Testing growing media pH and EC

Testing growing media pH and EC

A test method comparison

September 19, 2012
Premier Tech Horticulture team

There are various ways to test growing media to know your ph and EC. This is a short review of the five different growing medium testing methods , how to prepare the growing media samples and the appropriate way to test the pH and EC of each sample.

Saturated media extract, 1:2 dilution and 1:5 dilution
Regardless of the method, when collecting growing medium for testing, take the samples from plants that are the same variety, age, container size and fertilized the same. Not following this can result in values that are inconsistent, making it difficult to interpret the test results and make adjustments to your fertilizer program, if necessary. The Saturated Media Extract (SME), 1:2 and 1:5 (parts media:parts water) require collecting growing medium from the root zone and soaking it in varying volumes of distilled water.

If the growing medium is obtained from potted plants, select six to 10 plants and remove the growing medium from the two-thirds of the outside edge of the root zone. Avoid taking the top ½ inch (1.3 centimeter) of the root ball as it contains high salts that can distort test results. For bedding plants, use the lower two-thirds of the root ball from six to 10 plants. For plugs, use the whole cell; however, try to avoid the top layer of growing medium. The goal is to obtain up to 2 cups of growing medium per sample, place it into a plastic bag and mix it together.

Next, add distilled water to the growing medium sample. The amount of water depends on the test. For the SME Method, add enough distilled water until the growing medium glistens, but no water puddles on the surface. For the 1:2 and 1:5 test methods , add one part growing medium to either 2 parts distilled water or 5 parts distilled water, respectively. Regardless of the test method, allow the sample to sit for one hour. If the pH and EC meters have probes, stick the probe in the “mud” to obtain the results. If not, squeeze water from the “mud” with a coffee filter and measure the pH and EC of the extracted solution. Samples containing controlled release fertilizer should be handled carefully to avoid breaking the prills. As a special note, the SME Method is used by the majority of horticulture laboratories. If you use this method, you can compare your pH and EC results with laboratory results.

Three growing media samples prepared using the SME, 1:2 or 1:5 testing methods. All three samples have the same amount of growing medium, but different volumes of distilled water.

SME test method - measuring EC in the mud.

SME test method - Measuring pH in the mud.

Pour-through and squeeze methods
These methods are non-destructive and involve extracting water from a container or cell. The pour-through method is used for cell packs and larger pots while the squeeze method is for plugs or liners smaller than a 50-count tray. Standards for both methods are not as well established, but it is better to use these testing procedures than not test at all. Both the pour-through and squeeze methods require saturating the growing medium one hour before testing. If you constant feed, use the fertilizer solution to thoroughly water the plants or if you pulse feed, use clear water.

For the pour-through method, pour distilled water over the growing medium surface, as listed in the table below, and collect the leachate that flows out of the bottom of the container. The goal is to obtain 50 milliliters (2.5 ounces) of solution per sample. Do not exceed 60 milliliters (3.0 ounces) as the sample will be too diluted and result in lower EC values.

For the squeeze method, collect the solution from the plug by either pressing down on the surface or removing the plug and squeezing the solution out of the growing medium. The volume of solution needed depends on the pH and EC meters used for testing.

For both the pour-through and squeeze methods, the pH and EC are taken directly from the collected solution. The pour-through method is preferred if controlled release fertilizer has been incorporated into the growing medium, since it does not break the prills. Do not use these methods for sub-irrigated crops as excessive surface salts will be leached out into the collected solution and affect EC and pH results.

The table below summarizes the interpretive values of various extraction methods:

* Levels are unacceptable for seedlings

Remember to calibrate your ph and EC equipment before any testing to be sure the results obtained are accurate. With these test results, you can verify if your fertilizer program is delivering the correct nutrients to your crop or you can make adjustments if needed. Please contact Premier Tech Grower Services for additional information and support.