Plant Sap Analysis as a Tool to Optimize Fertilizer Application for Sustainable Citrus Production

Final report for OS21-148

Project Type: On-Farm Research
Funds awarded in 2021: $20,000.00
Projected End Date: 03/31/2024
Grant Recipient: University of Florida
Region: Southern
State: Florida
Principal Investigator:
Lorenzo Rossi, Ph.D.
University of Florida
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Project Information


Over the past decade, Florida’s iconic citrus industry has been devastated by a disease known as citrus greening (Huanglongbing; HLB). Although no cure is currently available, growers have found some success in managing the disease, and research has shown that increased nutrient applications have extended the productive life of affected trees. Using leaf nutrient analysis to determine the nutritional requirements of the plant has become a standard practice for citrus growers in the United States. However, it is difficult to detect nutritional deficiencies in the early stages because traditional leaf nutrient analysis shows nutrient accumulation for the entire season, reducing the overall precision of fertilizer management.

One management tool gaining attention is sap analysis. This approach can increase the sustainability of citrus operations by improving the precision in determining tree nutrient status and fertilizer application. For this reason, sap analysis is considered by many as a real-time analysis method, indicating the concentration of nutrients available to the plants during the season. This method analyzes the nutrient concentration of the fluids from the xylem and phloem in the leaf (sap).

In this study, the treatments tested were one citrus variety (‘Ray Ruby’ grapefruit), two planting densities (standard single row and high-density staggered row), and two analysis methods (standard plant leaf tissue analysis and plant sap analysis) arranged in a completely randomized design with four replications. Leaf samples were taken during the most important phenological stages (i.e., flowering, fruit set, fruit fill, and pre/early harvest) from 8:00 am to 11:00 am, including both new and old growth. Similarly, standard soil and plant leaf tissue analysis was conducted. The results for N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn were compared to the standard leaf nutrient concentration ranges currently used for citrus production in the area.

Project Objectives:

The main objectives of this study were to (i) compare the efficacy of leaf sap analysis of micro- and macro-nutrient concentrations to traditional plant leaf tissue analysis and (ii) assess the differences in these analyses between standard and higher planting densities. This was done by collecting leaf samples from grapefruit trees in a commercial grove and analyzing foliar macro- and micro-nutrient concentrations using plant leaf tissue analysis (Waters Agricultural Laboratories Inc., Camilla, GA) and plant sap analysis from two different laboratories (New Age Laboratories, South Haven, MI, USA, and NovaCropControl, Netherlands) in the HLB-endemic region of the Indian River Citrus District.


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  • W. Cody Estes, Sr. - Producer


Materials and methods:
The project aimed to improve fertilization efficiency by determining real-time nutrient status in the plants. The experiment was conducted at a commercial grove in Vero Beach, Florida (27°35'03.6"N, 80°37'31.4"W). In this study, the plant material consisted of 12-year-old ‘Ray Ruby’ grapefruit grafted onto a ‘Sour Orange’ rootstock. The experimental design was a randomized complete block design with four blocks, where the treatments tested were two planting densities (single and staggered rows) and two analysis methods (standard plant leaf tissue analysis and plant sap analysis conducted at two independent laboratories). The leaf samples were taken monthly during spring/summer and every other month during fall/winter, comprising 15 data collection points over two years.

The measurements and methods pertaining to each objective are discussed below:

Objective 1 - Compare the efficacy of leaf sap analysis of micro- and macro-nutrient concentrations to traditional plant leaf tissue analysis.

Leaves for sap analysis were collected from 8:00 AM to 11:00 AM, including both new and old growth, with a minimum of 80 g of tissue collected for each sample. Sampling was performed on the East side of the tree (sunny side), and all samples were kept in a cooler with ice until shipped to the respective laboratories, New Age Laboratories and NovaCrop. Two laboratories were used to control for differences in methodologies used to analyze leaf sap.

The procedures for leaf tissue sampling involved collecting one mature leaf from each tree quadrant—corresponding to the cardinal directions—with each plot containing 21 trees, resulting in 84 leaves per sample. After sampling, the leaves were shipped to Waters Agricultural Laboratories, Inc.

Objective 2 - Assess the differences in these analyses between standard and higher planting densities.

The grove consisted of raised beds containing both a row of standard-density single trees and a row of high-density staggered trees, allowing for the sampling of both treatments under nearly identical conditions. 

Research results and discussion:

SAP Sare grant graphs

Leaf nitrogen (N) was found to be at optimal levels by both leaf tissue analysis and plant sap analyses, with only one discrepancy between the plant sap laboratories. New Age Laboratories identified a deficiency in April 2022. Phosphorus (P) levels were optimal according to leaf tissue analysis; however, both plant sap analyses indicated potential P deficits during multiple months. Differences were also seen between the sap analyses: New Age Laboratories reported potential P deficits in April and July 2022, whereas NovaCrop reported deficits throughout most of the sampling periods.

Potassium (K) levels were generally optimal or slightly surplus in both leaf tissue and sap analyses throughout the samplings, with only minor differences between the methods. However, there were stark differences in calcium (Ca) and magnesium (Mg) levels between leaf tissue and sap analyses. Leaf tissue analysis found Ca to be at optimal levels throughout the samplings, whereas NovaCrop consistently found Ca deficits, and New Age Laboratories identified a Ca deficit in May 2022 and from late 2022 to early 2023. Similarly, Mg was deemed optimal by tissue analysis, but both sap analyses showed surpluses and deficits at different times. New Age Laboratories found Mg deficits in April and July 2022 and surpluses from late 2022 to early 2023, while NovaCropControl identified a deficit in August 2022 and a surplus in March 2023.

Data analysis revealed little to no differences between single-tree standard-density rows and staggered-tree high-density rows.


The analysis of leaf nutrient levels revealed several key insights into the effectiveness and discrepancies between traditional leaf tissue analysis and newer plant sap analysis methods. This detailed evaluation provides a comprehensive understanding of nutrient dynamics and highlights the potential benefits and limitations of each method.

Nitrogen (N): Both leaf tissue and sap analyses generally indicated optimal N levels, which is critical for the growth and productivity of the grapefruit trees. The single discrepancy found by New Age Laboratories in April 2022 underscores the need for multiple analysis methods to ensure accuracy and reliability in nutrient management.

Phosphorus (P): Phosphorus is essential for energy transfer and root development. While traditional leaf tissue analysis suggested optimal P levels, sap analyses revealed potential deficits during various months. This discrepancy suggests that sap analysis might be more sensitive in detecting transient P deficiencies, providing a clearer understanding of nutrient availability over time.

Potassium (K): Both analysis methods indicated that K levels were generally optimal or slightly surplus, highlighting effective nutrient management practices. Potassium is crucial for water regulation and enzyme activation, and maintaining its optimal level is vital for tree health.

Calcium (Ca) and Magnesium (Mg): The stark differences between leaf tissue and sap analyses for Ca and Mg are particularly noteworthy. Tissue analysis showed consistent optimal levels, whereas sap analyses revealed significant variability. Calcium is important for cell wall structure and signaling, and magnesium is a central component of chlorophyll. The discrepancies in Ca and Mg levels suggest that sap analysis may detect short-term fluctuations more effectively, providing insights into periods of deficiency that tissue analysis might miss.

Planting Density: The preliminary analysis of nutrient levels in single-tree standard-density versus staggered-tree high-density rows showed little to no differences. This finding suggests that planting density might not significantly impact nutrient uptake and status, though further investigation could provide more definitive conclusions.

Overall, this study emphasizes the importance of using multiple nutrient analysis methods to obtain a comprehensive view of tree health and nutrient status. By integrating leaf tissue and sap analyses, growers can improve their fertilization strategies, ensuring optimal nutrient levels and enhancing the sustainability and productivity of their citrus groves.

Participation Summary
1 Farmers participating in research

Educational & Outreach Activities

5 Consultations
1 Curricula, factsheets or educational tools
1 On-farm demonstrations
1 Published press articles, newsletters
1 Tours
2 Webinars / talks / presentations

Participation Summary:

15 Farmers participated
50 Ag professionals participated
Education/outreach description:

2022 – 2nd Congress of the International Society for Citrus Huanglongbing and Phloem-Colonizing Bacterial Pathosystems (ISCHPP)

Gave a poster presentation to 60-70 extension agents, researchers, and students at the 2nd Congress ISCHPP. The event was organized by the University of Florida and took place at the Sheraton Sand Key Resort in Clearwater Beach, FL, on October 26, 2022.


2023 Florida State Horticultural Society (FSHS)

Gave a 15-minute presentation to 30-50 extension agents, researchers, and students at the 136th Annual Meeting of the Florida State Horticultural Society (2023). This event was organized by the Florida State Horticultural Society and took place at the Daytona Beach Hilton Oceanfront Resort in Daytona Beach, FL, on June 12, 2023.


Other Outreach

Published press articles in the Southern Sustainable Agriculture Research and Education Magazine.

Learning Outcomes

5 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key changes:
  • nutrition management

  • nutrients uptake

Project Outcomes

15 Farmers changed or adopted a practice
2 Grants received that built upon this project
4 New working collaborations
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.