The why and how to testing the Electrical Conductivity of Soils.
In the soil, the Electrical Conductivity (EC) reading shows the level of ability the soil water has to carry an electrical current. The EC levels of the soil water is a good indication of the amount of nutrients available for your crops to absorb.
Think of it like this, all the major and minor nutrients important for plant growth take the form of either Cations (positively charged ions) or Anions (negatively charged ions). These ions that are dissolved in the soil water carry electrical charge and thus determine the EC level of your soil and how many nutrients are available for your crops to take in. Knowing your soils EC can allow you to make more educated farming decisions.
To support these claims, Researchers at Clemson University documented the correlations between EC and different crop inputs, documenting these at multiple sites over multiple years. They found unmistakable evidence showing that yield data have consistently supported the EC correlations with water, fertilizer, and pesticide use.
Using EC data to develop zones, in six on-farm tests, they overlaid yield maps developed after the crops had been harvested over EC maps developed before the crops were planted and found that the two maps match perfectly.
They also found that where EC levels were high (More available nutrients) less fertilizer is needed but more weed control in places where they had a morning glory problem. For example on sandier soils with low EC ratings, it took only a quarter-pound of active ingredient in the herbicide to get 80 percent control morning glory. On heavier soils with higher EC ratings, it took up to five times that amount to achieve the same level of control.
Other factors also contribute to soil EC variability include the connectivity of the soil water through soil density, soil structure, water potential, precipitation, timing of measurement, soil aggregation, electrolytes in soil water (e.g. salinity, exchangeable ions, soil water content, soil temperature). Also the conductivity of the mineral phase affects the EC reading for example the types and quantity of minerals, degree of isomorphic substitution, and exchangeable ions. Regardless of what these multiple causes of EC variability are, what still remains is that EC measurements are consistently correlated to soil properties that affect crop productivity, including soil texture, Cation Exchange Capacity (CEC), drainage conditions, organic matter level and salinity, so knowing your soils EC level is a great predictor of your plants health.
For example if the soil EC is too high, it can be indicative of excess nitrogen based fertilizer or a high level of exchangeable sodium. Soils with an accumulation of exchangeable sodium are often characterized by poor tilth and low permeability making them unfavorable for plant growth. Soil EC is also related to specific soil properties that affect crop yield, such as topsoil depth, pH, salt concentrations and water-holding capacity. Thus EC is a great tool for explaining what your yields could be and taking action to get better yields.
The way that Electrical conductivity can be measured is using an EC meter. The probe or sensor consists of two metal electrodes and a constant voltage is applied across the electrodes resulting in an electrical current flowing through the sample. Since the current flowing through the water is proportional to the concentration of dissolved ions in the water, the electrical conductivity can be measured. The higher the dissolved salt/ion concentration, the more conductive the sample and hence the higher the conductivity reading.
The unit of measurement for Electrical Conductivity is microSiemens per centimeter (µS/cm). Up until about the late 1970's the units of EC were micromhos per centimeter (µmhos/cm) after which they were changed to microSiemens/cm (1µS/cm = 1 µmho/cm). Also a 1000 microsiemans is equal to 1 millisieman (1MS/cm)
Interestingly, the unit "mhos" derives from the standard name for electrical resistance reflecting the inverse relationship between resistance and conductivity - the higher the resistance of the water, the lower its conductivity. This also follows from Ohm’s Law, V = I x R where R is the resistance of the centimeter of water. Since the electrical current flow (I) increases with increasing temperature, the EC values are automatically corrected to a standard value of 25°C and the values are then technically referred to as specific electrical conductivity. A good EC meter will have ATC (automatic temperature compensation) so you can get accurate results regardless of sample temperature.
To get a soil extract we recommend a similar method as we do for testing pH so that both EC and pH measurements can be taken at the same time.
- Gather a fresh soil sample in a plastic zip-loc bag. Try to get a profile from the top 6” of soil that the plants will grow in and take care not to contaminate the sample by touching with anything.
- Open the bag and let it air-dry for a few hours until it is mostly dried.
- Mix the soil in the bag to ensure a homogenous sample and then use a sieve with approximate 2mm spacing to remove any large soil clumps.
- Measure out ½ of a cup of the dried soil and put into a glass beaker.
- Measure out ½ of a cup of distilled water and put this into the glass beaker with the soil.
- Stir the mixture gently for 30 seconds. Do not mix to harshly as you may destroy the humus structure and the soil may give up elements that it otherwise would not do in nature.
- Let the soil-water suspension stand for 30 minutes.
- Stir water gently again before taking the EC measurement.
- Insert the EC meter into the beaker and swirl it gently around in the soil-water extract.
- After approximately 30-60 seconds or after the EC reading has stabilized, read the digital display on your meter.
It is difficult to say what your ideal EC levels will be because there are so many variables affecting the EC level that it almost depends on your individual conditions which if you analyze over time, will give you a meaningful set of data based on the performance of your crops and the changes you have made to your fertility program.
As a general guideline however, a good soil EC level will be somewhere above 200 µS/cm and 1200 µS/cm (1.2 MS/cm). Any soils below 200 means there is not enough nutrients available to the plant and could perhaps show a sterile soil with little microbial activity. An EC above 1200 µS/cm may indicate too much high salt fertilizer or perhaps a salinity problem from lack of drainage so keeping your EC within this range. Also watch to see how EC changes over the growing season, you may see it increase as microbes are releasing more nutrients from the soil or you may see a decrease as your crops use up all the available nutrients. Either way you can fertilize accordingly.
Water purity testing: Water purity testers are nearly always conductivity meters. Pure distilled water as very very low EC as it has no contaminants in it. Generally good distilled water is < 20 μS whereas good tap water is < 200 μS. If you find your drinking water above these levels then it is not a good water source.
Compost: You can also use your EC meter to monitor your compost pile and analyze how well your pile is doing. Compost in early stages may have an E.C. number of 10,000 μS as the pile becomes active and At the peak of breakdown it can even reach >100,000 μS. High quality finished compost should have an E.C. number of approximately 1,500 - 2,000 μS.
Foliar Fertilizers: You can use the EC meter to ensure your mixture is not to potent and to also ensure you get a consistant foliar spray potency each time. The potency of your spray should depend on what is in your spray, but as a rule of thumb, 15,000 – 35,000 is good for a normal spray mixture that you use only occasionally.
Leaf sap testing: Measure the leaf sap to find out how many ions are being processed into sugars. As the EC level goes down you can expect Brix levels (sugars being produced by photosynthesis) to go up before being transported around the plant. Expect leaf sap to be between 2,000 - 12,000 µS.
To check out our expanding range of Electrical Conductivity meters, click here.
- Martin Capewell
- Agriculture Solutions LLC