PROBLEMS WITH UREA-N FOLIAR FERTILIZATION OF AVOCADO

In Egypt, urea sprays resulted in an increase in ‘Fuerte’ fruit yield, size, and weight (no leaf analysis reported).

In field experiments conducted at the University of California, Riverside, no significant effect of concentrations of urea sprays on the total N in ‘Hass’ avocado leaves could be demonstrated.

Using detached leaves of ‘Fuerte’ avocado, researchers in Israel reported substantial uptake of foliar urea-N when applied with surfactants. They subsequently reported the translocation of ¹⁵N from foliar-applied urea to vegetative and reproductive sinks of both ‘Fuerte’ and ‘Hass’ avocado.

Recent research at the University of California, Riverside, provided evidence that the leaf NH₃-NH₄⁺ content of ‘Hass’ avocado on clonal 'Duke 7' rootstock 2 years from budding was not increased by foliar application of urea at the same concentration that increased citrus leaf NH₃-NH₄⁺ content two-fold. Maximum uptake of ¹⁴C-urea by ‘Hass’ avocado leaves was physiologically insignificant (2.1% of the total urea applied) at the end of 2 days. Over 96% of the ¹⁴C-urea applied was recovered from the leaf surface even after 5 days. Maximum uptake of ¹⁴C-urea by leaves of ‘Gwen’ and ‘Fuerte’ was less than 7% and did not increase significantly after the first 4 h over a 24-h period. ‘Hass’, ‘Gwen’ and ‘Fuerte’ leaves exhibited an active urease in the presence of 2 mM urea. Urease activity was linear after an initial 1 h incubation up to 6 h at 30°C.

This report will be directed to practical application of available information. Factors considered in present discrepant results are: geographical, environmental, concentration, surfactant, leaf age and source, and experimental design.

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