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Soil Physics

Influence of Sodicity, Clay Mineralogy, Prewetting Rate, and Their Interaction on Aggregate Stability

^a Colegio de Postgraduados, Campus San Luis Potosí, Iturbide 73, Salinas, S.L.P. 78600, Mexico
^b Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521, USA

^* Corresponding author (Laosheng.wu{at}ucr.edu)

Sodicity adversely affects soil physical conditions reflected by weak structural stability. Soil clay mineralogy influences the degree of aggregate disruption induced by sodicity. The main objective of this research was to evaluate the interactive effects of clay mineralogy, prewetting rate (PWR), and sodicity on soil aggregation. Three soils with predominantly clay minerals of smectite, vermiculite-smectite, and kaolinite were equilibrated with NaCl–CaCl₂ solutions having sodium adsorption ratio (SAR) values of 0, 20, and 50 and electrical conductivity (EC) = 3.0 dS m^–1. After air-drying, the treated samples were packed and prewetted at rates of 2 and 30 mm h^–1 with the NaCl–CaCl₂ solutions. Aggregate-size distribution, macroscopic swelling, and surface soil dispersion were determined after the packed samples were equilibrated to a matric potential of –0.1 MPa. The kaolinitic soil showed the lowest inherent aggregate stability when subjected to slow PWR and the lowest SAR. Furthermore, aggregate stability of the kaolinitic loam soil was not significantly affected by increasing SAR. The Millox (smectitic) soil, on the other hand, was most susceptible to aggregate slaking, whereas the Malibu (vermiculitic) soil was most susceptible to differential swelling (at slow PWR). When SAR was low, aggregate slaking by fast PWR was the main cause of the aggregate breakdown. At SAR 20, swelling and dispersion became more important to the structural stability.

Abbreviations: EC, electrical conductivity • ESP, exchangeable sodium percentage • OM, organic matter • PWR, prewetting rate • SAR, sodium adsorption ratio • TEC, total electrolyte concentration