Autumnal Avocado Orchard Testing
Recently we were asked about the best time of year to collect soil & leaf samples from avocado orchards for nutrient testing. Maintaining a regular soil test history from samples collected at the same time of year each year is recommended, and it makes sense to do this at the same time as leaf testing. The conventional time to do leaf sampling has always been autumn, as that is a period when the summer flush growth is finished and the tree is in a “quiet state”. Our Crop Guide describes the standard tests to request and the sampling protocol to follow. This season already, we have also seen a number of jobs with additional total copper and/or total cadmium requested, where orchardists are seeking to monitor any potential heavy metal accumulation in the soil as part of a sustainability program.
The original question prompted the laboratory to analyse recent leaf test data for samples submitted outside of the traditional autumn period. It was interesting to see that the median level for most of the elements in a Basic Plant leaf test seemed to be generally consistent across the months. The data for leaf nitrogen and leaf boron from samples submitted in 2021 are shown below. It is important to note that for some months there were only a very small number of samples, and the laboratory cannot verify the sampling method used so this does not represent a controlled survey. The whisker box plots below show the median result value and standard deviation, with the dots shown as data outliers. Within some months there was quite high variation, but the median result for Nitrogen across all months was reasonably well-aligned with the accepted interpretive range of 2.4 to 2.9% . For Boron, the median result data was a little more variable both within a month and across the months, with a decline during the winter months. The accepted interpretive leaf level in autumn is 30-50 mg/kg, and it is based on the autumn sampling convention.
Fig 1: Avocado leaf Nitrogen results from samples analysed at Hill Laboratories in 2021.
Fig 2: Avocado leaf Boron results from samples analysed at Hill Laboratories in 2021.
This desktop exercise tends to reinforce the recommended practice of sampling avocado leaf in autumn, although it is suggestive that the accepted interpretive ranges would be generally applicable for spring also. Data from several years would be needed before any change in the conventional sampling practice is recommended, but additional leaf sampling in spring may be useful to diagnose any suspected nutrition problems at that time.