Electrical Conductivity

Electrical conductivity (EC) is a measure of the salinity of the soil or the nutrient solution in hydroponics. That is, it is used to determine how many salts are present in the soil or nutrient solution. As a comparison, purified or distilled water doesn’t conduct electricity and will always have an EC reading of zero. The greater the amount of dissolved mineral ions (fertilizer) in the water, the higher the electrical conductivity will be. Therefore, high EC readings mean high levels of nutrients, and low EC readings are a determination of low concentrations of fertilizer salts in the water.

EC readings are important since they reflect the extent to which the salts present in the solution affect the way that plants will grow; measuring EC means that you can ascertain if the plants fertilizer needs are being met. It can also determine if there is an over-supply of nutrients in the water. The most common cations (positively charged ions) are calcium, magnesium, sodium and potassium, and the most common anions in hydroponics (negatively charged ions) are nitrates, sulfates and phosphates. Fertilizers and manures can contribute to ,others, such as ammonium.

As salts are dissociated into ions in solution they carry a positive or negative charge (e.g . KNO2 dissociates to K+ + NO3 – ) which can transmit electricity. Pure water will not transmit electricity, but as soon as salts are added, the ability of the solution to conduct electricity increases. This conductance increases with increasing solution strength. CF (conductivity factor) and EC (electrical conductivity) are a measure of this characteristic of nutrient salt solutions. While EC seems to be a very convenient measure, there are problems associated with relying only on EC to control hydroponic nutrient formulae. For example:

The EC will be roughly the same regardless of the elemental content of the solution. A nutrient solution with an EC of 2.0 cannot be distinguished from a solution of sodium chloride with an EC of 2.0.

Different nutrient salts show different capacities to conduct electricity when in solution so depending on the nutrient rations and the individual salts used, the EC may give a very different indication of the true ionic strength of the solution. A solution of potassium nitrate at EC 2.0 will be approximately half the strength (in ppm) of a solution of magnesium sulfate at EC 2.0. This is because potassium conducts almost double the amount of electricity at the same ionic strength as magnesium sulfate (shown in the table below).

Even if the nutrient content of the formula was known accurately at the start, once the solution has been recirculating through a growing crop for a few weeks the elemental content changes – the EC may well stay the same.

Conductivity of some common hydroponic nutrients at 2000 ppm

The EC of a nutrient formulation is a combination of the EC contributed by all the dissociated nutrient salts from the A and B stock solutions as well as impurities from the water supply and is not really any indication of the quality of the formula, just as estimate of the strength. In hydroponics, the way to determine the nutrient makeup of a formulation is to have a complete mineral analysis done, use a range of specific ion meters, or to calculate the nutrients in advance and use these in drain to waste systems. Outside of hydroponics, or in organic hydroponics, EC may not even be a measure of strength of a formulation as a range of compounds such as organic nutrient complexes and chelates do not conduct electricity.

Note: When testing the EC of a solution, there is no differentiation between the positive and negative ions, it only measures the total number of ions; EC does not measure individual solutes (dissolved compounds) within the nutrient solution. It cannot identify proportions of, say, potassium to magnesium in the solution. When a nutrient is used by the plant, some of its original compound is left in the solution. This increases the EC of the solution, which makes EC a useful diagnostic tool because it can be used to help estimate how much of the nutrients in a solution have been used over time. 

Managing EC

EC can be managed, at appropriate times, to generate vegetative growth or flowering and fruiting. It can also be used to reduce stress in plants due to environmental conditions. Plants can stress or alter growth patterns in changing environmental conditions so EC management is helpful in reducing that. When conditions are cool and light levels are low, plants tend to move towards the light causing elongation of the internodal spaces (long sappy weak growth of the stems). By raising EC levels, growth can be restricted which reduces this problem. Conversely, in hot and dry conditions EC levels may be lowered to compensate and reduce plant stress caused by these types of conditions. 

TDSEC and TDS (total dissolved salts in a solution) are both terms used in hydroponics. They both refer to measures of electrical conductivity.

  • EC readings are displayed in millimhos or mmhos/cm; the units of conductivity per centimetre centimeter
  • TDS readings are displayed in ppm  (part per million) – these meters use a conversion factor that then displays an EC reading as a TDS reading. The conversion rate is usually 700:1 so a TDS reading of 700ppm will have an EC reading of 1.

Conductivity Meters

Conductivity or EC meters are one of the least expensive items of soil/ solution testing equipment. Conductivity meters are devices used to measure the salinity of the water. An electrical device generates an electrical signal which travels between two electrodes. The ability of water to conduct electricity between these two electrodes is an indication of the dissolved salt – if the electric current flows more, then there will be more dissolved salts. Low electroconductivity (EC) readings indicate low concentrations of nutrient salts. Very high EC readings can indicate toxic levels of nutrients.

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