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The Ultimate Guide to Plant Sap Testing: How To Test Plant Sap For Nutrients In The Field

Updated: May 20


introductory thumbnail on plant sap analysis blog post showing laquatwin meters and tomato leaf


Plant sap analysis measures the amount of nutrients available in your plant.

It is typically done in the field by extracting plant fluid or sap (i.e. xylem carrying soil nutrients and dissolved minerals) from petioles or leaves, which is then run through various testing equipment to measure the sap for specific nutrients.


In the end of testing, you obtain a comprehensive picture of your crop nutritional status, which gives you the opportunity to detect any nutritional deficiencies or excesses before any changes in your plants become apparent.

 

Plant sap analysis is a precise method of testing and is often compared with lab-based foliar analysis. Numerous studies have shown a strong correlation between these two methods.


While reliance on plant sap testing labs was very common in the past, advancements like the introduction of pocket nutrient ion meters with ion-selective electrodes and photometers now allow growers to measure nutrient quantities precisely and conduct testing themselves. With these technologies it is possible to measure macronutrients (NPK and Calcium), two essential parameters (pH and Electrical Conductivity) and even Brix values, all in the field.


While relatively recently introduced, it is becoming more widespread, this plant sap testing method is one of the best diagnostics you can give your crop in real-time.

The application for plant sap testing is vast: open field crops, greenhouse crops, hydroponics, regenerative and organic agriculture.


Field plant sap testing has been rapidly gaining popularity since there is a raising demand from growers to perform testing themselves, receive instant results and own their own nutrient trends.


This post will give you an insight on the importance of plant sap testing, its application, benefits, best practices, will introduce you to plant sap testing equipment and give you some bonus tips (like reference nutrient levels for strawberries for all growing stages).


Skip through the topics as needed!



1. What is plant sap testing? What is the difference with soil testing?

Plant sap testing is a method used to analyze the nutrient levels within plant tissue.

Unlike soil testing, which provides information about the availability of nutrients in the soil, sap testing offers insights into how well plants have absorbed nutrients from the soil and fertilizer, which is a critical aspect for crop nutrition management.


Plant sap testing acts like a real-time diagnostic test for plants, indicating the presence and concentration of various parameters within the plant at the given moment.


In general, plant sap analysis is used as a tool for:


· Obtaining precise data about the nutritional state of a crop.

· Identifying any excesses or deficiencies in nutrients.

small plant with soil showing nutrient concentration difference within soil and plant sap

Here is a simple illustration showing the varying levels of nutrient concentrations in soil and plant sap. Despite the significant difference, both values fall within the optimal range.



2. Application & Benefits of Plant Sap Analysis


Sap testing is particularly important for greenhouse crops and hydroponic farming, where fertilizers are applied as a nutritive solution.


There are numerous benefits associated with plant sap testing:


1) Understanding your plant’s nutritional state



Plant tissue analysis will provide you with crucial information about the plant's nutrient concentration status and potential deficiencies, so you can take an immediate decision (and adjustment, if needed) to your fertilization strategy.


This is done by comparing testers' results with the reference tables. Reference tables would depict optimal levels of nutrients for different crops (by the way, these tables are available from us when you get any of the Agronomist Kits. At the moment we have data for 29 crop types).


2) Cost control and optimized use of fertilizer


The most effective method to control your fertilizer expenses is to know the nutritional requirements of your crop at each stage of its growth cycle. Plant sap analysis saves you money by allowing you to use fertilizer more sustainably, aligning it with real-time plant needs.


3) Proactive vs reactive crop management


Unlike lab analysis, you can conduct all the testing yourself and respond promptly to any necessary adjustments in your fertilization program.

Plant sap analysis provides instant information about the nutritional state of your plant. Field analysis enables proactive diagnosis of excesses or deficiencies before observing any physical effects on plants, fostering proactive rather than reactive crop management.


3. How does field plant sap testing compare with traditional foliar lab analysis?


It's important to note that fresh plant tissue is a good matrix to measure mobile ions absorbed by the plant, (transported from the roots to the leaves) which represents the amount of nutrients available for the plant growth.

Many studies over the years have shown a strong correlation between plant sap analysis and lab based foliar analysis.


Field plant sap testing and traditional foliar lab analysis offer distinct approaches to assessing plant nutrient levels. Let's compare these two methods.


Traditional Foliar Lab Analysis:


  • Involves sending fresh whole leaf samples to a laboratory for analysis.

  • Requires samples to be dried, ground into powder, and measured dry.

  • Reflects both nutrient content and structural nutrients present in the plant cell.

  • Typically yields higher concentration readings (in the % range for macro elements) compared to field sap analysis (in 'ppm' range).

Field Plant Sap Testing:


  • Utilizes fresh plant tissue, typically extracted from the petiole of the Most Recent Mature Leaf (MRML).

  • Measures mobile ions absorbed by the plant, representing available nutrients for growth.

  • Provides real-time insights into nutrient levels, allowing for immediate adjustments to fertilization programs.

  • Offers a mobile and immediate solution, enabling growers to conduct testing on-site without relying on external laboratories.


While both methods have shown a strong correlation in numerous studies, it's essential to consider the differences in concentration readings. The decision to send plant sap samples to the lab depends on factors such as immediacy of results and your goals as a grower.



What does it mean for growers? Should they still rely on plant sap testing laboratories?


For micronutrients, we recommend continuing to send leaf samples to the lab, as they are present in extremely low concentrations that can be difficult for field instruments to detect.


For macronutrients, we suggest utilizing LAQUAtwin and Hanna meters in the field due to their ease of use, value, and prompt delivery of results.


TIP: If you use a laboratory, ask them to share with you sufficiency ranges to help you with interpretation and building your own data. While many labs may not disclose this information, some will provide it upon request. This data will make it easier for you to compare results between different growing cycles.


If you are a professional grower, cultivate multiple crops at once, or manage high-value crops (like marijuana), then you want to set the frequency of your testing and, importantly, to establish and own your own dataset.


4. Optimal nutrient concentration range in plant sap: what level is sufficient for my plants?

optimal nutrient concentration in plant sap for strawberry

First, it's crucial to recognize that plant nutrient concentrations can vary significantly from one farm to another due to several factors:


· Region-specific climate and altitude

· Season and photo-period

· Quantity of fertilizer applied to the soil

· Soil chemical and physical properties

· Phenological stage of the plants

· Expected yield set by the grower


As a result, the optimal concentration of nutrients in plant sap is not a universally applicable range across all farms.


If you are new to sap analysis, a good starting point would be to develop your own farm-specific monitoring system. This approach will help you establish farm-specific sufficiency levels.


Comparing meter readings with reference tables for cucumbers

Reference table for optimal nutrient level in strawberry plant sap

If you are unsure where to begin, you can compare your results with levels obtained by other growers (available in our guide that comes free with every NutriCheck kit, and we also share some reference levels on our social media platforms). These references can serve as an initial point of comparison.


5. How can you test well and build strong crop nutrition knowledge?


Performing regular testing across the season will reveal how nutrient concentrations fluctuate during the growing cycle. Plant sap analysis results can be compared with soil analysis findings to understand the nutrient absorption efficiency and the delay between fertilizer application and plant absorption.


We recommend comparing results from week to week, across different varieties, or between various lots. It may take several full growing cycles until you collect enough data to perfectly understand crop requirements and absorption patterns.


The key to having full confidence in your data is to consistently follow the same sampling and testing protocol, thereby ensuring uniform conditions across all the samples.


6. Plant Sap Analysis Method: Best Practices for Nutrient Measurement in Plant Tissue


Sap is typically extracted from fresh petioles, which is the most representative part to reflect the plant nutrient absorption. The time of day and temperature can influence nutrient levels in the plant sap, so to ensure representative results, a rigorous sampling protocol should be followed.

Sampling 'Most Recent Mature Leaf'


· Sample when the plant is active: in the morning before 11 AM when the ambient temperature is between 58-82°F (14°C & 28°C), and relative humidity is between 60-85%.

· Randomly collect enough samples to represent the entire field or lot for analysis; we recommend collecting at least 20 leaves per hectare.

· Ensure the collected leaves are of the same age, consistently aiming for the most recent mature leaf.

· Avoid collecting leaves from plants showing signs of nutrient deficiencies, disease or insect infestation.

· If leaf samples cannot be tested immediately in the field, place them in a zip bag and store them in a cool environment until testing.

leaves collection protocol showing how to collect samples for plant sap testing

Sample collection and preparation


  • Utilize the pruner to separate the petiole from the stem, cutting the petiole into pieces approximately ½ inch in length.

  • Fill the sap press with the pre-cut petioles.

  • Squeeze and collect the plant tissue extracts into a beaker or test tube.

  • For optimal representability, squeeze all collected leaves into a single container.

  • Mix the petiole sap obtained; now the sap is ready for analysis.


cutting petioles from tomato leaves and extracting plant sap for testing
Cut - Prepare - Squeeze

7. How to measure Nitrate, Potassium, Calcium and Sodium in plant sap


LAQUAtwin meters are widely used for measuring nutrients in plant sap and numerous Crop Science papers have utilized data obtained from these instruments.


These ion meters (Nitrate, Potassium, Calcium and Sodium) offer a wide measurement range, which means you can measure your samples directly, without any prior dilution.


When you have to perform regular analysis, this can really save time, and improve precision + efficiency of your testing procedures.


In rare instances where the concentration exceeds the range, the meter will display an OR message. In such cases, we recommend diluting the sample twice (e.g., mixing 5 ml of sample with 5 ml of deionized water).


The resulting value must be multiplied by the dilution factor to accurately reflect the actual concentration prior to dilution.


Calibrate LAQUAtwin meters before the measurement for extra precision:

1. Place the solution onto the sensor of the meters.

2. Wait until the meter displays a stable reading before your record the results.

3. If you want best accuracy, take 3 measurements of the same sample and calculate the average.

LAQUAtwin meters and phosphorus meter being used in plant sap testing protocol

8. How to measure phosphorous in plant sap with Hanna Checker


The Phosphorus Checker has a limited measurement range (up to 15 ppm) and operates on a color-change principle. Samples should be diluted until transparent or exhibit very light coloration to fall within the meter's measurement range.


For sap samples, the required dilution factor varies based on crop type and sap color. We suggest starting with a 1/50 dilution (0.2 ml of sap with 9.8 ml of deionized water).

If the meter displays L.Lo, indicating high color intensity, further sample dilution is necessary. Try a 1/100 dilution (0.1 ml of sample with 9.9 ml of deionized water).


diluting plant sap sample to prepare for phosphorus testing
Undiluted sap sample - Dilution - Final sample, ready to be tested

To reflect the actual phosphorous concentration in the sample prior to dilution, the meter reading will have to be multiplied by the dilution factor.

How to calculate the dilution factor: Dilution 1/50 (0.2 ml of sample in 9.8 ml of deionized water) => dilution factor = 50 Dilution 1/100 (0.1 ml of sample in 9.9 ml of deionized water) => dilution factor = 100

Testing procedure:

guide on using hanna phosphorus meter HI706

Blank Calibration:


· Pour the diluted sample in the cuvette, insert the cuvette in the meter and close the meter cap.

· Press the button to perform the blank calibration.

· Remove the cuvette from the meter, add 10 drops of the reagent A and the powder from reagent B pouch.

· Close the cuvette and and shake until all reagent B (powder) is dissolved completely.

· Place the cuvette back into the meter and hold the button; a timer with a countdown will appear on the meter display

· After 5 minutes the results will be displayed on the screen. Results are displayed in mg/l (ppm) of Phosphorus (P).

· Multiply the meter result by the relevant dilution factor.



If you are seeking a mobile plant sap analysis kit that will test up to 8 parameters (and is ideal for plant sap testing), explore our TestAgro NutriCheck kit below.



Conclusion


Now that you've learned how to measure key nutrients in plant sap, it's time to build your own dataset to gain a deeper understanding of your crop's nutritional requirements and to utilize fertilizers more effectively.


If you're new to the world of precision agriculture, we encourage you to read more about pH, EC testing and soil testing protocols, to further enhance your growing knowledge and practices.


-Seb



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