Petiole Sap Testing

Goal: Test the plant sap to evaluate nutrient levels within the plant (nitrogen and potassium) and compare with standards. This will enable to adjust your nutrient solution, if needed.

Why is it important?

Lack of an essential nutrient will result in deficiency symptoms. Nitrogen, in particular, is used in large quantities by plants and is fundamental for many processes and structures. Nitrogen is part of the chlorophyll, amino acids and, nucleic acids.

How to do it

Step 1. Requirements

  • Requirements
    garlic press
  • knife
  • cutting board
  • delicate task wipes
  • distilled water
  • nitrogen and potassium meters
  • calibrating solutions

Before extracting the sap from sampled leaves, calibrate the meters. A two-point calibration is preferred. Become familiar with the equipment, follow manufacturer's directions and calibrate often. It is recommended to re-calibrate every six samples. 

Step 2. Sample selection

MRML

Older leaves translocate nutrients (source) to young leaves (sink) which makes nutrient concentrations not reliable. Nutrient levels are the most stable in the most-recently matured leaves (MRML). A leaf that is not mature yet will have a lighter green color and will be smaller than fully expanded leaves. To identify the MRML, look for leaves that have stopped expanding and that have a dark green coloration.

Cut the entire MRML, remove the blade immediately and keep the intact petiole only. Collect as many leaves as you consider will be a representative sample for your area. Select leaves that appear normal; avoid sampling leaves that have pest or disease issues.

If the leaf of your vegetable crop is compound, such as tomato, the petiole can be several inches long whereas simple leaves, such as pepper or basil, will have short petioles. The shorter the petiole, the more leaves will be needed to collect the minimum sap volume required by the sap meters.

Step 3. Sample storage (optional) and other sampling recommendations

Sampling

Make your sampling as consistent as possible. Sap testing can be performed from 9 am to 4 pm always from the MRML. Taking samples around the same time every time you test the sap, will provide results that are more consistent.

The nutrient levels in the sap will only be reliable for a couple of minutes after the sap is squeezed from the petioles. Therefore, if large greenhouses or multiple locations need to be sampled, storing the petioles of the MRML inside a plastic bag is recommended. Petioles (not chopped) can be stored at room temperature for up to 2 hours, on ice inside a cooler for up to 8 hours or frozen overnight. If petioles have been stored under cold conditions, they need to be warmed to room temperature before extracting the sap for reading nutrients.

Step 4. Sap extraction and measurement

Sap extraction

Ideally, take the reading of the sap under a shaded location to avoid exposing the meter sensors to direct sunlight. Most standards provide guidelines of nitrogen levels as nitrate-nitrogen (NO3-N). Therefore, before you extract the sap, make sure that the sap meter is set up to provide results in the nitrogen unit provided by the standard. 

If you have a fresh sample, remove the leaf blades and chop the petioles into smaller pieces. If you have stored petioles under cold conditions for several hours or from the day before, remember to warm them to room temperature before chopping them to analyze the sap. Chop petioles and insert them inside the garlic press. Gently squeeze the sap out of the plant tissue and deposit a few drops of sap on the sensor. 

How to interpret it

Meters

Compare the results with the standard. Be mindful to compare your results with the standard that is appropriate for your crop developmental stage (i.e. first blossom, fruit 2-inches long, etc.). For interpretation, you can consult comprehensive documents such as https://edis.ifas.ufl.edu/cv004. By comparing your results with the standard, you will be able to determine if your nutrient solution is achieving the targeted plant nutrition or if you need to adjust your hydroponic solution.

For more information: Contact authors Tatiana Sanchez and Paul Fisher of University of Florida IFAS Extension. Register for our online certificate courses to increase your greenhouse knowledge. April 16 2020.