Friday, 14 July 2006 - 5:00 PM

Effects of Salinity, Sodicity, and Clay Mineralogy on Soil Physical and Hydraulic Properties.

Victor M. Ruiz Vera, Colegio de Postgraduados, Campus San Luis Potosi, Iturbide 73, Salinas, San Luis Potosi 78600, Mexico and Laosheng Wu, Univ of California-Riverside, Dept of Environmental Sciences, Riverside, CA 92521.

Understanding the relationship among soil properties, irrigation water quality, and soil management practices is vital to the sustainability of irrigated agriculture. The objectives of this study were to evaluate the interactive effects of clay mineralogy, PreWetting Rate (PWR), salinity, and sodicity on soil aggregation, water infiltration rate, soil water characteristics, and bulk soil electrical conductivity (ECa). Six soils with predominant clay minerals of smectite, vermiculite-smectite and kaolinite were equilibrated with solutions of Sodium Adsorption Ratio (SAR) = 0, 20, and 50 and Electrical Conductivity (EC) = 3.0 dS m-1. After air-drying, the samples were sieved and repacked in brass or Plexiglas rings. The soil columns were prewetted at rates of 2, 5, and 30 mm h-1, after which infiltration rates and aggregate stabilities were determined. In other soil columns with similar treatment (except PWR), the soil water characteristic curve and ECa were determined. When SAR increased from 0 to 50 (at EC = 0.6 dS m-1 and low PWR), the aggregate stability decreased from 0.63 to 0.19. In addition, the steady-state infiltration rate (i) in the vermiculitic and smectitic clay loam soils decreased by 87% and 92%, respectively. For a given matric potential, the amount of water retained in the smectitic (except the Cotharin soil) and vermiculitic soils increased when SAR increased from 0 to 50 (EC = 3.0 dS m-1). The air-entry value decreased 678% for the smectitic silty-clay loam soil and 301% for the smectitic silty-clay soil, which was attributed to a reduction in macroporosity due to high sodicity. The physical and hydraulic properties of the kaolinitic loam soil were not significantly affected by SAR increase. The ECa of the smectitic clay loam soil increased from 1.14 to 2.42 dS m-1 when SAR increased from 0 to 50 (soil solution EC ≤4 dS m-1). At low EC, aggregate slaking by fast PWR was the main cause of aggregate disruption and lower infiltration capacities when SAR was low whereas swelling and dispersion become more important to structural stability when SAR was ≥20.

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