EFFECTS OF SODIUM ACCUMULATION ON SOIL PHYSICAL-PROPERTIES UNDER AN EFFLUENT-IRRIGATED PLANTATION

Effluent irrigation commonly results in increased soil sodicity, becau se of the medium-to-high salinity and high sodium concentrations of ma ny effluents. The exchangeable sodium percentage (ESP) of soils at the Wagga Wagga Effluent Plantation Project increased from <2% to >25% at some depths within the surface 0.6 m of soil, after 5 seasons of irri gation with either (i) treated sewage effluent or (ii) bore water with similar salinity and sodium adsorption ratio (SAR). A survey of dispe rsion index (DI) and saturated hydraulic conductivity (K-sat) was carr ied out to investigate the effects of increased soil sodicity on soil physical properties. The K-sat was measured after 5 irrigation seasons at 2 depth intervals (0.15-0.45 m and 0.5-0.8 m) using both bore wate r and distilled water, and compared with measurements made at the same sites using bore water before irrigation commenced. Both DI and ESP w ere 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 wat er or effluent, DI was very low and not related to ESP. K-sat was sign ificantly lower after 5 irrigation seasons, and significantly lower in distilled water than in bore water. However, there was no relationshi p between Ksat and ESP. It was concluded that the increased ESP caused an increased tendency for soil dispersion but was not shown unequivoc ally to lead to decreased K-sat. The increased sodicity does not seem likely to affect continued use of the land for effluent irrigation, bu t may affect its suitability for alternative uses that involve physica l disturbance of the soil, for example, by cultivation.