EC in soil: The complete guide

Electrical conductivity (EC) is an important parameter that affects the health and growth of plants. We cover the key information you need to know about EC in soil, including how to test this parameter, the types of electrodes and the instruments available for measuring EC. 

For more information about pH in soil, see our companion guide here. If you would like advice on the most suitable EC testing solution for your requirements, call our experts on 01525 850 855 or email us directly at sales@hannainstruments.co.uk.

What is electrical conductivity?

Electrical conductivity, or EC, is a measurement of how effectively a substance transmits an electrical current. In gardening and agriculture, the EC in soil provides insight into whether plants have the correct amount of nutrients by measuring the number of ions available.

Different plants have preferred EC ranges, requiring accurate monitoring to ensure healthy growth. The typical measurement for EC is milliSiemans per centimetre (mS/cm).

Why test soil EC?

Testing your soil is the ideal way to get a clear idea of the nutrients available to your plants. In combination, pH and conductivity measurements provide information about available nutrients present within the soil. 

While plants have varied tolerances and requirements for nutrient concentrations, there is a correlation between better crop yield and growth in soil that is well-balanced. This balance can be achieved using information from EC and pH testing. 

pH and EC

pH tells you how alkaline or acidic a material is by measuring the hydrogen ions within the soil. The further away pH is from neutral - 7.0 on the pH scale - the more ions there are, resulting in a higher EC measurement. 

Measuring pH and EC together provides insight into the potential cause of an outlying measurement of either parameter, allowing for adjustment to bring both measurements within a suitable range.

Total dissolved solids and EC

Total dissolved solids, or TDS, is a measurement of inorganic and organic materials dissolved within a liquid. Like EC, TDS provides information about the levels of nutrients in the soil.  TDS is measured in PPM, which is commonly used for nutrient solutions to determine how many nutrients to use in the soil or hydroponics.

You can choose a meter or tester that includes both measurements, such as the HI-98318 tester or convert EC into TDS using a conversion factor. The most common conversion factors for TDS are 0.5 and 0.7. Determining the correct conversion factor for your tester allows you to convert EC to TDS accurately.

Here is an example of the type of conversion table you might use:

What affects electrical conductivity in soil?

Changes in temperature

As temperature increases, the ions within the soil become more active, while cooler weather slows them down. The more ions move, the more effectively they conduct an electrical current.

Soil type

Soil with less porosity, such as sandy soil, enables ions to stick and bind to other particles, leading to a lower electrical conductivity reading. By contrast, clay-rich soils have a higher conductivity due to increased porosity. 

Moisture levels

Moist soil has higher levels of electrical conductivity as the ions are able to move freely within the soil. Soil that holds moisture well or is consistently saturated has a higher EC reading than dry soil.

Irrigation

Salts are highly conductive and raise the EC of your soil. When soil is irrigated through a system or by rainwater, the level of salt may increase or decrease, whether by washing away salts or introducing them. 

Fertilisers

As you introduce salts, minerals and nutrients through fertilisers, EC increases. Fertiliser use should be balanced and measured once introduced to the soil to ensure plants maintain the correct EC and pH levels for healthy growth.

Soil depth

Upturned earth or ground where clay and bedrock are closer to the surface can impact EC, affecting growth. Measuring the soil composition within and beneath the top level of soil can provide the insight to resolve high or low EC.

Testing methods for electrical conductivity

Direct measurement

Best suited for: General gardening

Testing the soil directly is a good option for general gardening. The HI-98331 Groline direct soil tester is a suitable solution for quick, direct measurement for flowers and vegetables that have less specialist requirements or for personal growing.

How to test the soil directly

1. Insert the probe into the soil
2. Take an EC measurement
3. Clean the probe with suitable cleaning solution for the next use

Measuring pore water

Best suited for: Greenhouses, hydroponics and aquaponics

Measuring pore water provides insight into what your plants receive from the soil or directly from a water source at the root level. You can also learn more about how nutrients and salts are draining through the ground from this sampling method.

A suction lysimeter, such as the HI-83900-30, allows you to extract water for your sample from the soil. This tool is a long tube with a non-reactive, porous ceramic cap to ensure no interference in readings. 

How to measure pore water

1. Extract water from the soil using the lysimeter
2. Pour water into a clean beaker with enough height to submerge a probe
3. Rinse your probe with deionised water
4. Measure the EC of the extracted water

Measuring saturated soil extract

Best suited for: Agriculture and the management of salt deposits

A saturated soil extract takes more preparation to produce a suitable sample, requiring the creation of a soil slurry. However, the accuracy of the results and the ability to quantify salinity in the soil makes this method a suitable choice for agriculture specialists, farmers and serious hobbyists. 

How to measure a soil slurry step-by-step

1. Take your soil sample
2. Add sample to container that has been rinsed with deionised water and dried
3. Mix deionised water into your soil sample until it becomes a muddy paste
4. Continue mixing the slurry until homogenous
5. Run your slurry sample through a filter to remove any debris
6. Pour a small amount of the filtered sample into a clean beaker
7. Rinse the probe with deionised water, insert into the sample and take measurement

Measuring bulk EC

Best suited for: Continuous measuring and in-field testing

Bulk electrical conductivity is a measurement of the total conductivity within the soil, air and moisture of our sample. Bulk EC is a foundation for calculating more precise readings as required using the pore water and saturated extract methods. 

This method is useful for measurement alongside the use of permanent monitors and meters for EC in agriculture.

How to measure bulk EC

1. Rinse the testing probe with deionised water and allow to dry completely
2. Ensure the sample area soil is moist
3. Use an auger or ruler to make a hole at a consistent testing depth
4. Insert your probe into the soil and take your measurement

What instruments can you use for EC testing?

Our range of EC meters and testers at Hanna include:

• Direct soil testers: Handheld instruments that allow you to check the EC of soil directly in the garden, such as the HI-98331 EC tester.
• Hydroponics testers: Portable testers used for the measurement of EC in hydroponics, such as the HI-98318 EC Tester or the HI-9814 EC/pH meter.
• Portable EC meters: Laboratory-grade instrumentation for testing in the field, often covering multiple parameters such as the HI-9814 multiparameter meter.
• Benchtop meters: Multiparameter instruments for testing soil samples within a laboratory, such as the HI-5522 meter.
• EC monitors: Permanent meters that provide continual readings of EC through a probe inserted into the soil or hydroponics system, such as the HI-981420 monitor.

The most suitable EC testing instrumentation will depend on your requirements and goals for testing. An EC tester is a suitable solution for gardens and smaller growing environments, while EC monitors are an effective choice for plants that need continual, accurate monitoring to grow. 

If you are unsure which instrument is most suitable for your requirements, our experts at Hanna can provide insight and guidance. Call 01525 850 855 or email directly to speak to our team.

Which conductivity probe should you use for soil?

There are two options for conductivity probes:

Two-electrode conductivity probes

Two-electrode probes are also known as amperometric electrodes, providing a solution for easy and practical electrical conductivity measurement within specific parameters. The two electrodes are insulated from each other and do not touch. Two-electrode conductivity probes work best within a specific range of conductivity below 5 mS/cm.

Four-ring conductivity probes

Four-ring conductivity probes, also known as potentiometric probes, use a series of four metal rings around the inner body of the probe to measure EC. The two middle rings act as sensing electrodes, and the two outer rings act as drive electrodes, supplying power to the inner two. 

When a four-ring probe is introduced to a sample, the electrode voltage drops proportionally to the conductivity, providing a measurement of EC. A four-ring probe covers a wide EC range, making this probe suitable for a variety of environments and plants.

How to maintain your EC electrode

Cleaning

Residue on probes can lead to inaccurate readings, making it essential to keep your electrodes clean for accurate testing.

How to clean an EC/TDS or EC/TDS/Salinity probe

1. Rinse the probe with deionised water
2. Use a soft lint-free cloth to remove any remaining residue
3. If there is still residue, rinse the probe again and gently wipe with the cloth
4. Perform a final pass by dampening your cloth with deionised water
5. Rinse the probe for a final time to remove any fibres or particulates

How to clean a pH/EC/TDS/Temperature probe 

1. Rinse the probe with deionised water
2. If residue remains, use a cleaning solution specifically designed for soil
3. Submerge the probe in the cleaning solution for 15 minutes
4. Remove the probe and rinse with deionised water
5. Place the probe in a storage solution for at least one hour before reuse

Tip: If you have a probe that measures additional parameters, such as pH, it is important to consider those factors in your cleaning process.

Calibration

Regular calibration helps to achieve long-term accuracy when testing EC. Using a calibration solution in disposable packets is recommended to avoid contamination when testing soil. 

How to calibrate your EC probe with single-use packets

1. Rinse down the probe with deionised water
2. Cut open the packet of calibration standard
3. Enter calibration mode and select the correct calibration standard on your EC meter
4. Rinse the probe with some of the calibration standard to prevent contamination
5. Submerge the probe in the packet
6. Wait for the reading to stabilise and accept the standard
7. Remove the probe and rinse with deionised water

Tip: Hanna multiparameter instruments use quick calibration solutions to complete calibration in a single step.

How to calibrate your EC probe with bottled calibration standard

1. Rinse down the probe with deionised water
2. Pour standard into a dry, clean beaker
3. Add a stir bar and place the beaker on a stir plate
4. Stir the standard and pour it out
5. Refill the beaker with standard in enough volume to submerge the probe
6. Enter calibration mode and select the correct calibration standard on your EC meter
7. Rinse the probe with some of the calibration standard to avoid contamination
8. Submerge the probe into the beaker
9. Wait for the reading to stabilise and accept the standard
10. Remove the probe and rinse with deionised water

Tip: If your EC tester also measures pH, you should follow the additional calibration steps necessary for accuracy in all measurements.

Storage

The exact method for storage will depend on the probe. EC/TDS and EC/TDS/Salinity probes can be stored with a storage cap or protective sleeve. Probes that measure pH have stricter storage requirements, where a storage solution is required to ensure the longevity of the probe.

Buy EC instrumentation for soil from Hanna Instruments UK

Our Groline range at Hanna Instruments includes instrumentation to suit many environments, from garden hobbyists to agricultural growing. Browse Groline online today for EC meters, pH testers and multiparameter solutions.

For advice and guidance on the most suitable EC testing instrumentation for growing, call our experts at 01525 850 855 or email our team directly.