Optimum planting densities for the production of eucalyptus oil from blue mallee (Eucalyptus polybractea) and oil mallee (E. kochii)

Abstract Mallees have the potential to become a new industrial crop for much of the cereal growing areas of Australia, providing alternative income and environmental salvation for farmers with land at risk to salinisation. Two species of mallee, blue mallee ( Eucalyptus polybractea ) and oil mallee ( E. kochii] ), were tested at Condobolin, New South Wales over a five year period to assess biomass and oil production. Plots were established at five planting densities (between 2000 and 9000 plants/ha) and harvested annually, each August. Leaf oil concentration was measured by steam distillation at each harvest and used with biomass to calculate total oil production. Biomass and oil production were not significantly different for either species between 3300 and 9000 plants/ha in any year, but significantly greater than plantings at the lowest density. Leaf oil concentration did not differ significantly with each harvest nor was it affected by planting density. Oil mallee had significantly higher leaf oil concentration than blue mallee. Oil quality and quantity determinations were made on leaf samples which were solvent extracted and analysed by GC. Small samples of leaf were taken every three months for the first three years then at the fourth harvest. For both species, total oil concentration was significantly higher in six and nine month-old regrowth than the other sampling periods. Cineole was by far the most abundant component for both species and its concentration in leaf from 12 month-old regrowth was significantly higher than that from three, six or nine month-old regrowth. For maximum oil production per unit area of plantation, blue mallee is better suited to the Condobolin environment than oil mallee. The normal planting density of 5000 plants/ha currently used by growers appears optimum, but this may be modified after considering seedling costs, projected longevity of the crop and management strategies.