Effect of Saline Irrigation on Early Growth of Eucalyptus gomphocephala and Acacia saligna
Authors: Haas Jennyfer 
Published: August 2009

An investigation is in progress in Formentera, Balearic Islands, to see whether satisfactory growth-rates can be achieved for trees of certain species — using small quantities of highly saline irrigation water, but relying on winter rainfall alone for leaching, and at the same time avoiding detrimental long-term environmental effects. Climate and soils in Formentera are similar to those of many semi-arid portions of the Mediterranean region. Two species, Eucalyptus gomphocephala and Acacia saligna, were tested with four irrigation treatments: no water, best available water (less than IdS/m), and waters of 5 and 10 dS/m. At Site 1, sea-salt water was mixed with BAW to achieve the desired conductivities, and at Site 2, highly saline ground-water was used for mixing. Sodium absorption ratios (SARs) were 84.8 for 5dS/m mixes and 152.1 for 10dS/m mixes at Site 1, and 13.06 for 5dS/m and 23.2 for 10dS/m at Site 2.

Trees were raised from seed in a local nursery, and were planted out in a grid pattern at 1.5 × 2 m spacings. Water requirement was calculated from a balance between pan-evaporation and tensiometer readings. Soil samples were taken at the beginning and end of each irrigation season. Trees were watered by hand-held hosepipe for two seasons. Irrigation was then withdrawn from half of the sample-trees. The remaining half were watered for two further seasons. Thus it was possible to compare the performances of trees receiving two years and four years of irrigation. In the first two years the three irrigated classes did significantly better than the trees which had received no water, but there was no significant difference between the classes due to salinity. At four years of age, the trees receiving continuing irrigation had grown more than those which received only two years of irrigation, although the latter continued to grow at a slower rate. However, where soils were extremely shallow (Site 2), withdrawal of irrigation affected survival rates, especially among the eucalypt.

The Acacia survived better than the Eucalyptus, even on extremely shallow soils. At the end of the second season it was clear that the establishment and vigorous early growth of the trees had been successfully achieved. At the end of the fourth year, it was also clear that trees would continue to grow without continuing irrigation — with the proviso that, on extremely shallow soils (20 cm or less), the Acacia survived significantly better than the Eucalyptus.

Analyses show that the winter rain appears to be effectively leaching the soil. Exchangeable sodium percentages (ESPs) rise during the irrigation season and fall during the winter. All ESPs have fallen off considerably when irrigation has been withdrawn. It appears that satisfactory growth can be achieved, and environmental protection established, with these small quantities of water at a purely temporary environmental cost (the short-lived increase in soil salinity). So far, the experiment demonstrates a low cost and effective technique for rapid tree establishment with low-quality water. Monitoring continues of soil nutrients, organic matter, litter, and soil fauna, to determine the sustainability of benefits.