Press technique: Determining substrate pH and electrical conductivity of plugs and liners

Utilize the press technique to determine substrate pH and electrical conductivity (EC) of plugs and liners.

The press technique
An example of the press technique to determine substrate pH and electrical conductivity of plugs and liners. Photo by W. Garrett Owen, MSU Extension.

In-house nutritional monitoring methods such as the PourThru, Saturated Media Extraction (SME) or 1:2 Dilution procedures are used to determine substrate pH and soluble salts, referred to as electrical conductivity (EC), levels. Most often, these procedures performed on plants produced in their final containers and rarely on plugs or liners. How do you monitor substrate pH and EC of plugs and liners? Either conduct a PourThru or utilize the press technique or plug press.

To conduct a PourThru on plugs, refer to the video, “PourThru Monitoring: Sampling Plugs.” To conduct a plug press, you will need the following supplies for the following seven steps.

Supplies: Distilled water, collection saucer or cup, 3- and 12-ounce cups, pH/EC meter and calibration solutions, and record sheet.

  1. Irrigate at least five plug trays of the sample species and cultivar/variety one hour before testing.
  2. Within a plug tray, choose the plugs to sample. Note: The number of plugs or liners to sample depends on the volume of the plug cell. You may have to sample fewer plugs per tray if the plug cell volume is larger compared to smaller plugs with smaller plug cell volume, thus requiring more plug press sampling.
  3. Place the collection saucer or cup under the selected plugs of the plug tray.
  4. Collect the leachate by gently pressing on top of the plug with your index finger (see photo). The leachate will press out of the plug tray drainage hole. If conducted correctly, the plug will be pushed over and will recover within days. Note: The amount of leachate will be determined by the plug cell volume (Step 2) and the type of pH/EC meter, e.g., submersible probe-type or flip top sensor-type pocket pH meters.
  5. Calibrate the pH/EC meter. Note: Calibrate pH/EC meters before each use because measurements are only as accurate as the last calibration. Ensure your calibration solutions have not expired. Furthermore, proper pH/EC meter probe maintenance and storage should be practiced following the manufacturer’s instructions. Test the leachate for pH and/or EC. Average the values.
  6. Interpret the values. If a corrective action is needed, please refer to e-GRO Alert 7.02: “Corrective Procedures for Modifying Substrate pH and Electrical Conductivity (EC).”

The American Floral Endowment is gratefully acknowledged for funding to create and establish all available materials. 

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