Accurate EC soil testing made easy.
The HI98331 Direct Soil EC Meter is a convenient tool that can be used not only in soil but in soil-less media like rock wool and coco coir. This guide will explain how to interpret your results. Thus, helping to stop any confusion if you have been previously measuring soil EC (electrical conductivity) with other methods.
The Ability To Test Electrical Conductivity
When using the HI98331 Direct Soil EC Meter, it is important to be aware that the moisture content of your soil/media will significantly affect the measured EC (electrical conductivity) value. To ensure accurate measurement, it is recommended that readings are taken when soil moisture is close to field capacity. As a general guideline this optimal measurement time is approximately 15 minutes after an irrigation is complete. The soil/media will remain at the proper moisture level for close to an hour; depending on the media and amount of drainage it provides. The 15 minute window allows time for water to drain from larger pores (spaces) in the media and some air to enter which evens out the moisture content for consistent readings.
Note: It is recommended that you take three readings and average them. EC is not homogeneous throughout the media. It is important to pay attention to the depth you are measuring. Keep that consistent throughout your tests.
Different soil, soil mixes, and soilless growing media hold various amounts of moisture when at field capacity. Rock wool, peat moss, and coconut coir all hold more water. Meanwhile, soil mixes with more pine bark, perlite, and sand hold less water. The amount of water a media holds at field capacity will affect the EC value when measured with the HI98331. The larger the difference in water holding capacity, the harder it is to compare the direct EC measurements of multiple medias. For example, comparing measurements from a rock wool cube to sandy loam from a field.
Direct soil EC measurements will also differ from saturated media extract, 1:2 dilution (slurry), and pour-through testing method results. There are complex formulas that exist to convert results between these testing methods but the relationship is only linear if testing the exact same soil or soilless media.
Note: The HI98331 measures in mS/cm. dS/m, mS/cm, mmhos/cm, and EC are used interchangeably when referring to measurements of the salinity of soil.
1 decisiemens per meter (dS/m) = 1 milisiemens per cm (mS/cm) = 1 EC = 1 mmhos/cm
Many growers ask for a list of plant salinity tolerances to compare their results. Growers could use them as a guideline with the following limitations in mind: soil EC measurements are affected by moisture content, soil composition, and testing method... the numbers in these lists will likely not match up to your results without interpretation. For example, the measurements in this salinity tolerance list were taken using the saturated media extract method and the soil tested was Utah’s fine sandy loam from a field. Those measurements will only be comparable with direct soil EC measurements taken with the HI98331 if you are measuring the same or a similar soil type (sandy loam) and using a conversion formula or testing method comparison chart.
Results from different measurement methods could be quickly and roughly compared using the following chart so long as the media sampled is similar in composition. Direct soil measurements with the HI98331 are closest to the 1:2 results on this chart and are often slightly lower.
| 1:2 | SME | Pour Thru | Indication |
| 0 to 0.03 | 0 to 0.8 | 0 to 1.0 | Very Low |
| 0.3 to 0.8 | 0.8 to 2.0 | 1.0 to 2.6 | Low |
| 0.8 to 1.3 | 2.0 to 3.5 | 2.6 to 4.6 | Normal |
| 1.3 to 1.8 | 3.5 to 5.0 | 4.6 to 6.5 | HIgh |
| 1.8 to 2.3 | 5.0 to 6.0 | 6.6 to 7.8 | Very High |
| >2.3 | >6.0 | >7.8 | Extreme |
For example a reading of 5.0 mS/cm using the SME (saturated media extract) method to measure Utah’s fine sandy loam would be roughly relatable to a 1.8 mS/cm reading using the HI98331 to measure the same or a similar soil composition.
So, What Does All This Mean?
It is best to forget comparing your results to lists and charts. It may be time consuming or not possible to find data on your specific soil composition or soilless media. Your comparisons and conversions will be rough before factoring in unquantifiable variables such as genetics and environmental differences. The answer is to simply build your own data set.
By collecting weekly data with the HI98331 growers can associate visual signs of plant health with the direct EC measurement data. Once this data is acquired the salinity can be maintained without having to rely on visual signals from plants. Only the HI98331 measurements are needed.
When a plant is visibly stressed from high salinity, the damage has been done and the plant will not be able to reach its full potential in time for harvest or sale. Using the HI98331 growers can maintain a healthy root zone and correct salinity issues before they damage the plant's roots, immune system, and ultimately stunt growth.
The HI98331 is especially helpful in greenhouse and indoor growing facilities. The growing media in these facilities often has ample drainage which allows the grower to reduce the media’s salinity quickly when needed. This is accomplished by flushing with low salt water or a much lower fertilizer concentration.
With tools such as the HI98331 Direct Soil Meter and the HI99121 Direct Soil pH Meter growers can stay ahead of soil pH and EC issues before they become apparent and avoidable damage is done.
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Got Questions?
For more information regarding how Hanna Instruments can help you with your soil testing needs, contact us, as sales@hannainst.com or 1-800-426-6287.
