Research Report No. 357

Tree and shrub performance and soil conditions in a plantation irrigated with saline groundwater.  J. Morris, R. Bickford and J. Collopy.  January 1994.  37pp. (unpubl.)

SUMMARY

A field trial of 22 tree and shrub species was conducted over 10 years on a highly saline-sodic site in an irrigation farming area of northern Victoria. The plants were drip irrigated for the first five years, and sprinkler irrigated thereafter. Half of the site was watered with saline groundwater, diluted to an electrical conductivity of 6.5 to 13.3 dS m-1 (equivalent to 4000 to 8000 mg L-1 dissolved salts). Soil and groundwater conditions, plant growth and foliar salt uptake were monitored annually or more frequently; root distribution beneath Eucalyptus camaldulensis was assessed at 42 months after planting. Salt tolerance was assessed by four survival and growth criteria, and the rankings of species compared with the results of glasshouse and growth room screening trials.

Species which displayed satisfactory tolerance of the site conditions were Eucalyptus camaldulensis, E. largiflorens, E. occidentalis, E. leucoxylon, E. incrassata, E. griffithsii, Casuarina cristata, C. cunninghamiana, Melaleuca lanceolata, M. armillaris, Acacia saligna and Callistemon salignus. Glasshouse screening was only partly successful in predicting the relative field performance of these species. Some species which showed satisfactory early growth and survival declined markedly during the period of the trial, emphasising the need for field evaluation trials to continue over 5 years or longer.

Salt storage in the soil profile decreased substantially during the trial, and no increase in salinity of soil or groundwater was observed as a result of tree water use or irrigation with saline water. Roots of drip irrigated E. camaldulensis were found to occupy a leached zone extending to below the watertable observed in shallow bores. While foliar chloride concentration was not suitable as a salt tolerance index, it varied with root zone salinity and may be useful in diagnosing causes of tree decline.

The observations of tree performance in relation to site conditions support the potential use of salt tolerant tree plantations for water table control in saline discharge areas, and for disposal of saline wastewater or pumped groundwater. The salinity of soil water in the root zone was estimated to be greater than 20 dS m-1 throughout the trial. However, the observed reduction of growth and hence water uptake with increasing salinity indicates that the efficiency of plantations established for these purposes will be greatest where salinity is relatively low, even though the trees are able to tolerate more saline conditions. Thus, plantations for groundwater discharge control should be located up-catchment from discharge areas, where the groundwater is accessible but concentration of salts by surface evaporation is minimal; and saline wastewater applied to trees should be shandied or alternated with fresh water irrigation if possible, to maintain a vigorous canopy capable of maximum water use.