Effects of sodium accumulation on soil physical properties under an effluent-irrigated plantation

Effluent irrigation commonly results in increased soil sodicity, because of the medium-to-high salinity and high sodium concentrations of many effluents. The exchangeable sodium percentage (ESP) of soils at the Wagga Wagga Effluent Plantation Project increased from 25% at some depths within the surface 0·6 m of soil, after 5 seasons of irrigation with either (i) treated sewage effluent or (ii) bore water with similar salinity and sodium adsorption ratio (SAR). A survey of dispersion index (DI) and saturated hydraulic conductivity (Ksat) was carried out to investigate the effects of increased soil sodicity on soil physical properties. The Ksat was measured after 5 irrigation seasons at 2 depth intervals (0·15-0·45 m and 0·5-0·8 m) using both bore water and distilled water, and compared with measurements made at the same sites using bore water before irrigation commenced. Both DI and ESP were measured at each of 3 depths in the surface 0·6 m of soil. The DI in distilled water was positively correlated with ESP, with finer soil textures having higher DI for the same ESP. When measured in bore water or effluent, DI was very low and not related to ESP. Ksat was significantly lower after 5 irrigation seasons, and significantly lower in distilled water than in bore water. However, there was no relationship between Ksat and ESP. It was concluded that the increased ESP caused an increased tendency for soil dispersion but was not shown unequivocally to lead to decreased Ksat. The increased sodicity does not seem likely to affect continued use of the land for effluent irrigation, but may affect its suitability for alternative uses that involve physical disturbance of the soil, for example, by cultivation.