Soil structural stability affects the profitability and sustainability of agricultural systems. Different-sized structural units have different stability mechanisms and respond differently to such external factors as rain, wind, irrigation and management. A comprehensive analysis of the soils structural stability requires its characterization at the macroand micro-aggregate scales. We determined the aggregate stability of 36 soils at the macro-aggregate scale using wetsieving methods and of 20 soils at the micro-aggregate scale using laser-ray diffraction techniques. All the tests gave consistent estimates of aggregate stability. Most soils were homogeneous and quite stable at the macro-aggregate level as determined by the «water stable aggregate» parameter, but differed significantly among them and were quite unstable at the micro-aggregate level as determined by the «mean weight diameter of micro-aggregates» parameter. Slaking induced by the fast wetting of aggregates was the main destabilizing mechanism in these soils (88% of the soils had slaking stability index values < 0.5), whereas most soils were quite tolerant to the mechanical shaking of aggregates (89% of the soils had stirring stability index values > 0.5). The combination of the macro-and micro-aggregate stability tests is a consistent way for describing the structural stability of the studied soils.Key words: soil structural stability, wetting, stirring, aggregate-breakdown, slaking stability index, stirring stability index. ResumenDeterminación de la estabilidad de macro-y micro-agregados del suelo mediante una combinación de técnicas de tamizado en húmedo y difracción de rayos láser La estabilidad estructural del suelo afecta a la rentabilidad y sostenibilidad de los agrosistemas. Los agregados de distintos tamaños son estabilizados por mecanismos diferentes, y responden de forma diferente frente a la lluvia, el viento, el riego y otras prácticas agronómicas. Un análisis completo de la estabilidad estructural de los suelos requiere su caracterización a nivel de macro-y micro-agregados. En este trabajo se ha determinado la estabilidad de macroagregados de 36 suelos mediante métodos de tamizado en húmedo, y la estabilidad de microagregados de 20 suelos mediante técnicas de difracción de rayos láser. Todos los ensayos estimaron de forma consistente la estabilidad de agregados. La mayoría de los suelos se comportaron homogéneamente y de forma bastante estable al nivel de macroagregados, de acuerdo con los valores del parámetro «agregados estables en agua», mientras que se comportaron de forma muy diferente y muy inestable al nivel de microagregados, de acuerdo a los valores del parámetro «diámetro medio ponderado de los microagregados». La desagregación inducida por la humectación rápida de los agregados (slaking) fue el mecanismo más desestabilizador en estos suelos (el 88% de los suelos presentó índices de estabilidad frente a slaking < 0,5), mientras que la mayoría de los suelos fueron bastante estables frente a la agitación mecánica de lo...

Surface sealing and crusting is a common feature in many irrigated soils that negatively affects their productivity and sustainability. Soil crusting is provoked by the mechanical breakdown of aggregates and/or by the physical and chemical dispersion of soil clays. Our objectives were to examine the clay-dispersive behavior in 36 illitic soils of the middle Ebro river basin (Spain) as affected by irrigation and rainwater quality, soil wetting and mechanical stress, and identify major soil properties affecting clay-dispersion behavior. The applicability to these soils of the Sumner et al. (1998) flocculation-dispersion nomogram was also analyzed. Three laboratory tests (i.e., flocculation, spontaneous dispersion and mechanical dispersion of clays) were applied to the air-dry, ground and sieved soil samples (0-20 cm depth). Based on their flocculation values, 10 and 32 soils will tend to chemically disperse, respectively, when equilibrated with the electrolyte concentrations of the irrigation (4 mmol c L -1 ) and rain (1 mmol c L -1 ) waters. None of the studied soils were susceptible to spontaneous clay dispersion when wetted in the absence of mechanical stress, whereas nearly 75% of them were mechanically dispersive and thus potentially sensitive to crusting by impacting raindrops or intensive cultivation. The Sumner et al. (1998) flocculation-dispersion nomogram classified adequately the clay-dispersive behavior in 92% of the studied soils, demonstrating its appropriate predictive capability for these illitic soils. Various conceptually-consistent relationships were found between some soil properties and the dispersive behavior of soil clays, but their statistical significances were insufficient for predicting purposes.Key words: clay dispersion, crusting, water quality, wetting, mechanical disturbance, soil properties. Resumen Dispersión química, espontánea y mecánica de arcillas en suelos de zonas áridasEl encostramiento del suelo es un problema frecuente en muchos regadíos, que afecta negativamente a su productividad/sostenibilidad. Es producido por la ruptura mecánica de agregados y/o por la dispersión física y química de arcillas. Nuestros objetivos fueron analizar el comportamiento dispersivo de 36 suelos ilíticos de la cuenca media del Ebro (España) frente a la calidad de las aguas de riego y lluvia, la humectación y el estrés mecánico, e identificar las principales características del suelo responsables de dicho comportamiento. Asimismo se analizó la aplicabilidad a estos suelos del nomograma de floculación-dispersión de Sumner et al. (1998). El grado de dispersibilidad de los suelos se determinó mediante tres pruebas de laboratorio (tests de floculación y de dispersión espontánea y mecánica de arcillas) aplicadas a muestras de suelo (0-20 cm de profundidad) secas al aire y tamizadas. En base a los valores de floculación, 10 y 32 suelos fueron susceptibles de dispersión química cuando se equilibraron, respectivamente, con las aguas de riego (4 mmol c L -1 ) y lluvia (1 mmol c L -1 ). Ningún suel...

Darcy's law is applied to describe the steady flow processes in which the flux remains constant with time along the conducting system. Due to the dispersion and migration of colloidal particles and lodging in the soil pores the reduction in hydraulic conductivity occurs with time in particular when the soil and the percolating solution are affected by electrolyte concentration. Hence, the aim of this study is to find empirical equations that can be used to predict the flux with time. Data for the effluent volume versus time (up to 6 hours) which was collected for three soils (located at Quevedo-Los Rios region) treated by two salt solutions (5 and 50 meq/l) with different SAR values were used to test certain mathematical forms of equations. Only four empirical equations were found to perfectly fit the data (flux vs time) whereas, fitting the calculated and measured data of the hydraulic conductivity for all soils produced regression factors R 2 ≥ 0.99. So, these equations can be applied to predict the hydraulic conductivity and to characterize the flow process at saturated conditions of the studied soils with great confidence. The Hoerl function model was the best of all equations for application as the fitting degrees were almost perfect for all studied soils at 5 and 50 meq/l. It was observed for all equations that one of the fitting parameters would always represent the initial hydraulic conductivity (K o s) that was evaluated graphically at zero time by extrapolation.