Plug plant production presents many challenges to growers—seedlings are sensitive, plug volume is small and any mistake might cause critical delay or even loss of plants. Proper moisture content and nutrition are crucial to successful production.

Once a problem occurs with plug plants it is very difficult to correct, so prevention is the key to successful plug production. Regular onsite testing and proper management can detect potential problems before they occur.

Prior to planting, it is recommended that you perform a complete chemical analysis of your irrigation water and make sure that your fertilizer injectors are calibrated. Fertilizer injectors should be calibrated once a month and substrate properties, such as pH, salt content and moisture—should also be known.

After planting you should test salinity and pH, check the fertilizer delivery system and closely monitor and manage the moisture level in the plug substrate.

In this article we will discuss the best way to perform these onsite tests.

In-house testing of substrates used for plug plants

Testing substrate pH and EC using in-house testing methods is both quick and inexpensive. Here are some of the common testing methods, with guidelines for their interpretation.

Regardless of the testing method you choose to use, make sure that the samples you take are representative of the crop to be tested. Do not mix species, as different species have different nutrient and pH requirements.

Wearing gloves will eliminate the danger of contaminating the plugs with your hands.

Two-to-one (by volume) water-to-substrate

  • Step 1: Take five to 10 plug plant cells as a representative sample and mix the sample to ensure uniformity.
  • Step 2: Air-dry the sample at room temperature. Unless the sample is very wet, it should be sufficiently dry in 24 hours.
  • Step 3: Measure a known volume of your substrate sample in a beaker or a cup. The substrate in the cup should be slightly more compressed than it was in the plug.
  • Step 4: Add two equal volumes of distilled water into the cup and swirl. Allow this to stand for 30 minutes.
  • Step 5: Measure pH and EC directly in the extract.

Saturated media extract

  • Step 1: Take 10 or more plug plant cells as a representative sample and mix sample to ensure uniformity. You should collect four to eight ounces of substrate.
  • Step 2: Place the sample in a cup and slowly add distilled water while stirring the sample continuously. Add water until the sample is saturated. This determination must be made visually—the sample should behave like a paste and the surface should glisten, but there should be no free water on the surface of the sample.
  • Step 3: Allow the solution to stand for 60 minutes.
  • Step 4: Extract the solution from the substrate by squeezing it through a paper towel—a vacuum system could also be used.
  • Step 5: Measure EC and pH in the extract.

Leachate pour-through method

This is probably the most convenient method for testing your substrate chemical parameters. Another advantage of this method is that the procedure does not involve damage to the plants tested since the substrate is never disrupted.

Results may vary substantially, though, depending on the leachate percentage of the total irrigation amount. Many growers don’t take the percentage of leachate into consideration and so might receive less reliable results.

  • Step 1: Irrigate the crop, making sure the substrate is thoroughly wet. Allow the substrate to drain for 30 to 60 minutes.
  • Step 2: Add enough distilled water to get two ounces of leachate.
  • Step 3: Measure EC and pH in the leachate.

Squeeze method

This method for testing the nutritional status of plug plants was developed by North Carolina State University. It is simpler and faster to perform than the two-to-one extract and the saturated media extract, since it is not necessary to dry the sample. It is also considered to be more representative than the other methods, since no subjective addition of the correct amount of water is involved.

  • Step 1: Irrigate the crop with fertilizer, making sure the substrate is thoroughly wet. It is very important to perform this test only after fertilization.
  • Step 2: Allow the substrate to drain for one to two hours—it is necessary to wait at least one hour in order to allow the fertilizer to come into equilibrium with the substrate. On the other hand, waiting too long may result in depletion of nutrients from the substrate by the older seedlings.
  • Step 3: Sample substrate from at least five plug trays and mix the sample to ensure uniformity.
  • Step 4: Place the collected sample in a paper towel or cheesecloth and squeeze the solution from the substrate into a cup.
  • Step 5: Measure EC and pH directly in the extracted solution.

Interpretation of results

Your interpretation of the results will depend on the method you used. The table below gives general guidelines for interpreting the results, using each of the described methods.

Interpretation of Soluble Salt Levels (Electrical Conductivity) of Substrates (mS/cm)

Rating Two-to-one method Saturated media extract method Pour–through method Squeeze method
Very low salt levels, indicating very low nutrient status 0-0.25 0-0.74 0-1.0 0-1.0
Low fertility. Suitable for seedlings and salt-sensitive plants 0.25-0.75 0.75-2.0 1.0-2.5 1.0-2.5
Acceptable range for most established plants 0.75-1.25 2.0-3.5 2.5-4.5 2.5-5.0
High fertility. May be suitable for high-nutrient-requiring plants 1.25-1.75 3.5-5.0 4.5-6.5 5.0-6.0
Can cause root damage >2.5 >6.0 >8.0 >8.0

Interpretation of pH Levels for Substrates

(pH levels are the same for all of the described testing methods)

Acceptable range
Iron-inefficient plants 5.4-6.2
General 5.8-6.4
Iron-efficient plants 6.0-6.6