Soil testing method provides steady results

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

The saturated medium extract (SME) soil-testing method is the standard technique used in commercial and university soil testing labs for growing media. Some growers also use the SME method for in-house testing. The advantage of this method is that it provides consistent results and has well-established standards. The disadvantage is that it is not as rapid as some other methods.

SME method steps

Step 1. Collect a small amount of growing medium from the bottom two-thirds of the container. For plugs or flats, use the entire cell as a sample. Take samples from 10 or more plants randomly distributed throughout the group of plants to be sampled. When a sufficient amount of medium is collected, thoroughly mix the sample to ensure uniformity.

Step 2. Place 4-8 ounces (120 to 240 ml) of the collected medium in a cup. When taking a sample that is to be sent in for laboratory analysis, 2 cups (400 ml) of medium is required. Constantly stir the sample with a spatula or knife while slowly adding distilled water until the sample has reached a consistent moisture level.

The saturation point is determined when the sample behaves like a paste, the surface glistens with water, but there is no free water on the surface of the sample. Allow the solution to equilibrate for 60 minutes.

Step 3. Measure the pH directly in the slurry.

Step 4. Extract the solution from the medium by squeezing the slurry through a paper towel or a coffee filter. Measure the electrical conductivity (EC) in the extracted solution.

Reasons for different results

The standard SME method is to measure pH in the slurry and electrical conductivity in the filtered solution.

Some testing labs extract the solution using a vacuum pump. Vacuum extraction tends to raise the pH because carbon dioxide is removed. This and other slight differences in testing protocols mean that your in-house testing results will differ from a lab’s results even if you use the SME method.

The trial results below show that the way a sample is prepared will affect pH and EC readings.

	Vacuum extracted	Squeeze extracted	Measured in the slurry
pH	6.17b	5.7a	5.56a
EC	0.66a	1.39b	0.83a

Numbers within a row that share the same letter (i.e., a) are not significantly different from each other using a statistical test.

Laboratory pH and EC results are helpful because the equipment and protocols used are precise, but you should rely more on your own in-house figures if you are confident in your testing meter and protocol. Laboratory tests can also provide the concentration of individual nutrients or contaminants such as nitrogen or sodium. EC results only indicate the total concentration of all salts combined.

The advantages of in-house testing are that you are able to take additional samples, more frequently and easily track trends over time. In contrast, lab soil tests are more expensive, but are the only way to know how much nitrogen, phosphorus, potassium, sodium and other nutrients are in the medium.

Wait a consistent amount of time (60 minutes) after preparing the sample before you measure pH and EC. The table above shows pH and electrical conductivity measured in a SME slurry at different times after mixing water into the growing medium. The pH drifted slowly up over time, but the electrical conductivity remained unchanged. The trend over time may vary depending on factors such as reaction of slow-release fertilizer in the sample.

If the EC reading is very high, check to see how the sample was taken. Do not sample media from the top one-third of the container except when dealing with small plugs, in which case it is difficult to remove the upper media.

Media samples should be taken only from the bottom two-thirds of the containers, because salts accumulate at the top of the media, especially with subirrigation, due to evaporation from the media surface.

Consistency is the key

To make decisions, you need good data. Consistency starts with having a single, trained person performing the soil tests.

Interpretation of growing media electrical conductivity or soluble salt levels
	Saturated media extract method
Low fertility	0 to 1.0 mS/cm
Acceptable range	1.1 to 3.0 mS/cm
High fertility	>3.0 mS/cm
The units of measure for electrical conductivity can be mMho/cm, dS/m, mS/cm, µM/cm, or mMho x 10-5/cm. The relationship is 1 mMho/cm=1 dS/m=1 mS/cm=1000 µS/cm=100 mMho x 10-5/cm.

One source of error with the SME method comes from multiple people making different interpretations of “saturation.” For example, in a small trial to test sensitivity to errors in water measurement, we collected 120 mL (4 ounces) samples of a peat/perlite growing medium. We added enough water to bring the medium to saturation (100 percent water), and then also applied either 75 or 125 percent of that water volume to represent excessively dry or wet SME.

		pH (slurry)	EC (extract)
75% water	120 mL media/ 26.25 mL water	5.41a	2.07b
100% water	120 mL media/ 35 mL water	5.46ab	1.75a
125% water	120 mL media/ 43.75 mL water	5.51b	1.73a
Measurements in the pH or EC column that share the same letter (a or b) are not significantly different from other measurements in that column.

Varying the water volume had a slight effect on media pH, and a bigger effect on media EC (a low water volume led to a falsely high EC reading).

Purchase a good quality pH and electrical conductivity meter, and calibrate the meters before use. Calibration solutions have an expiration date and should be discarded when that date is reached.

- Paul Fisher, Amy 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.

{sidebar id=1}

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.