The Plug squeeze method to test substrate-pH and electrical conductivity (EC) for plugs and liners

Goal: Use the plug squeeze method to test substrate pH and electrical conductivity (EC) for plug seedlings and liners. 

Why are pH and EC important?

Young plants are very sensitive to substrate-pH and EC.

Low substrate-pH level may lead to micronutrient toxicities, whereas deficiencies in micronutrients are common at high pH.

Plants will be short of nutrient supply if substrate EC remains low. High EC can lead to salt burn of roots.

How to do it?

Step 1. Requirements

Requirements

You will require:

  • A pH meter for pH measurement
  • an EC meter for testing EC,
  • a beaker for collecting soil solutions,
  • and the target plug seedlings or liners.

Step 2. Irrigate to container capacity

Irrigate the crop to container capacity using the fertilizer or clear water solution that you are applying for your crop one hour before testing. Make sure the substrate is thoroughly wet, but drained at the time of testing.

Step 3. Sample collection

Sample collection

Collect samples from 10 or more cells distributed in different trays throughout the crop. Combine the samples into one container.

Squeeze the solution from the cell by either pressing down on the plug surface, or removing the plug and squeezing it into a collection container. The volume of solution needed will depend on the type of pH or EC meter used for testing. pH and EC meters that require a small sample volume are ideal for this method.

Step 4. Measure pH and EC

Measure and interpret

Measure the pH and EC in the extracted solution.

How to interpret the results

Acceptable pH range

  • Iron-inefficient 'Petunia' group: 5.4 to 6.2
  • General group: 5.6 to 6.4
  • Iron-efficient 'Geranium' group: 6.0 to 6.6

EC value (mS/cm)

  • Low fertility: 0 to 0.9 (more fertilizer may be required)
  • Moderate fertility: 1.0 to 2.7 (suitable for young plants)
  • High fertility: >2.7 (high for young plants, may need to leach or reduce fertilizer concentration)

Low EC levels do not necessarily indicate a problem in plugs and liners, because vigorously growing plants can rapidly deplete nutrients (within hours). Young roots are also sensitive to high salt levels.

The Plug Squeeze Method is ideal for small plugs and liners, where a large number of cells would be destroyed if a 1:2 or saturated paste extract method is used.

The Plug Squeeze method should not be used on containers larger than a 50-count 10' x 20' (25 x 51 cm) tray because substrate test results tend to be variable in larger pots.

For more information: Contact authors Jinsheng Huang and Paul Fisher of University of Florida IFAS Extension. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fafard et Frères Ltd (Canada), Fine Americas, Greencare Fertilizers, Pindstrup, PremierTech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. August 23 2014.