Whether greenhouse or high tunnel plants are grown in soil such as ground beds or raised beds, or grown in soilless media in containers, it is important to regularly test electrical conductivity (EC) to monitor soluble salts and adjust fertility programs.
What are Soluble Salts?
Soluble salts are dissolved inorganic solutes. In growing media, common soluble salts are calcium, magnesium, sodium, chloride, sulfate and bicarbonate. Smaller quantities of potassium, ammonium, nitrate and carbonate are also found. Sources of soluble salts in soils and soilless media include commercial fertilizers, animal manures (usually high tunnels), soil organic matter, composts, runoff from areas where salt or ice-melt products have been used and irrigation water that is high in dissolved salts.
High Soluble Salts
Soluble salts above the normal range for a prolonged period may cause root injury, leaf chlorosis, marginal burn and sometimes wilting. When soluble salt levels are above the normal range, growers are advised to identify the source for the high levels.
Reducing high soluble salt levels may involve reducing the concentration and frequency of the fertilizer being applied and watering thoroughly to leach out excessive salts. To prevent soluble salts from reaching above normal levels, match fertilizer applications with plant nutrient needs as the plant grows. Consider plant growth rate and environmental conditions as part of a nutrient management program. To avoid soluble salt buildup in high tunnels, incorporate high tunnel soils thoroughly and flush with irrigation water after removing summer crops. If tunnels will remain unplanted over the winter, growers are advised to consider planting a cover crop or removing the plastic to allow rainfall to leach soluble salts. Proper nutrient management and monitoring soluble salts are best management practices that can reduce fertilizer costs and prevent ground water contamination which can result from excessive leaching due to high soluble salts.
Measuring Electrical Conductivity
Measuring the electrical conductivity (EC) of either the soil solution or a soil-water extract is the common method to quantify the concentration of soluble salts in growing media. Electrical conductivity is the ability of a solution to conduct an electrical current. As soluble salts increase the media, the solution becomes a better conductor of electricity and the EC increases.
Extracting Solutions and Test Results
Soil or media samples themselves are not actually analyzed during a soil test, but rather plant-available nutrients are pulled out or "extracted" from the sample using an "extracting solution".
There are three commonly used methods of extracting soluble salts based on the use of water as an extracting solution. They are: Saturated media extract (SME), 1:2 dilution method, and Leachate PourThru.
The amount of water used to extract plant-available nutrients and other details of the tests can give large differences in the results of the tests. This is illustrated by a comparison of soluble salt values in the tables. Note that the 1:2 method results in the lowest levels. Results of SME and leachate pourthru (soilless media) are closer, but some differences exist.
Always use the interpretative data that match the test you made otherwise you could make an incorrect interpretation of the results. For example, a result of 2.6 for soilless media would be "extreme" (too high) for the 1:2 method, "normal" for SME, and "low" for leachate pourthru. What a difference in how you would think about your results!
There is also a difference in the data depending if the media is a soilless media or contains 20% or more soil. Table 1 contains interpretation values for soilless media and Table 2 for soil.
Saturated media extract (SME). SME is currently "the" method of testing soilless greenhouse media and it is almost universally done by commercial and university labs, including the UMass Soil and Plant Tissue Testing Lab. In this test a paste is made using soil and water and then the liquid portion (the extract) is separated from the solid portion for pH, soluble salt, and nutrient analysis. Special skills and laboratory equipment are required to perform this test. This method is a more representative measurement of total soluble salts in the solution.
1:2 dilution method. This method has been used for many years and has good interpretative data to back it up. In this test an air-dried sample of soil and water are mixed together in the volume ratio of 1 part soil to 2 parts water (e.g., using a measuring cup, 1 fl. oz. of soil + 2 fl. oz. of water). The liquid extract is then separated from the solids using laboratory grade filter paper or a common coffee filter. The extract is then ready for analysis. This is a very easy test to master and quite suitable for on-site greenhouse testing of pH and soluble salt using the so-called pH and EC "pens" available from greenhouse suppliers. The 1:2 method is a very good choice for occasional pH and soluble salts testing by growers on-site.
Leachate PourThru. Leachate PourThru is a method that would be used for plants grown in containers. One of the major advantages to leachate pourthru is that there is no media sampling or preparation. Unlike SME and 1:2 methods, plants do not have to be sacrificed or disturbed for testing because the extract is the leachate collected from the container during routine irrigation. The leachate can be analyzed on-site using the pH and EC pens or it can be sent to a commercial laboratory for a complete nutrient analysis.
Leachate pourthru is best used for continuous monitoring and graphical tracking of pH and soluble salts. To make this method work best an irrigation and leachate protocol must be established and carefully followed when sampling takes place. Leachate pourthru is not a good choice for casual checks (use 1:2 method for this). Some growers like to "whip out" the old pH or EC pen now and then and check the drainage from some pots. Unfortunately, with casual use like this, the "numbers" are often quite variable, inconclusive, and probably unreliable.
Table 1. EC Interpretation Values (mS/cm) for Saturated Media Extract (SME), 1:2 and PourThru for Soilless Media
| Indication | SME | 1:2 | pourThru |
|---|---|---|---|
| Very low | 0-0.75 | 0.25 | 0-1.0 |
| Low | 0.76-2.0 | 0.3-0.75 | 1.0-2.6 |
| Normal | 2.0-3.5 |
0.76-1.25 |
2.6-4.6 |
| High | 3.5-5.0 | 1.26-1.75 | 4.6-6.5 |
| Very High | 5.0-6.0 | 1.76-2.25 | 6.6-7.8 |
| Extreme | >6.0 | >2.3 | >7.8 |
Source: North Carolina State University. 2000. https://content.ces.ncsu.edu/monitoring-and-managing-ph-and-ec-using-the-pourthru-extraction-method
Table 2. EC Interpretation Values (mS/cm) for Saturated Media Extract (SME) and 1:2 for Soil
| Indication | SME | 1:2 |
|---|---|---|
| Non-saline | 0.1-2.0 | <0.40 |
| Very Slightly Saline | 0.40-0.80 | |
| Moderately Saline | 2.1-4.0 | 0.81-1.20 |
| Saline | 1.21-1.60 | |
| Strongly Saline | 4.1-8.0 | 1.61-3.20 |
| Very Strongly Saline | 8.1-16.0 | >3.20 |
Source: Gartley K.L. Recommended Methods for Measuring Soluble Salts in Soils. Recommended Soil Testing Procedures for the Northeastern United States. Chapter 10. 5/2011.
These units are commonly used for values when measuring EC:
1.0 mmhos/cm = 1.0 dS/m = 1.0 mS/cm = 1000 EC (µS/cm)
References
Cox D. 2005. Current Methods of Greenhouse Media Testing and How They Differ
UMass Extension Greenhouse Crops and Floriculture. Soil Testing
The Northeast Coordinating Committee for Soil Testing. 2011. Recommended Soil Testing Procedures for the Northeastern United States, Northeastern Regional Publication No. 493, 3rd Edition, Agricultural Experiment Stations of Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont and West Virginia.
North Carolina State University. 2000. Monitoring and Managing pH and EC Using the PourThru Extraction Method.
Sanches Elsa. Dealing with High Soluble Salt Levels in High Tunnels. Penn State Extension