The electrical conductivity of water is actually a measure of salinity. Excessively high salinity can affect plants in the following ways:
Specific toxicity of a particular ion (such as Sodium)
Some plants are more susceptible to the electrical conductivity than others and each specie has an electrical conductivity threshold, beynod which yield is decreased.
Experiment showing the effect of different EC levels on crop
Most EC meters have the following information indicated on them, or in their manuals:
The measurement units - different electrical conductivity meters may use different units, e.g. ds/m or μs/cm.
The range of the measurement.
Whether the instrument automatically compensates for temperature. If it does not, the measurement of the water EC should be taken at 25oC.
In order to have a reliable reading, it is important to take the reading correctly.