Prewetting rate and sodicity effects on the hydraulic conductivity of soils

Exchangeable sodium deteriorates the hydraulic conductivity of soils. The s usceptibility of soils to sodicity depends on the soils' inherent propertie s, and is known to increase with an increase in clay content and the presen ce of 2:1 clay minerals, and decrease with an increase in sesquioxides cont ent. The objective of this study was to determine the effect of an extrinsi c time-dependent property, such as soil prewetting rate (PWR), on the hydra ulic conductivity (HC) of smectitic soils varying in their exchangeable sod ium percentage (ESP) between 1 and 10 and in clay content. Five soils were studied: a loamy sand, a loam, a sandy clay, and 2 clay soils. Air-dried so il samples with ESP of 2, 6, and 10 were packed in plexiglass columns and w ere prewetted from below at 3 rates (1.7, 4.25, and 50 mm/h) with deionised water. The columns were then leached at constant hydraulic head with deion ised water, and flow rates were measured. The reference hydraulic conductiv ity (HC0) of the loamy sand was not affected by both PWR and sodicity. In t he loam, HC0 was not affected by rate of wetting. However, increasing the E SP from 2.1 to 9.5 decreased HC0 from 15.8 to 5.0 mm/h for the fast wetting . Similar decreases were noted in the other 2 wetting rates. The hydraulic conductivity of the sandy clay and the 2 clay soils were effected by PWR. I n the sandy clay with ESP 5.5, increasing PWR from 1.7 to 50 mm/h resulted in a decrease in HC0 from 78.7 to 3.2 mm/h The decrease in HC0 in these soi ls with the increase in wetting rate was ascribed to aggregate slaking. The effect of PWR on HC0 was more notable, especially at the low ESP, as clay content increased. The hydraulic conductivity of the sodic soils leached wi th distilled water decreased more steeply and to lower values with increasi ng PWR. Fast prewetting, which increased breakdown of soil aggregates, incr eased the susceptibility of the soils to sodicity.