An evaluation of foliar nitrogen and zinc applications to macadamia
Authors: Huett D. O. , Vimpany I. 
Published: September 2006

The efficiency and effectiveness of foliar nitrogen (N) applications as urea to macadamia leaves were investigated in field experiments at Alstonville, New South Wales. The first experiment (August 2000) evaluated the effect of 1–8% urea solutions on the evidence of leaf burn. A 2% urea solution produced negligible leaf burn (1% leaves with lesions) whereas the incidence of leaf burn increased with urea concentration; 20% of leaves were affected with an 8% urea solution. In the following month, a second experiment was conducted using a 2% urea solution that was painted onto both sides of leaves to measure N uptake efficiency. The urea was enriched with the stable isotope 15N, which allows a direct measure of urea uptake, a common method for tracing plant N uptake. Leaves were sampled after 3 and 6 days, and cellulose acetate was then applied to remove urea adhering to the surface of leaves. Leaves adjacent to urea-treated leaves were also sampled to account for any transport out of treated leaves. The experiment was repeated in September 2001. In 2000, a mean of 31% of the urea was absorbed by the mature leaves and this increased the N content by 2.2%. In 2001, a mean of 38% of the urea was absorbed by the mature leaves and this increased N content by 1.9%. Leaves from part of a large mature macadamia tree were stripped to provide an estimate of leaf biomass. From this, the increase in leaf N uptake for a mature orchard was calculated to be 3.98 kg/ha in 2000 and 4.57 kg/ha in 2001. The efficiency of application and hence leaf N uptake from a commercial spray would be expected to be lower than that of the present study. Commercial foliar urea applications are unlikely to meet the N requirements of a productive macadamia orchard. In a separate study, the efficiency of zinc (Zn) fertiliser as soil (5–20 g Zn/m2 canopy ground area) and foliar applications were examined at a mature commercial orchard near Alstonville on a Ferrosol soil. In August 2001, a 2% solution of zinc sulfate heptahydrate was thoroughly applied to the canopy of trees using a backpack misting machine and mature leaves were sampled 4 weeks later. Non-sprayed control trees were also sampled. Cellulose acetate was applied to sampled leaves to remove foliar-applied Zn adhering to the surface of leaves. The leaf Zn concentrations were increased (P<0.05) from 11 to 52 mg/kg 4 weeks after foliar Zn application. Sprayed leaf Zn concentrations were stable 18 weeks later, and flush leaves that emerged after spray application had similar (P>0.05) Zn concentrations to control leaves 12 months later indicating that little if any remobilisation of Zn had occurred over these periods. Soil Zn application had no effect (P>0.05) on leaf Zn concentrations 1 and 2 years after application. The effectiveness of a commercial foliar Zn application was evaluated in September 2001 using a low set orchard sprayer and a 1% Zn solution. After 4 weeks, leaf Zn concentrations were increased from 12 to 26 mg/kg. Foliar Zn applications can be recommended to increase leaf Zn concentrations in macadamias despite evidence in the literature for only 1% uptake efficiency.