Long-term effects of saline irrigation water on 'Valencia' orange trees: relationships between growth and yield, and salt levels in soil and leaves

This study documents changes in yield, growth, soil salinity (ECe) and leaf sodium (Na) and chlorine (Cl) concentrations in mature Valencia orange [Citrus sinensis (L.Osbeck)] trees on sweet orange (Citrus sinensis) rootstock in response to increased levels of Na and Cl in irrigation water. Four levels of salt, ranging from the river-water control (0.44 dS/m) to 2.50 dS/m, were applied over a 9-year period through an under-tree microsprinkler system to trees in the Sunraysia area of the Murray Valley in south-eastern Australia. A salt-balance model showed that evapotranspiration was reduced by salinity, whereas leaching fractions increased from an average 24% in the control to 51% in the most saline treatment. The high leaching fractions were achieved as a result of freely draining soils and good irrigation management, and allowed us to maintain low to moderate levels of soil salinity throughout the trial and minimised the effect of salt treatment on fruit yield. Soil salinity increased almost linearly in response to irrigation-water salinity during the first year, and fluctuated seasonally thereafter; however, very few readings exceeded 3 dS/m, even in the highest treatments. By contrast, leaf Na and Cl concentrations in the highest salt treatment continued to increase over the first 4 years. The relationship between yield and soil salinity was extremely weak, but yield did decrease as foliar concentrations of Na and Cl increased: in Year 9, leaf Na in the highest treatment relative to the control was associated with a predicted reduction of 17% in yield and 59% in annual trunk-diameter growth.