Sap and Traditional Plant Nutrition Monitoring: Understanding the Opportunities and Limitations
As growers seek to improve fertiliser efficiency, optimise yields and build resilient production systems, attention is returning to ‘getting the fundamentals right’. The fundamentals are the core principles of nutrient management: understanding soil fertility, monitoring plant nutrient status and applying nutrients at the right rate, time and place.
Part of getting the fundamentals right is assessing the tools available to help growers make informed decisions, including leaf, petiole and sap analysis.
At Hill Labs, we are increasingly asked about sap testing. While enthusiasm for sap analysis has grown in recent years, there are a range of views on its application within commercial crop production. While we continue to monitor developments in this area, leaf and petiole testing remain our recommended approach due to of their established scientific basis and interpretation frameworks.
Different Tests Answer Different Questions
With plant testing, different plant tissues provide growers different information.
Analysis of the different plant tissues is a unique tool that provides different perspectives on plant nutrition.
Why Sap Testing Has Attracted Attention
Interest in sap testing is driven by its intention of providing earlier insight into nutrient trends. Advocates suggest sap analysis may help:
Real-time, in-field sap analysis can support nitrogen management in some crops. However, in-field measurements generally provide semi-quantitative results, prioritising timeliness over accuracy.
Laboratory analysis can be used alongside rapid testing methods as a tool for calibration and verification. While it typically takes longer, it provides a high level of analytical accuracy. Although this level of precision may not always be required, understanding the uncertainty associated with alternative methods and periodically validating results through laboratory testing can help inform more confident crop management decisions.
These are beneficial insights, particularly in high-value cropping systems where small nutritional adjustments can influence crop performance. In-field, real-time nutrient analysis has appeal and is useful for nitrogen management for some crops. In-field results are semi-quantitative, trading accuracy for timeliness.
Where the Debate Continues
While interest in sap testing is growing, there is also ongoing discussion within the scientific community about its application and interpretation.
Researchers have noted that sap testing methods vary considerably between providers, including differences in:
Sample collection remains a significant source of variation in sap testing results, with factors such as time of day, plant moisture or irrigation and temperature all influencing results. Careful sample handling, including transport to the laboratory, is also critical.
A challenge is that interpretation frameworks for sap testing are proprietary and not always publicly available, making it difficult to compare recommendations between providers or understand how nutrient targets have been developed.
Our understanding is that further standardisation and validation are needed before sap testing can be universally applied across crops and growing conditions.
Learning from International Experience
The discussion around sap testing is not unique to New Zealand.
Some laboratories overseas offer sap testing, particularly for intensive greenhouse and horticultural production systems. Others have adopted a more cautious approach, citing challenges associated with interpretation, repeatability and standardisation. Internationally, sample collection remains a key challenge, as differences in sampling methods can significantly affect the results and their interpretation.
This diversity of views demonstrates that sap testing is an evolving field and industry understanding continues to be developed.
Why Hill Labs Continues to Provide Leaf and Petiole Testing
Leaf and petiole testing remain the cornerstone of plant nutrient assessment for Hill Labs as the methods are supported by decades of scientific research. These methods have:
For many crops, petiole analysis already provides valuable information on nutrient movement within the plant, while leaf analysis provides a comprehensive assessment of overall nutrient status. Together they offer a balanced and well-understood approach to nutrient monitoring.
| Sap Testing | Leaf & Petiole Testing | |
| Measures current nutrient movement | Primary focus | Petiole testing provides insight into mobile nutrients |
| Scientific validation | Developing | Extensive research history |
| Sampling protocols | Vary between providers | Widely standardised |
| Interpretation ranges | Often proprietary |
Petiole - Well established for some crops Leaf – Well established for most crops |
| Usefulness for long-term nutrient planning | Emerging | Well proven |
| Suitable for routine crop monitoring | Potentially | Widely accepted and validated |
Our Current Position
At Hill Labs, we remain interested in developments in sap testing and continue to monitor new research and industry experience.
However, before introducing any new analytical service, we need confidence that the methodology is robust, interpretation frameworks are scientifically validated and recommendations can be consistently translated into practical outcomes for growers.
For now, leaf and petiole testing remain our recommended approach because they provide reliable, well-understood and actionable information that is supported by extensive scientific research and local interpretation data.
As the science evolves, we will continue to evaluate new evidence and keep our clients informed.
