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Plant Sap Analysis: Complete Guide for Modern Growers

Updated: 7 days ago

plant sap analysis blog post with laquatwin meters

This guide covers everything from plant sap analysis, its benefits, application, essential sampling techniques to equipment recommendations and best practices so you can develop and manage your crop nutrition program yourself.

1. What is Plant Sap Analysis?


Plant sap analysis is a powerful, real-time diagnostic method for measuring nutrient levels present in your plant, giving growers an in-depth look at crop health right in the field.


This process is done by extracting sap (i.e. xylem carrying soil nutrients and dissolved minerals) from petioles or leaves, then analyzing it with specialized equipment.


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.


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.


Field plant sap analysis 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.


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.


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


2. Application & Benefits of Plant Sap Analysis


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


There are numerous benefits associated with plant sap testing:


a) Real-time insight on your crop nutritional state


girl checking cucumber leaves on a farm
Testing for nutrients with field equipment

Plant sap analysis done in the field will provide you with real-time data about the plant's nutrient concentration status.


Unlike conventional lab-based foliar analysis, plant sap testing enables you to conduct testing yourself and detect deficiencies or excesses before they manifest visually.


With field plant sap analysis, there is no time delay to receive the results: you can take an immediate decision (and adjustment, if needed) to your fertilization strategy.


b) 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.


c) Onsite testing and independent decision making


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

Proactive crop management


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?


Many studies over the years have shown a strong correlation between plant sap analysis and lab based foliar analysis. (see illustration below)


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.


table showing laquatwin testers results compared with lab measurements
Correlation between LAQUAtwin measurement data and ion chromatography

Field plant sap testing and traditional foliar lab analysis offer distinct approaches to assessing plant nutrient levels.

Let's compare these two methods.


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.


4. Working with Laboratories: When to Use Them and When You Can Conduct Testing Yourself


For micronutrients, we recommend continuing to send leaf samples to the lab, as they are present in extremely low concentrations that can be difficult (but not impossible) for field instruments to detect.


For macronutrients, we suggest utilizing field instruments or kits 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, then you want to set the frequency of your testing and, importantly, to establish and own your own dataset.

5. How to Sample Your Plants for Field Plant Sap Analysis


girl prepares for plant sap testing cutting leaves
Sampling 'Most Recent Mature Leaf'

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.


 · 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

6. [VIDEO] How to Measure Nitrate, Potassium, Calcium and Sodium in Plant Sap


LAQUAtwin meters are widely used across the world 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

7. 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 = 50Dilution 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.


8. I Have My Results: What Level Is Sufficient for My Plants?

girl checking nutrient levels after plant sap analysis
Comparing meter readings with reference tables for cucumbers

You can compare your results with optimal nutrient reference ranges developed by other growers or researchers. (Kit NutriCheck LAQUAtwin comes with a guide with 36 crops).

Some crops will have more information available than others.


Here is an example for strawberry at different growing stages for Nitrate, Potassium, Calcium and Phosphorus. These references can serve as an initial point of comparison.


Reference table for optimal nutrient level in strawberry plant sap

However, 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.


9. Building on Your Results and Developing 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.



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 LAQUAtwin 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.

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