Get Cultured: Monitoring electrical conductivity of irrigation water and rooting media

by Donald J. Merhaut

The rooting media in container production is influenced by three main factors: substrate types, water quality and temperature.  There are two main variables, pH and electrical conductivity (EC), which can be measured to indirectly determine nutritional status of the rooting media and impacts on production from water sources.  How to measure pH and how it relates production processes is the subject for another newsletter.  In the following article, I will focus on the EC only, what it means to your production, how to properly measure EC and what action to take if EC levels are high

Irrigation Water Electrical Conductivity (EC)

The electrical conductivity (EC) is a measure of the ability of water to conduct electricity. Different laboratories may use different units of measure.  These units and their conversions are given in table 1.

Table 1.  Common units of measure for electrical conductivity and their conversions.

Conversions: 1 dS/m = 1 mS/cm = 1 mho/cm = 1000 µmho/cm = 1000 µS/cm = 700 ppm

Deionized water cannot conduct an electrical current because all the ions have been removed. The EC of water increases when positively and negatively charged ions, such as sodium (Na⁺), fluoride (F^-), calcium (Ca²⁺), sulfate (S0₄^2-), nitrate (NO₃^-) and ammonium (NH₄⁺), are added to the water.  Potable water has some salts, such as F^-, Na⁺ and NO₃^-, which will increase the EC, usually in the range of 0.5 to 0.8 dS/m.  For other water sources, such as well water, the EC may be elevated due to the presence of carbonates (CO₃^2-) and bicarbonates (HCO₃^-), the presence of which also contributes to alkalinity of the water.  Alkalinity is the measure of water’s ability to increase rooting media pH and will be addressed in a future newsletter. In brief, there is no direct correlation between alkalinity and EC of water sources, so EC measurements cannot be used to estimate water alkalinity or the functionality of acidification processes to reduce alkalinity (see example in table 2).

Table 2.   The effects of acidification to reduce alkalinity on irrigation water.  Even though pH and alkalinity decreased with acidification, there was no effect on electrical conductivity.

From Albano, et al. 2017.  Irrigation water acidification to neutralize alkalinity for nursery crop production: substrate pH, electrical conductivity, and nutrient concentrations; and plant nutrition and growth. HortScience. In Press.

Other sources which may increase well water EC include salt water intrusion or other dissolved minerals.  If a water source has a relatively high EC (> 1.0 dS/cm), it may cause production problems, especially if fertilizer, which will increase EC, is injected into the irrigation system.

Management practices such as blending with fresh water of lower EC, or filtration, such as reverse osmosis, may be necessary.  High salts in secondary water sources, such as treated municipal water (reclaimed water) and recycled irrigation water, are common, and more so in summer months when evaporation rates are high which concentrates the salts.  In extreme cases, high EC in irrigation water can cause crusting of salts on the container surfaces (fig.1) and the inside of the southwest sides of containers where drying of media occurs first, especially those plants on the border rows of production beds.  In addition, drip emitters and sprinkler heads can become clogged with salt buildup.

Electrical Conductivity of Irrigation Water Fortified with Fertilizer

When injecting fertilizer into the irrigation system, measuring EC can provide an easy way of monitoring that fertigation system to ensure that it is functioning properly.  There are a few rules to follow when utilizing EC to estimate proper fertilizer injection:

1. Keep a log book of EC readings.
2. Let the irrigation system run before taking a sample.
3. If water sources change, the EC of the new water source needs to be considered.
4. If fertilizer formulations change, new EC readings need to be established, as different fertilizer types can change the EC, even if the nitrogen rates are similar (table 3).

Table 3.  The electrical conductivity of fertigation water containing different fertilizer types.^*

^*In this example, deionized water was used, which has an EC of 0.00 dS/m.  All chemicals used were reagent grade.  Commercial fertilizers may result in a higher EC due to impurities. ^†While urea has an EC of near 0.0 dS/m, in production systems it can quickly break down to ammonium (NH₄⁺) and ammonia (NH₃), which can be rapidly taken up by plants and can be toxic in high rates.  This is especially a problem during high summer temperatures when this chemical conversion is faster.

When water sources change, it is likely that the EC will also change.  For example, some municipalities rely on surface water sources during winter and spring, then will switch to well water later in the summer months.

When fertilizer is injected, it is critical that EC measurements be confirmed with the new fertilizer source.  For example, a grower may be injecting 100 ppm nitrogen with one fertilizer brand during the summer months, and then switch to a different formulation that is more water soluble for the colder winter months.  Even though nitrogen concentrations may be the same, EC can be different.  Table 3 lists the EC of different fertilizer sources.

There are two major “take home” messages from the fertilizer examples shown in table 3.

1. Fertilizer source can have a large impact on EC, even if nutrient concentrations are similar. Even though there are six different nitrogen sources in this example, all containing 100 ppm nitrogen, the EC ranged from 0.03 (urea) to 0.78 (ammonium chloride).
2. Only compounds that carry a positive or negative charge or dissociate (break apart) into ions in water will increase EC. Unlike molecules such as ammonium chloride (NH₄Cl), which break apart into the charged ions of NH₄⁺ and Cl^-_, urea does not dissociate (“break up”) into charged molecules when dissolved in water. Therefore, urea, an uncharged molecule, does not contribute to the EC of the water.  Thus, even at 500 ppm nitrogen from urea, the EC is still near 0.0 dS/m.  Note that in the soil, urea eventually will break down into ammonia and ammonium. 
3. Similar to urea, chelates are also uncharged molecules, so the contribution to the EC is negligible, as seen with this iron chelate example.

Monitoring Rooting Media EC

While it is critical to monitor EC in the irrigation water sources and fertilizer injection systems properly, it is also important to monitor rooting media EC.  Some commercial rooting media contain fertilizer or you may add granular fertilizer types into the media as part of your production programs.  In addition, as organic components break down, other salts may be released from the media.  Over time there can be a buildup of salts in the container, and in some cases this can be severe (fig. 1).  In normal production practices, it is usually recommended to have a leaching fraction of 25%, so if you add 1 liter of water to a container, 0.250 liter drains from the container bottom.  By conducting the following “PourThru Method” (see below), one can monitor salt buildup through EC measurements and changes in rooting media pH with a pH meter.  Both EC and pH meters are available in portable pen-sized electrodes, making it much easier to perform on site measurements.

Instructions for Monitoring EC and pH in Container Media

This method is adapted from Cavins et al., 2000, Monitoring and managing pH and EC using the PourThru Extraction Method, NC State University Leaflet 590 (https://content.ces.ncsu.edu/monitoring-and-managing-pH-and-ec-using-the-pourthru-extraction-method):

• Irrigate containers as you would do in your standard production practice, using the water source you usually use, even if you inject with fertilizer. This can be drip, overhead irrigation, or hand watering. The container should be thoroughly saturated.  If the substrate contains a high percentage of peat moss or other substrate which is hydrophobic (does not rewet easily when dry), use other containers that do not have this problem. 
• Allow containers to sit approximately one hour for drainage to occur.
• Place the container in a bucket or saucer. A small paint bucket works well for #1 container (fig. 2 and 3).  It is also recommended to place an inert object such as a PVC ring in the container to elevate the pot above the bottom of the collection bucket.  This insures that all solution drains from the container and prevents pots from sitting in the drainage water.
• Add only enough distilled water so that you can collect about 50 millilters of leachate from the pot. It is preferred to use distilled water since this will have an EC value of 0.0 dS/m.  However, potable water may be used, but subtract out the EC contributed by potable water.  As a side note, if the potable water has high alkalinity, it will interfere with pH readings.  The important part of this step is to not add too much water, because this will dilute the sample, causing erroneously low EC values.
• Allow the pots to drain into the saucer or collection bucket, approximately one hour, depending on media type and size of containers.
• Measure the EC with a portable EC meter, following the instructions for the specific meter that you are using.

 Some key notes regarding the PourThru methods:

1. Perform a few test runs to determine the amount of water needed to collect approximately 50 milliliters of leachate. This will vary with container size and type of rooting media.
2. Perform the method on at least three to five pots and get the average reading for those containers.
3. Keep the containers level so that the water flows through the media uniformly.
4. Choose plants for the test carefully. The growing media conditions from container to container throughout a production block can be quite variable. For example, containers that are on the outside of the southwest side of the production bed tend to dry out faster and will give higher EC readings.  The PourThru method can be used to see if excess salt accumulation is indeed occurring on these production bed perimeters.
5. Be consistent with sampling procedures each time the test is conducted. It is recommended to also record the volume of leachate collected.  This way, you can tell if testing procedures are uniform, especially if different individuals conduct the tests.  If you decide to weigh the leachate rather than measure the volume in a beaker, 1 milliliters of water weighs 1 gram.
6. In the summer, you may need to do tests every month or every other week. Also, more frequent testing will be necessary if plants show signs of salt stress, such as leaf necrosis on the outer edges of older leaves or plant wilting even after an irrigation event.
7. Completely rinse and dry the paint buckets or leachate collection trays. Any algae buildup and dirt can give erroneous readings.
8. Keep records of the EC measurements, current fertilizer programs (especially when brands of fertilizer change or when fertilizer stocks are made), weather conditions and canning date of production block.
9. If media shrinkage is a problem, keep records of levels of rooting media in containers. If media rooting media levels have decreased significantly, it may be an indication of incomplete composting of organic materials.

Once the readings have been determined, the EC range will indicate what, if any, action is required to improve the production process.  

Some ranges of EC are listed in table 4. 

Most propagation programs perform better under EC values of less than 1.5 dS/m since new roots of young plants can be particularly sensitive to high salts.  The medium range of EC is usually recommended for crops with low fertilizer requirements such as camellia, azalea, ferns and blueberries.  The high EC values usually are recommended for woody perennials with higher fertility requirements.  If readings are above the ranges listed, it may be necessary to leach containers with irrigation water to remove excess salts.  Other options are to decrease or turnoff fertilizer injection for several days.

Table 4.  General guidelines for electrical conductivity utilizing a PourThru method in containerized crops.  In conjunction with these guidelines, observe plant performance:  necrosis on the outer margins of older leaves and plant wilting, even after an irrigation event may be a sign of salt accumulation in the rooting media.

Don Merhaut is UC Cooperative Extension Specialist for Nursery and Floriculture Crops, Department of Botany and Plant Sciences, UC Riverside.