Plants need plenty of sun, air, water, and nutrients to grow. But how can you make sure your plants have enough nutrients? Measuring different aspects of soil can tell you exactly what you need and what your missing, and help you to foster strong and healthy plants.
Testing the pH, moisture content, and temperature of your soil are a good start for healthy soil. Monitoring phosphates, nitrates, calcium, and potassium are all primary components to plant growth. Other minor nutrients are needed as well.
One way to help keep track of all these nutrients is by testing the electrical conductivity of your soil. Electrical conductivity can tell you if you need more nutrients, or if you have too much. This will save you time and money when managing your plants.
First, let's go over the basics. (Or use one of the links below to skip to a topic!)
- What is Electrical Conductivity?
- Things That Affect the Electrical Conductivity of Soil
- Soil pH & Electrical Conductivity
- Why You Should Test Soil EC
- Choosing an EC Testing Method
- Choosing Your Conductivity Probe
- Soil EC Testing Options
- Care & Maintenance of Your Soil EC Electrode
What is Electrical Conductivity?
Electrical conductivity (EC) measures how well a substance can transmit an electrical current. Small charged particles, called ions, help to carry the electrical charge through a substance. These ions can be positively or negatively charged. The more ions available, the higher the conductivity; fewer ions would result in lower conductivity. EC is typically reported in milliSiemans per centimeter (mS/cm).
Total Dissolved Solids (TDS)
Total dissolved solids (TDS) is the amount of dissolved substances in solution. This measurement reads all the dissolved inorganic and organic substances in a liquid. Results from this reading are displayed as milligrams per liter (mg/L), parts per million (ppm), grams per liter (g/L), or parts per thousand (ppt).
Measuring TDS is a long process. First you extract all the water from a soil sample, then evaporate the water and weigh the remaining residue after evaporation. It's much easier to measure the electrical conductivity of substance, and then convert the reading into TDS with a conversion factor. The trick here is to make sure you use the correct conversion factor!
Something to keep in mind when choosing a conversion factor is that not all dissolved solids conduct electricity. For example, if you measured the conductivity of a glass of water and then add table salt, the conductivity will go up. But, if you took that same cup of water, measured the conductivity, and then added sugar, the conductivity would not be affected.
This is because table salt breaks apart into charged ions when put into a solution. Sugar does dissolve, but it does not break apart into charged ions. However, if you were to measure the TDS of the two glasses of water they would be affected by the addition of either salt or sugar.
Most common conversion factors between EC and TDS are 0.5 and 0.7. The 0.5 conversion factor is based off of how EC and TDS relate to sodium chloride. The 0.7 conversion factor is based off of how EC and TDS relate to a mixture of sodium sulfate, sodium bicarbonate, and sodium chloride. To use the conversion factor, simply multiply your EC reading by the conversion factor to calculate the TDS.
Example Conversion Table
(Click to Enlarge)
Things That Affect Electrical Conductivity of Soil
Many things can affect the electrical conductivity of your soil. The most common factors are temperature, soil type and its moisture level, salinity, irrigation and fertilizers, and the depth of the soil.
Temperature Fluctuations
The temperature of air, water, and soil will affect your electrical conductivity readings. Remember that EC of soil involves measuring the ions in the sample. These ions get very excited when the temperature gets warmer, so they bounce around and have greater activity.
More activity means the ions are better able to conduct an electrical current. So, the conductivity of the soil increases. As temperatures cool down, ions calm down and move around less. Less activity means the ions have a harder time carrying electrical current. This decreases the conductivity of the soil.
Soil Type and Moisture Levels
The texture of the soil influences the amount of moisture that's available. This affects the soil's EC. Ions like to stick and bind to other particles (like the particles in soil). When they are all bound up, the ions can be harder to read. Moisture, or water, helps to free up the ions so they can be read.
The texture of the soil also influences how much space there is for water to be in the soil. This is called porosity; different sizes of soil particles create different spaces for air and water.
Sand does not hold onto moisture well, so it has a lower conductivity. Silty soil, similar in texture to the wet mud on a river bank, has a middling base conductivity. This type of soil is able to hold onto water relatively well.
Soils rich in clay have a higher conductivity due to how well they are able to hold onto moisture, and ones with a middling conductivity tend to have the greatest crop yield. They are able to hold in just enough water, while at the same time draining away excess.
Another property that relates to EC and soil texture is called cation exchange capacity (CEC). CEC relates to the amount of clay and organics in soil. Clay has higher electrical conductivity, so the higher the CEC, the higher the conductivity is.
Irrigation and Fertilizers
Usually people only think of things such as the ocean as salty, but did you know soil can be salty too? These salts can be a problem if the electrical conductivity, or total dissolved solids are too high.
Salts are very conductive and will raise the EC of your soil. Water used to irrigate crops will directly affect the quality of the soil by either increasing or diluting available salts and nutrients. This in turn affects the electrical conductivity.
Natural rains will dilute the amount of salt near the roots of plants. This helps to keep the plant from getting “burned” by excess salts and nutrients. This means that the plant's roots are essentially clogged by the salts and nutrients. They become unable to take up salts, which can stunt its growth.
If irrigation water has a high salt content it can accumulate in fields, increasing the salinity and electrical conductivity. Most crop fields are considered good for planting if the EC does not exceed 4 dS/m. However, this number will vary by what crops are to be planted.
The addition of fertilizer is a good way to encourage crops to reach optimal growth. It is possible to have too much of a good thing, though. Fertilizers introduce nutrients and salts into the soil. These ions will attribute to higher electrical conductivity of the soil. It's important to be mindful of the electrical conductivity of your soil. Add too much fertilizer and you can increase the salinity and EC past safe limits.
Depth of Soil
Last but not least, the depth of the soil can directly affect its electrical conductivity. Plants can only grow in topsoil, the nutrient-rich top-layer of soil. If bedrock, or clay is too close to the surface, this can raise the electrical conductivity of the soil. It's important to take note of what kind of land is around (and under!) the planting area.
Soil pH & Electrical Conductivity
When soil pH and the electrical conductivity of your soil interact, interesting things happen. The pH of your soil tells you how basic or acidic it is, which can influence the electrical conductivity results.
pH is the measurement of ions as well, but specific ions. Positively charged hydrogen ions cause a substance to be more acidic, while negatively charged hydroxyl ions cause a substance to be more basic. As these ions carry charges, they can also carry electricity.
The more acidic or basic something is, the more ions there are. The more ions, the higher the electrical conductivity is. Therefore, the more acidic or basic your soil is, the higher the EC will be. The closer your pH is to being neutral, the less it will affect the electrical conductivity of your soil.
Why You Should Test Soil EC
Testing your soil is all about making sure the nutrients are balanced. Measuring the pH of soil gives you an idea of how available the nutrients are, while EC clues you in to how much is actually there. Remember that EC is good at giving a measurement of the strength of the ions in the soil. This helps you to track the nutrients that are available to your plants.
There is a high correlation of better crop yield to the use of electrical conductivity soil maps. Like topography maps, there are maps that show the EC of various geographic areas. You can create an EC map of your own; test the EC of different areas and plot it on a map.
Plants have varied tolerances to dissolved salts and nutrient concentrations. Plants such as peas and beans are very sensitive to salts deposited in the soil (EC must be below 2 mS/cm). Wheat and tomatoes have a moderate tolerance for higher conductivity. Cotton, spinach, and sugar beets are examples of plants with very high EC tolerances; soil for these plants can go up to 16 mS/cm before damaging crop yield.* It's important to balance the EC of your soil to promote optimal plant health.
*This is referenced from a study that measured EC though a 1:1 and a 1:5 saturated soil extract.
Choosing an EC Testing Method
There are several available methods of testing the electrical conductivity of your soil. You can test the pore water (the water found in the soil), the total or bulk conductivity of the soil, or you can create a slurry to test the conductivity of soil.
Hanna Tip: When measuring EC in soil, take measurements right next to the plants as well as further away. Moisture, nutrients, and pH can vary greatly across a planted area. This means a little more work, but you will appreciate being able to get results that better represent your planting area.
Measure Your Pore Water
Best Uses: Greenhouse, Hydroponics, Water
Pros: You can see what nutrients are actually available to your plants
Cons: Need a pore water extractor or multiple measurements with a calculation
Measuring the electrical conductivity of pore water will give you the best idea of your plant's experience in the soil. Plants can only take up nutrients out of soil if they are dissolved into the water near their roots. EC measurements of pore water will also give information on how the nutrients and salts are draining out of your fields.
This could give you a way to tell how you may need to adjust irrigation and fertilization of the crops. All of these methods are accurate when you use a tester or probe with temperature compensation. This corrects your readings for the changes in ionic activity associated with temperature.
To measure the EC of your pore water, you first will need to extract the water from the soil. This is done with a pore water extractor, or a suction lysimeter. A suction lysimeter is a long tube with a non-reactive porous ceramic cap. The non-reactive cap is important so that the nutrients being drawn up with the water do not interfere with the readings.
Lysimeters create enough suction to break the water tension in the soil. Once the tension is broken, the water will naturally flow up into the lysimeter. We highly recommend using more than one lysimeter when sampling near plants due to the wide variations in nutrients between the surface and near the roots.
How To Measure Pore Water
- Set up the lysimeter.
- Extract the water from the soil at the same depth as you would usually sample.
- Once the water is extracted, pour some of the water into a clean beaker to rinse it.
- Fill the beaker with enough extracted water to submerge the probe.
- Rinse the probe with deionized water, and then a little bit of the sample.
- Take your measurement.
Measure the Bulk EC of Your Soil
Best Uses: Continuous Measurements, In-Field Tests
Pros: Total conductivity of air, water, soil. Easy to test & no extra equipment needed.
Cons: Cannot differentiate between the soil, air in the soil, or water in the soil
Bulk electrical conductivity of soil measures the total conductivity. Total conductivity includes the EC of the soil, air, and moisture in your sample. All these things carry charged ions that would read as EC. This reading is very useful; you can calculate your pore water conductivity and saturated extract conductivity from the result. You would need to know your water content to perform that calculation (how much water there is in your soil).
How To Measure the Bulk EC
- Pick your testing location.
- Rinse the testing probe with deionized water, and make sure it is dry.
- Check the soil and ensure that the soil is moist.
- Use a ruler or auger to make a hole in the soil. This keeps the testing depth consistent.
- Insert your probe directly into the soil, and take your measurement.
Measure the Saturated Soil Extract EC in a Slurry
Best Uses: Managing Salt Deposits, Agriculture, Fields
Pros: Soil salinity, what crops are best suited to the soil
Cons: More sample preparations, which are more time consuming
Using a saturated soil extract to test the EC of your soil involves a bit more sample preparation. But this method yields accurate results. This is a good way to quantify the salinity of your soil. This method is the more traditional way to test the electrical conductivity of soil. Soil is full of spaces between the grains of material. The pore space between the soil grains can contain air or water. To completely saturate a soil sample with water means to fill in all the pore spaces with water.
How To Measure in a Soil Slurry
- Take your soil samples from your field.
- Make sure the containers you use were rinsed before with deionized water and allowed to completely dry!
- Choose a sample and mix in deionized water until the soil becomes a sticky, wet paste. This paste should have enough water in it that the soil for it to become very muddy (thick slurry).
- Allow the slurry to mix.
- Run the sample through a filter over a funnel.
- Once the sample is filtered, pour some of the filtered sample into a clean beaker to rinse it. Afterwards, discard the sample used for rinsing.
- Fill the beaker with enough extracted water to submerge the probe.
- Rinse the probe with deionized water, and then a little bit of the sample.
- Take your measurement.
Hanna Tip: When taking a measurement, rinse the probe in extra sample before taking a reading. This can help you get a faster, and accurate, reading.
Choosing Your Conductivity Probe
Choosing the probe that fits your testing needs is as important as how you prepare your soil samples. There are two main types of probes used in EC testing: two-electrode probes and four ring probes. All types of probes must be properly maintained. (Click here if you want to skip a head to the maintenance section.)
Two Electrode Conductivity Probes
Pros: Inexpensive. Small sample volume. No fringe field effects.
Cons: Need a different meter for each testing range. Polarization effect.
Electrical conductivity can be measured using a two electrode probe. This is also known as an amperometric electrode. The probe is inexpensive and highly versatile. The two electrodes in the probe are made of a non-reactive material. This is important because you do not want them to corrode or react with your sample.
The electrodes are insulated from each other so they will never come in contact. They will only ever come in contact with your sample. The two electrodes measure a current passed through the ions in your sample. Due to this construction, you do not need much sample to submerge the probe.
There is a space between the electrodes that has to be stable. Bending the two electrodes in the probe would yield inaccurate results. Careful cleaning is required avoid any build up between the electrodes. The thin film of residue that can build up on the surface of the electrodes is enough to change the fixed distance between them. This will cause inaccurate readings.
Another issue that can arise when using this type of probe is the polarization effect. This is especially common in two electrode probes that have stainless steel electrodes. An electrical charge can build up between the pins and cause your EC readings to be lower than they should be. You can minimize polarization by using a probe with graphite pins. Graphite electrodes are also less reactive than stainless steel electrodes.
When using a two electrode probe, it is important that you are aware of amount of conductivity in your sample. The fixed distance between the electrodes in the probe means that the probes perform best within a certain range. You can tailor your purchase of probe and calibration solutions.
Four Ring Conductivity Probes
Pros: One probe covers the whole testing range. Greater accuracy in higher ranges. No polarization effect.
Cons: Fringe field effect. Larger sample volume. More of a financial investment.
The four ring conductivity probe, or potentiometric probe, works differently than the two electrode probe. This probe works by using four metal rings around the inner body of the probe. The two middle rings work as sensing electrodes, and the two outside rings act as drive electrodes. The drive electrodes supply the electrical voltage that the inner rings monitor. When introduced to a sample, the voltage drops proportionally to the conductivity. This change is converted to conductivity.
The construction of the four ring probe allows it to be used in a wide range of samples. However, for the probe to work, the vent holes above the four metal rings must be fully submerged. This means that when using a four ring conductivity probe you need a larger sample size to have accurate measurements.
Four ring EC probes are useful in that you only need one probe to cover all sample ranges (up to 1 S/cm). When measuring over a wide conductivity range, a four ring probe is a better option than a two electrode probe. This probe is more accurate in samples with higher electrical conductivity.
Although this probe does not have polarization effects, it does have the fringe field effect. The fringe field effect happens when the electrical field around the probe contacts sample container. Something like the sides or bottom of a cup will cause your EC readings to be erratic. You can avoid this effect by placing the probe so there is an inch of space between it and the sides of the container. Due to the materials used in constructing a four ring conductivity probe (typically platinum), they are more expensive than a two electrode probe.
Soil EC Testing Options
The meters used for testing are as diverse as the probes. To meet your testing needs, you can use digital direct soil conductivity testers or direct soil portable conductivity meters. Each of these categories possesses many features and options to help you complete all of your testing needs.
Keep in mind; it's always best to get a meter with temperature compensation. Temperature can change the behavior of your soil’s conductivity, and it can change the performance of your conductivity probe. A meter with temperature compensation would be able to adjust the electrical conductivity reading to these changes.
Digital Direct Soil Conductivity Testers
Pros: Easy to use. Pocket sized. Inexpensive.
Cons: Many are two-electrode probes, so if you have a wide range of EC you will need multiple probes.
Direct soil conductivity testers are pocket-sized, simple, easy to use probes. Many of these probes are two-electrode probes. Some testers such as the Soil Test™ Direct Soil EC Tester use a four-ring probe for measuring EC of soil. They are great for bringing accurate electrical conductivity testing to the field and are also great in soil slurries.
A variety of options enables testers to be tailor fit to your testing needs. Check what the tester is made from. Durable plastic or steel bodies help to ensure a long useful life for the probes. Different types of plastic work best in protecting your probe from fertilizer concentrates. There are waterproof options available. You no longer have to worry about accidentally damaging the tester.
Many digital direct soil conductivity testers are also combination testers. They can test more than one quality in soil. Most have both electrical conductivity and total dissolved solids (TDS) modes. Other testers can also measure the pH of your soil. These features are useful, as you only would need to carry one tester to perform your field tests.
The testers can warn you of low battery. This will keep you from inaccurate readings by preventing readings when the power is too low. Many of the testers can be calibrated to a single calibration point. Some combination testers can be calibrated using Quick Cal Mode. This enables the testers to calibrate the different testing electrodes (e.g. pH and EC/TDS) all at once.
Portable Electrical Conductivity Meters for Soil
Pros: Laboratory accuracy in the field. More multiparameter options. Customizable
Cons: More technical to use. More expensive.
Portable conductivity meters for soil are the next step up. They bring laboratory accuracy to the field. These meters vary in design and function. Some have a simple two-button design, while others have detailed menu access. Most soil portable conductivity meters are able to test more than one parameter at once. Waterproof and water-resistant options help to keep your meter properly functioning.
Testing more than one parameter at once helps you have an all-in-one solution with laboratory grade results. When testing different parameters, converting from EC to TDS or Salinity is easy. You may even be able to choose your preferred EC/TDS conversion factor. This helps you get desired results with ease. Some meters can be calibrated with a quick calibration solution, just as with the testers. Another feature on these direct soil portable meters is an amplified probe. Amplified probes help to minimize electrical noise in the samples. Many things can cause noise, or electrical interference. These things include motors, pumps, and grow lights.
If you need to track and/or report your results, a direct soil conductivity meter is an excellent choice. Select meters are able to give you Good Laboratory Practices (GLP) data. The data include information such as time, date, calibration data, and logged measurements. This gives you traceable data to report.
These portable meters take a bit more expertise to operate than the direct EC testers. Some do come with a dedicated HELP button which prompts on-screen tutorials. The portable conductivity meters are a bit larger than the conductivity pocket testers. These meters are more of an investment than the little conductivity testers. Always check the operating range on the meters before buying one. This will ensure that you are going to be using a meter suitable to your conductivity ranges.
Care & Maintenance of Your Soil EC Electrode
Proper care and maintenance of your conductivity probe is paramount for accurate readings. Cleaning, calibrating, and appropriate storage will extend the useful life of the probe. Be sure to consider probes that measure more than just EC; the pH portion of the probe will also require care.
Cleaning Regularly
Keeping your soil conductivity probes clean is the first step to getting accurate results. This step also extends the useful life of your probe. Improper cleaning can change how the probes respond in samples. Residues on probes can cause the EC meter to receive a reading that is either too low or too high. Cleaning your probe properly between readings is important for obtaining stable readings. Some meters will alert you to when your probe may need to be cleaned. Your probe type will influence how you clean it.
For an EC/TDS, or an EC/TDS/Salinity probe:
- Start with rinsing the probe with deionized water.
- Residue is stuck to the probe, use a soft cloth to remove particulates.
- Be very careful with this step! Some probes have a glass body and care should be taken when handling the probe.
- You do not need to use a lot of pressure; this can bend two-probe probes. Instead, rinse the probe and gently use the cloth again. Dampening the cloth with deionized water can help remove particulates.
- If a cloth is used, make sure to rinse the probe again, any fibers stuck to the probe could interfere with readings.
- Rinse the probe again with deionized water.
For a pH/EC/TDS/Temperature probe:
- Fill a squeeze bottle or spray bottle with deionized water.
- Rinse the probe with deionized water.
- If there is still residue on the probe, DO NOT wipe down the probe! Instead, use a cleaning solution specifically made for soil.
- There are multiple cleaning solutions, including ones for general cleaning, agriculture, humus deposits, and soil deposits.
- When using a cleaning solution.
- Rinse the probe before submerging it.
- Let the probe soak in cleaning solution for 15 minutes.
- Remove the probe from the cleaning solution.
- Rinse the probe with deionized water.
- Place the probe in storage solution for at least 1 hour before using it again.
For more information, and step by step guidelines, on proper combination probe (specifically probes that can also measure pH), maintenance, please reference The Ultimate Guide to Testing Soil pH.
Calibrate Often
Calibrating a soil conductivity probe can be tricky. This is due to the calibration standards used for the EC probes do not have any buffering capacity. No buffering capacity means that the calibration standards are easily contaminated. Contamination can come from the deionized water used to rinse the probe. It can also come from other standards, storage solution from a pH probe, or residuals from samples. The contamination would change the calibration enough to cause inaccurate calibrations.
Avoiding contamination is much easier when using disposable packets of calibration solution. The single use packets ensure that you use a completely fresh standard for each calibration. Another way to cut down on contamination is to use a little of the standard to rinse the probe. Using the calibration standard to rinse the probe removes residue from the probe.
Hanna Tip: Some probes can use a Quick Calibration Solution to calibrate multiple measuring parameters at once.
Conductivity Calibration Steps:
- Fill a squeeze or spray bottle with deionized water.
- Using the bottle, rinse down the probe.
- If using a disposable one-use-only packet of calibration standard, tear or cut open the packet.
- Enter the calibration mode on your EC meter.
- Make sure you have the correct calibration standard selected.
- Rinse the probe with some of the calibration standard (Pour some of the standard over the probe to avoid contamination.)
- Insert the probe into the packet, making sure that it is properly submerged.
- Let the reading stabilize, and accept the standard.
- Remove the probe from the standard sachet and rinse with deionized water.
- If using a bottle of calibration standard.
- Pour some standard into a dry and clean beaker.
- Place a stir bar in the beaker and put the beaker on a stir plate.
- Stir the standard in the beaker, and pour that standard out.
- Fill the beaker with enough standard to submerge the probe.
- Enter the calibration mode on your EC meter.
- Make sure you have the correct calibration standard selected.
- Rinse the probe with some calibration standard.
- Pour some of the standard over the probe to avoid contamination.
- Insert the probe into the beaker until the probe is properly submerged.
- Let the reading stabilize, and accept the standard.
- Remove the probe from the beaker and rinse the probe with deionized water.
- Repeat these steps for other electrical conductivity standards.
Additional calibration steps may be needed if the probe can measure other parameters, such as pH.
Condition Always
Storage of an electrical conductivity probe differs by probe type. One thing that never changes is that the probe should always be stored clean. Rinse the probe with deionized water to remove all residues from the surface.
For an EC/TDS, or an EC/TDS/Salinity probe:
- Clean the probe. Follow the cleaning directions above for more in-depth instructions.
- Place the probe into its storage cap or protective sleeve.
For a pH/EC/TDS/Temperature probe:
- Clean the probe. Follow the directions above for cleaning with an agriculture specific cleaning solution.
- Once the probe is clean, store it in a storage cap containing storage solution, or pH 4.01 buffer.
Looking for some troubleshooting tips for your electrical conductivity probe? Check out our blog post on the 8 Common Mistakes When Taking Conductivity Measurement.
Soil may be complex but...
...choosing the best electrical conductivity testing solution doesn't need to be! Use this guide to electrical conductivity testing to help you narrow down your options. For help in choosing the best option for your electrical conductivity testing needs, please contact us using one of the channels below.
