Propriedades reológicas de solos afetadas pela matéria orgânica, área superficial específica e salinidade

Abstract

Soil structure consists of the geometric arrangement of particles in aggregates. It is a dynamic property and (especially) together with texture influences several soil physical processes. For aggregation and structure formation, soil particles must be flocculated, which depends on some factors such as soil organic carbon (SOC) content, specific surface area (SSA), and concentration and type of cations. The objective of the study was to evaluate the effect of SOC, cation type, and of SSA on the resistance of soil microstructure evaluated by rheometry. Three separate studies were carried out. In study I to verify the effect of SOC, samples from ten Archaeological Black Earths (ABE) were collected in the Amazon region. The samples were submitted to four levels of SOC oxidation by the addition of 0 (C0), 40 (C1), 80 (C2) and 120 (C3) ml of concentrated hydrogen peroxide to approximately 100 g of soil. In study II, to verify the effect of ASE, samples of 13 soils with different granulometric compositions and great variation in ASE were collected in Rio Grande do Sul. To verify the effect of cation type in study III, samples of 16 horizons of four salinized soil profiles were collected in Rio Grande do Norte. The treatments in this study were: soil saturated by capillarity with saline solution KCl (+K), CaCl2 (+Ca) and MgCl2 (+Mg) in the concentration of 0.1 mol L-1, Leaching of soluble salts (LS) by successive leaching with alcohol 60% and untreated soil (control). All the treatments were submitted to an amplitude sweep test with controlled deformation in a modular rheometer equipped with parallel plate measuring device. As a result, the following rheological variables were obtained: strain (γLVR) and shear stress (τLVR) at the end of the linear viscoelastic range (LVR), strain (γYP) and storage modulus value (G’YP) in yield point, the maximum shear stress (τmax) and integral Z (Iz). In study I, the SOC oxidation affected all rheological variables, reducing the elasticity and resistance of soil microstructure. The effect of SOC loss was different among ABEs, according to the amount and quality of SOC lost. In study II, the increase in SSA increased the elasticity verified by γLVR, γyp and Iz, but the formation of very stable microaggregates (pseudosands) in soils with high SSA provided reduction of these variables. The microstructural stiffness, evaluated by τLVR, G’YP and τmax, was not correlated with SSA, but was influenced by particle size distribution, normal force acting on the sample, and cation concentration. In study III, soil desalination caused an increase in microstructural elasticity verified in LVR and reducing elasticity in YP when compared with the control, whereas the saturation by cations, mainly by K+, provided an opposite effect, increasing the microstructural stiffness. Salt leaching and saline saturation increased soil resistance.