Muhammad Asad Ullah, Lachlan Casey, Richard Webb, Daryl Joyce*

Mineral nutrition of horticultural crops is crucial for high productivity and good quality produce. Well-fertilised plants are more resilient against adverse environments and disease (Lahav and Kadman 1980).

The importance of mineral nutrients, including calcium (Ca), is well documented towards improving crop yield potential and product robustness, including for avocados (Hofman et al. 2005; Tonetto de Freitas and Mitcham 2012; Joyce 2021).

Moreover, mineral nutrient deficit during fruit growth and development contributes to postharvest issues, such as stem end and body rots (Joyce et al. 2021).

Timely measurement of Ca and other nutrients in soil, leaf, and developing and mature fruit samples is useful to predict avocado fruit quality and inform marketing. For example, to send fruit into relatively complex export versus comparatively simple domestic supply chains.

Mineral analysis, such as by inductively coupled plasma spectrometry (ICP), typically involves wet chemistry under laboratory conditions. However, such conventional approaches are typically time and labour consuming. Moreover, turnaround times may exceed two weeks.

Additionally, time and cost limitations may encourage pooling individual samples. This practice can result in loss of potentially valuable temporal and spatial sampling information.

In this context, rapid, efficient, and cost-effective mineral analysis on-farm would be ideal for intensive monitoring towards informing site specific fruit quality improvement outcomes.

Recent technological advances have led to handheld X-ray fluorescence (XRF; Figure 1, LHS) to measure mineral elements, from fluorine (F) to uranium (Ur) in the Periodic Table, in a variety of materials, including plant samples.

Relatively user-friendly handheld XRF can be co-opted for relatively quick real-time quantification of Ca and some other plant nutrients, including potassium (K). However, its utility in horticulture is relatively unreported to date.

Handheld XRF is prospectively useful for water, soil, and plant (dried or fresh) samples. It does not require wet chemistry and, once calibrated, the instrument is simply ‘point and shoot’ for instant results.

Kalcsits (2016) tested handheld XRF to determine Ca and K concentrations in apple peel. Their levels, as reflected in the K/Ca ratio, are indicative as an index of predisposition to Ca deficit related bitter pit disorder.

Our preliminary investigation comparing novel handheld XRF versus conventional ICP for ‘Hass’ avocado skin revealed promising correlations for K concentration and the K/Ca ratio. Regression correlations accounted for 88% and 79% of the variation, respectively (Figure 2).

Figure 2: Linear regression correlations between handheld XRF and wet chemistry ICP analysis methods for
estimating K and K/Ca in dried and powdered avocado fruit skin samples (n = 50) reflecting 88% and 79% variability in dataset, respectively. 

For more detailed study, benchtop XRF (Figure 1, RHS) allows for spatial as well as quantitative analysis of XRF-‘visible’ elements. Using this benchtop technology, we imaged spatial distribution of Ca and K in avocado flesh. As depicted in Figure 3, variations in concentration and distribution are clear. In addition to general insight, spatiotemporal insight can be used to inform standardised sampling for XRF-visible minerals in fruit and other plant tissues.

Being relatively novel, there are some challenges in using XRF. Custom calibration is needed for specific sample types, including as there may be interference among ions, like K, Mg, or nitrates (NO3-) in soil solution, and/or organic substances, such as oil.

At an operational level, handheld XRF requires training and certification for use. Careful handling and maintenance of X-ray-related equipment is also needed.

Nevertheless, XRF is poised to become a time- and cost-effective alternative to traditional mineral analysis techniques for mineral estimation in water, soil, and plant tissue.

*Muhammad Asad Ullah, Lachlan Casey, Richard Webb, Daryl Joyce are researchers at The University of Queensland. Contact Daryl Joyce: d.joyce@uq.edu.au