Prewetting rate and sodicity effects on the hydraulic conductivity of soils

Abstract

Contribution from the Agricultural Research Organization (ARO), The Volcani Center, Bet Dagan, Israel. No. 604/2000 series.

Exchangeable sodium deteriorates the hydraulic conductivity of soils. The susceptibility of soils to sodicity depends on the soils’ inherent properties, and is known to increase with an increase in clay content and the presence of 2:1 clay minerals, and decrease with an increase in sesquioxides content. The objective of this study was to determine the effect of an extrinsic time-dependent property, such as soil prewetting rate (PWR), on the hydraulic conductivity (HC) of smectitic soils varying in their exchangeable sodium 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 soil samples with ESP of 2, 6, and 10 were packed in plexiglass columns and were 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 deionised water, and flow rates were measured. The reference hydraulic conductivity (HC₀) of the loamy sand was not affected by both PWR and sodicity. In the loam, HC₀ was not affected by rate of wetting. However, increasing the ESP 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. In 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 soils 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 with distilled water decreased more steeply and to lower values with increasing PWR. Fast prewetting, which increased breakdown of soil aggregates, increased the susceptibility of the soils to sodicity