Achieving Plug Plant Perfection

In plug plants, once a problem occurs, it is much more difficult to correct it, so preventing problems is the key to successful plug production. Regular on-site testing and proper management can detect potential problems before they occur and damage is done.

Prior to planting, it is recommended to perform a complete chemical analysis of the irrigation water and to make sure that the fertilizer injectors are calibrated. Fertilizer injectors should be calibrated once a month.

Properties of the substrate, such as pH, salt content and moisture should also be known in order to produce quality plug plants. After planting the plug plants, you should test salinity and pH, check fertilizer delivery and closely monitor and manage the moisture level in the plug substrate.

In this article we will discuss the recommended methods to perform these on-site tests for plug plants. These methods also apply for container plants.

In-House Testing of Substrates Used for Plug Plants

Testing substrate pH and EC using in-house testing methods is both quick and inexpensive. We will describe here some of the common testing methods used for plug plants and give guidelines for their interpretation.

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

Wearing gloves will eliminate contamination of the plugs from your hands.

2:1 (by volume) Water: Substrate

Step 1: Take 5-10 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 dry enough in 24 hours.

Step 3: Measure a known volume of the 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 volume of distilled water into the cup and swirl. Allow it 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 150-300 ml (4-8 oz.) 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. The determination of how much water to add is done 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 equilibrate for 60 minutes.

Step 4: Extract the solution from the substrate by squeezing it through a paper towel. A vacuum system can also be used.

Step 5: Measure EC and pH in the extract.

Leachate Pour-Thru Method

This is probably the most convenient method for testing the substrate chemical parameters. Another advantage of this method is that the procedure does not involve damage to the plants tested, since the substrate is not disrupted. However, results may vary a lot, depending on the leachate percentage of the total irrigation amount. Many growers to not take the per cent leachate into consideration and therefore might receive less reliable results.

Step 1: Irrigate the crop, making sure the substrate is thoroughly wet. Allow the substrate to drain for 30-60 minutes.

Step 2: Add enough distilled water to get 50ml (2 oz.) of leachate.

Step 3: Measure EC and pH in the leachate.

The 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 then the saturated media extract and the 1:2. It is also considered to be more representative than 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 60-120 minutes. 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 (cheesecloth) and squeeze the solution from the substrate into a cup.

Step 5: Measure EC and pH directly in the extracted solution.

Interpretation of the Results

The interpretation of the results will depend on the method 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)

Source: smart-fertilizer.com