Cohesive character in Alfisols, Ultisol and Oxisols in northeast of Brazil: Relationship with tensile strength and particle size

The objective of this study was to evaluate the contribution of the constituents sand, silt and clay to the tensile strength and, by extension, to the genesis of horizons with cohesive character, as well as to know the variability of these attributes in the soil horizon. Horizons with and without cohesive character were selected in six soils, in which samples were collected at the top, middle and base in blocks with dimensions of 0.15 × 0.20 × 0.10 m for width, length and height, respectively. Aggregates/clods were obtained from the collected material and used in the tensile strength tests. Additionally, the contents of sand (with fractionation in five classes: very coarse, coarse, medium, fine and very fine), silt and clay were quantified. 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). Sand fractionation data were analyzed using the microcomputer program PHI, which calculates different statistical parameters (relative frequency, cumulative frequency, mean and standard deviation). The degree of sorting of the sands was defined based on the standard deviation (σ) of the phi values, namely: very well sorted, well sorted, moderately sorted, poorly sorted, very poorly sorted and extremely poorly sorted. Regression analyses were also performed between the mean values of tensile strength and the particle-size fractions and standard deviation of the phi values relative to sand. Tensile strength was higher in horizons with cohesive character, reaching mean value of 49.9 kPa, followed by the values of soils without this attribute (mean value of 34.1 kPa). The horizons with and without cohesive character showed significant variations in total sand contents, but without differing in relation to the textural class – sandy clay loam for five of the soils evaluated. It was concluded that particle size alone does not explain the manifestation of the cohesive character in soils, but the constituents sand, silt and clay have a significant effect on its genesis; because it causes more closed packaging, poor sorting of sand grains is one of the primary factors in the genetic process of cohesive character in soils. There is a significant variation in the amount of sand and silt in at least one of the top, middle and bottom positions when the cohesive and non-cohesive B horizons are analyzed. In general, in horizons with a cohesive character the tensile strength of aggregates/clods decreases from top to bottom.