Electrical Conductivity (EC) measures a material’s ability to conduct an electric current and the conductivity of a solution is directly related to the amount of salts dissolved in it. The more salts, the higher the EC.
Why is this relevant to hydroponic gardening? Well, hydroponic nutrients are primarily composed of mineral salts, so Electrical Conductivity can give you an idea of how many nutrients are in your solution.
Each ion in the solution has a different specific conductivity and the ions that determine pH have conductivities hundreds of times larger than other ions, and thus have a disproportionate impact on the EC value. Because of this, the EC measurements of a solution at pH 5.5 will be completely different from that same solution when the pH is adjusted to 6.8. Many gardeners trying to replicate the same EC and solution concentrations week after week fail to make sure the pH is the same each time the EC is measured.
When mixing weekly nutrient solutions, if you can ensure the pH is the same at the time of every reading, EC meters can help you replicate past solutions to make sure your end result is as close to previous, successful results as possible.
On a daily basis, EC is useful because it can tell you if your solution has lost nutrients or water. If your solution’s EC becomes too high, you can add water to lower it to the original value. If the EC becomes too low (70% of the original value), your solution has been substantially changed in composition by the plant and needs to be disposed of and replace with a fresh one.
While EC is certainly useful for hydroponic gardeners, don’t get overly excited — EC does not measure the concentration of individual minerals (or ions) in the water, and thus cannot distinguish between phosphorous and table salt.