Use the 1:2 testing method for media-pH and EC

Terms including EC, PourThru and SME have been tossed around a lot lately, so here’s the last article in a 3-part series on the basics of soil testing.

The 1:2 soil testing method is one of the most popular ways to test pH and electrical conductivity (EC) in container growing media. It is rapid, reliable and has well-established guidelines and can help you improve nutrient management.

1:2 measuring steps

Step 1. Collect a small amount of growing medium from the bottom two-thirds of the container. The bottom two-thirds is typically where the roots are in a pot. Sampling from this area avoids fertilizer salts that can accumulate at the medium surface with all irrigation methods (not just subirrigation). For plugs or flats, use the medium from the whole cell as a sample.

Take samples from 10 or more containers distributed in the group of plants to be sampled. Combine the medium (about a spoonful from each pot) into one container for a combined sample. When a sufficient amount of medium is collected, thoroughly mix the sample to ensure uniformity. Each time you measure, you can select from different pots to ensure you are not removing too much medium or biasing the results.

Step 2. Measure out a known volume of medium from the combined sample into a beaker or cup. Usually 60-120 milliliters (2-4 ounces) is sufficient. The medium should be firmly compressed in the beaker so that it is slightly more compressed than when it was in the pot.

Step 3. In a separate container, measure a volume of distilled water equal to twice the volume of the medium sample.

Step 4. Mix the water into the medium sample and stir lightly. Allow the solution to equilibrate (30-60 minutes) before measuring pH and EC.

Step 5. Measure both pH and EC directly in the slurry.

Consistency is the key

To make the right decisions, you need good data. Consistency starts with having a single, trained person perform the soil tests. You will also need a quality pH and EC meter. Be sure to calibrate the meter before each use. Calibration solutions have an expiration date and should be discarded when that date is reached.

If you measure a very high or low pH or EC reading, check that the sample was taken correctly, and that the meter is calibrated. Retest before you decide to make a management change based on the soil test.

Tips for success

Tip #1. Measure medium and water volumes separately and then mix them. If you try to measure out water volume in the same beaker that already contains growing medium, you will end up over-applying water and diluting the sample, which will result in a falsely low EC reading.

A trial test was conducted using a peat/perlite/vermiculite medium. We measured soil (60 milliliters or 2 ounces) and water (120 milliliters or 4 ounces) volumes separately (correct method) with a resulting EC of 1.0 mS/cm. The left beaker in the photo shows the correctly measured volume of water in a 1:2 test (media and water measured separately).

The right beaker in the photo above shows excess water applied by measuring the water in the same container as the growing medium. Pouring water into the soil beaker to bring the soil/water level up to a total of 180 milliliters (incorrect method) required 140 milliliters of water (16 percent extra) and EC was 0.9 mS/cm (slightly lower than the true reading).

Tip #2. Compress the medium down slightly more in the beaker than it was in the pot. Too much or too little compaction will change the results by changing the amount of medium in the sample.

The three beakers shown above contain the same medium weight, but with different pressures to pack the medium down. The more the medium is packed down into the container, the more solid medium (and nutrients or other salts) will be concentrated in that given volume of medium.

We ran a trial in which 75 milliliters or 2 ounces of a moist peat/vermiculite/perlite commercial medium was packed down at the recommended level (slightly more compressed than in the pot), or as tight as we could by hand or loosely filled. After adding 150 milliliters of water to each beaker, we measured EC and pH (see Table 1). Media EC increased as compaction increased, but pH was not affected.

If the EC reading is very high, check how the sample was taken. Do not take the medium sample from the top one-third of the container, except where you have small plugs (in which case it is difficult to remove the upper medium surface). This is because salts accumulate at the top of the medium, especially with subirrigation, due to evaporation from the medium surface.

We have measured EC levels 20 times higher in the upper layer of the medium compared with the lower two-thirds of the container (where most of the roots grow).

Tip #3. Be sure not to break slow-release fertilizer prills when preparing the sample. Broken prills can result in an artificially high EC reading. When using slow-release fertilizer consider using the PourThru testing method.

- Paul R. Fisher, Amy C. Douglas and William R. Argo

Paul Fisher is associate professor in the Environmental Horticulture Department at the University of Florida; pfisher@ufl.edu. Amy Douglas is an assistant breeder at NovaFlora Inc., adouglas@novaflora.com. William R. Argo is technical manager, Blackmore Co., (800) 874-8660; bargo@blackmoreco.com.

The authors thank the University of New Hampshire Agricultural Experiment Station, and our Young Plant Research Center partners Blackmore Co., Center Greenhouses, Costa Nursery, D.S. Cole Growers, Ellegaard, Fafard, Four Star Greenhouses, Glass Corner Greenhouses, Greencare Fertilizers, Knox Nursery, Kube-Pak Corp., Lucas Greenhouses, Pindstrup, Pleasant View Gardens, Premier Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and Welby Gardens for supporting this research.

The authors and their organizations do not assume any liability for use of this information. Use of trade names does not imply endorsement, preference or recommendation for any products listed or omitted.

Effect of growing media compaction on 1:2 soil test

Interpretation of media pH levels for container-grown crops

Values are the same for all testing methods.