Changes in soil chemistry in effluent-irrigated Pinus radiata and Eucalyptus grandis plantations

Two tree species (Pinus radiata and Eucalyptus grandis) were planted in 1991 and irrigated weekly with either secondary-treated effluent or bore-water, in an effluent-irrigated plantation at Wagga Wagga, NSW. Changes in soil salinity, sodicity, alkalinity and the accumulation of potentially toxic elements after 2 and 4 irrigation seasons were studied. The average sodium adsorption ratio (SAR) and electrical conductivity (EC) of effluent were 4.8 and 0.79 dS/m, while in bore water they were 3.3 and 0.70 dS/m, respectively. Effluent was applied at 3 rates: the water-use rate of the plantation less rainfall (medium treatment, M), a nominal rate 0.5 times M (low treatment, L), and a nominal rate 1.7 times M (high treatment, H). The bore-water treatment (W) was irrigated at its water-use rate less rainfall. Salinity of saturation paste extracts in the surface 0.05 m increased from a pre-treatment value of 1 dS/m to 2-3 dS/m after 4 years of effluent irrigation. The increase in soil salinity was consistent with the salt loading for the different irrigation treatments. Similarly, exchangeable sodium percentage (ESP) increased in all treatments. The ESP was generally <2% at the start of the experiment and increased to a maximum of 25% in the 0.3-0.4m soil layer of treatment L. Values had changed throughout the 4 irrigation seasons and the soil profile does not appear to have reached a steady-state condition. Soil pH increased in all treatments. The largest increase (approximate to 0.7) occurred in treatment H, while treatment W had the smallest increase (approximate to 0.4). The increase in soil pH caused a decrease in exchangeable Al and Mn in the upper 0.4 m of the soil profile in all treatments. Effective cation exchange capacity (ECEC), approximated by the sum of the extractable cations, increased significantly in the upper 0.3 m of the soil profile after 2 and 4 irrigation seasons, but there was no significant change in ECEC below 0.3 m.