Granulometria de solos: horizontes com caráter coeso e sua resistência tênsil

Abstract

Soils of the Coastal Boards are an important factor of economic and social development in the coastal region of Brazil. However, these soils manifest some chemical and physical limitations. With respect to physical limitations, one of the most expressive is the occurrence of cohesive horizons, whose genesis is still not well understood. In this research, we hypothesized that a) the cohesive character in soils, whose magnitude of cohesion presents a direct and proportional correlation with the tensile strength of the aggregates/clods, has its genesis influenced by the granulometry and poor selection of the sand fraction and b) That there is variation in the tensile strength at the horizon of the soil. The objective of this study was to evaluate the contribution of particle size and particle distribution of the sand fraction in the genesis of cohesive horizons in the Coastal Boards of Ceará, as well as to know the variability of tensile strength in the soil horizon. The study was carried out in the city of Aquiraz-CE, where samples of two soils (Argissolo Vermelho-Amarelo Distrófico típico - PVAd; Argissolo Amarelo Eutrocoeso abrúptico - PAex) were collected at the Bt1 (cohesive) and Bt2 (non-cohesive) horizons at the top, middle and base of each horizon. Analyzes of granulometry (with sand fractionation) and tensile strength were performed in aggregates/clods. The data of each soil profile were analyzed, considering a completely randomized design, in a 2 x 3 factorial scheme (two degrees of cohesion: Bt cohesive and Bt non-cohesive), and three positions in the profile: top, middle and base, with five replications). They were submitted to the Shapiro-Wilk test for normality, the F test for the analysis of variance, and the Tukey test for the comparison of averages, all at 5% probability. The relationship between the tensile strength and each of the other physical attributes (sand, silt and clay fractions), considering all the data obtained, was measured from regression analysis (considering the linear regression model) and correlation. The t-test, at 5% probability, was used to evaluate the significance of the angular (b) and correlation (r) coefficients. The results showed that in all horizons (cohesive and non-cohesive) the sand fraction was predominant, defining with the silt and clay fractions the sandy clay loam in the horizons Bt1 and Bt2 of the Argissolo Vermelho-Amarelo Distrófico típico (PVAd) and sandy clay texture in the horizons Bt1 and Bt2 of the Argissolo Amarelo Eutrocoeso abrúptico (PAex). With regard to the fractionation of the sands, the predominance of medium sand was observed for all the horizons investigated, and in the three positions in the horizons (top, middle and base), followed by order of fine and very fine sand together, coarse sand and very coarse sand. As for the tensile strength values, they were higher in cohesive horizons, with decreasing the tensile strength of the aggregates/tops of the top towards the base. It was concluded that the sand and clay fractions influence the genesis of cohesive character in soils. Regarding the sand fraction, the poor selection of grains competes for more densely packed particles, contributing significantly to the cohesion of the aggregates/clods. There is significant variation of tensile strength in the soil horizon.