Chemical, spontaneous and mechanical dispersion of clays in arid-zone soils

Abstract

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 mmolc L-1) and rain (1 mmolc 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.