Tensile strength in horizons with and without cohesive character: Variability and relation with granulometry

Soils of Coastal Tablelands are important for the economic and social development of the coastal region of Brazil. However, these soils appear to have a number of chemical and physical limitations. One of the most expressive physical limitations is the occurrence of cohesive character, whose genesis is still not fully understood. Thus, considering that the tensile strength is a direct measure of the cohesion of particles, this research was undertaken to discover how the size of soil particles influences the genesis of the cohesive character in Bt horizons of the Coastal Tablelands of Ceará, Brazil. The study was carried out in the municipality of Aquiraz-CE, where samples of two soils were collected (Argissolo Vermelho-Amarelo Distrófico típico - PVAd; Argissolo Amarelo Eutrocoeso abrúptico - PAex) in the horizons Bt1 (cohesive) and Bt2 (non-cohesive) in the top, middle and bottom portions. The results demonstrated that the size of the particles led to sandy clay loam texture in horizons of PVAd and sandy clay texture in the horizons of PAex. Regarding sand sorting, for all investigated horizons and in the three positions (top, middle and bottom), the medium sand fraction prevailed, followed successively by fine and very fine sands together, coarse and very coarse sands. Tensile strength values were higher in cohesive horizons, with reduction in the tensile strength of aggregates from top to bottom. It was concluded that tensile strength was a useful indicator for differentiating cohesive properties in the different soil horizons. There was significant variation of tensile strength in the soil horizon defined by the pedologist - which suggests that it is necessary to include as a valuable addition to the diagnostic tools available to the working pedologist, a tensile strength scale for the identification of cohesion to avoid subjectivity in the definition of the cohesive horizon. For the studied set of horizons, tensile strength decreased from top to bottom. Sand and clay fractions, despite not being the only determinant factor, influenced the genesis of the cohesive character in soils. For the sand fraction, poor degree of sand sorting led to a denser packing of particles, significantly contributing to the cohesion of aggregates. Our experiment strongly suggests that the packing of particles allows the increase of cohesion provided there is a cementing agent even in small quantity.