend. The average values for the silicon content in horizons with cohesive character was 0,86 dg kg−1, in contrast with 0,54 dg kg−1 in non-cohesive horizons. The average value for the aluminum content was 4.18 dg kg−1 in horizons with cohesive character and 1.32 dg kg−1 in non-cohesive horizons. In general, it can be stated that all soils evaluated in this study had reduced iron contents, always below or close to 2 dg kg−1, except for the horizon without cohesive character in the Ultisol, evidencing the little expression of this chemical element in the genesis of the cohesive character in soils of Coastal Tablelands. It was concluded that the contents of silicon and aluminum associated with poorly crystalline compounds significantly contribute to the manifestation of cohesion as a diagnostic attribute; iron does not. There is significant variability of tensile strength, especially in the horizons described with cohesive character, with reduction of cohesion from the top to the base.