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

Project Overview

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


  • Fruits: citrus


  • Crop Production: fertilizers, nutrient management

    Proposal abstract:

    Citrus fertilization using leaf nutrient concentration as a reference has become a standard practice for citrus growers in the United States. Our approach is to increase the sustainability of this existing farming practice by improving the precision in nutrient status determination and fertilizer application. We propose to optimize nutrient management and plant nutrition using sap analysis as a complementary tool for soil and leaf analysis.

    Leaf sap analysis has been a useful tool for fine-tuning fertilization processes in tomatoes, potatoes, peppers, melons, strawberries, roses, sugarcane, vineyards, olives and nectarines (Hochmuth, 1994, 1999; MacKerron et al., 1995; Studstill et al., 2003; Cadahía, 2005; Hochmuth et al., 2012; Joris et al., 2014; Tei and Nicola, 2017) however, this tool has not yet been used in citrus. This method analyses the nutrient concentration of the fluids from the xylem and phloem in the petiole (sap). As an analogy, plant sap analysis is comparable to a blood test analysis for plants. The results from this test, together with the soil and leaf analyses, will increase the precision of nutrient status determination and nutrient input.

    We want to consolidate the sampling strategy for citrus in Florida (time of the day, leaf age, leaf part, and number of leaves), sap testing (equipment, sample storage, and reading time frame), testing options (commercial minilabs for in-house testing and outsource analysis in commercial lab), and establish the initial guidelines for N, P, K, Ca, Mg. S, B, Cu, Fe, Mn and Zn ranges in two citrus species, grapefruit and mandarin.

    Leaf sap nutrient concentrations vary among season of the year (Fall, Spring, Summer) (Hochmuth, 1999; Brust, 2008) . Therefore, it is important to determine the adequate ranges for each nutrient at each season. After determining the adequate sap nutrient ranges for each season, a more rational fertilization approach could be developed, based on what the plant needs at that specific time of the year. The sap analysis will give us a real-time nutritional status information to immediately correct any nutrient deficiency and/or excess. The comparison and correlation between sap analysis, leaf analysis and soil analysis will provide the tools to determine causes for nutrient imbalances and assist finding solutions quickly. With this approach we expect to improve fertilizer management, conserve natural resources, minimize negative environmental impacts and reduce production costs. An added value of sap testing is the possibility to monitor total sugars and brix in the plant (even before the fruit set), which can become an extra tool for growers to predict and improve internal fruit quality.

    Project objectives from proposal:

    In this project we will collect soil and leaf samples of grapefruit and mandarin trees in a commercial grove, and analyze soil and foliar macro and micronutrient concentrations using a low-cost stand-alone bench nutrient analyzer (iMETOS MobiLab; Pessl Instruments GmbH, Weiz, Austria), standard soil and plant leaf tissue analysis (Waters Agricultural Laboratories Inc., Camilla, GA), and plant sap analysis (New Age Laboratories, South Haven, MI) in HLB-endemic region in the Indian River citrus District. The citrus groves used for this series of trials are located in a farmer collaborator at the Indian River county in Vero Beach, FL.

    Our goal is to improve fertilization efficiency by determining real-time nutrient status and calculate fertilizer needs. We will establish a relationship between analysis methods, and hopefully identify adequate ranges of nutrient sufficiency for the plant sap analysis to allow for modifications in the current fertilization strategies from 1 to 3 times per year to more frequent applications on demand to reduce the fertilizer input for commercial citrus production, providing nutrients in the right time and at the right rate and reducing the potential for nutrient runoff and leaching to water bodies and groundwater.

    The treatments tested are two citrus varieties (‘Ray Ruby’ grapefruit and ‘Bingo’ mandarin) and three analysis methods (stand-alone bench nutrient analyzer, standard plant leaf tissue analysis, and plant sap analysis) arranged in a complete randomized design with 6 replications (see Figure 1 below). Leaf samples will be taken 6 times per year to match phenological stages at budding, flowering, fruit set, fruit fill, Pre/early harvest, and post/late harvest.

    Petioles for sap test will be collected from 8 am to 11 am, including both new and old growth. The comparison between new and old growth will help us to determine the mobility of each nutrient inside the plant at each stage, besides giving information about nutrients that may start to get depleted or accumulated in excess (Timmermans and van de Ven, 2012). The methodology for citrus sap sampling is the one suggested by NewAge Laboratories (United States) and NovaCropControl (Netherlands), which also matches with the methodology suggested by Hochmuth (1999) for vegetables in Florida. The procedures for soil and leaf tissue sampling are the ones suggested by Morgan et al., (2020). All samples will be kept in a cooler with ice until shipped to the respective laboratory.

    The standard soil analysis, plant leaf tissue analysis and plant sap analysis will be sourced out in commercial labs. Sap will be analyzed on site as well using a stand-alone bench nutrient analyzer to study the accuracy of a newly released portable system to be used on site.

    N, P, K, Ca, Mg. S, B, Cu, Fe, Mn and Zn results will be compared to the standard leaf nutrient concentration ranges currently used for citrus production in Florida (Morgan and Kadyampakeni, 2020). We will run a timeseries analysis to understand how each nutrient change during a whole season and generate correlations between sap results vs leaf and soil results to check for possible causes of nutrient imbalances. We expect to establish the initial guidelines for N, P, K, Ca, Mg. S, B, Cu, Fe, Mn and Zn ranges in two citrus species.


    Figure 1. Experimental design.
    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.