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Abstract
The ability of urea foliar sprays containing hydrocarbon or silicone surfactants to maintain leaf N concentration in mature prune (Prunus domestica L.) trees was tested in a non-irrigated commercial orchard near Young, New South Wales. In 1976-77, foliar-applied N was as effective in maintaining leaf N concentration as soil-applied N on a g for g N basis, but a silicone surfactant which enabled sprays to completely wet leaves and to spontaneously penetrate open stomata, was no more effective in terms of leaf N concentration than a conventional hydrocarbon surfactant. In 1977-78, a drought year, foliar-applied N was more effective than soil-applied N. However, the silicone surfactant was less effective than the hydrocarbon, as run-off was excessive and the spray film dried very rapidly. Inclusion of the humectant glycerol in the silicone sprays retarded the rate of drying and eliminated the difference between the surfactants. A combination of soil and foliar N is suggested as a means of maximizing control of tree N status in most years. However, conventional surfactants appear to be slightly superior to silicone surfactants in high-volume spraying.