It is important to understand the meaning of TDS in order to understand irrigation water EC (electrical conductivity). TDS is Total Dissolved Solids meaning the total amount of solids in water, mainly mineral salts. TDS is measured in ppm (parts per millimetre) or in mg/l.
The role of TDS in Electrical Conductivity:
Since it is hard to measure the TDS in the field, The EC of the water is as a measure for TDS.
Measuring water EC is quick, inexpensive, and can be determined using portable meters. Electrical conductivity reflects the capacity of water to conduct electrical current, and is directlyrelated to the concentration of salts dissolved in water, though the TDS. This is because the saltsdissolve into positively charged ions, as well as negatively charged ions, that conducts electricity.This is why distilled water has an EC of 0, and will not conduct electricity.
When the salt concentration reaches beyond a certain level, EC is no longerdirectly related to this concentration, because ion pairs are formed. Ion pairs weaken eachother’s charge, so that above this level, higher TDS will not result in equally higher EC.
Electrical conductivity of water is also temperature dependent meaning the higher the temperature, the higher the conductivity. Electrical conductivity of water increases by 2-3% for an increase of 1 ° C of water temperature. Many EC meters nowadays automatically standardise thereadings to 25 ° C.
The Reason EC Should be Measured
- We can say that EC is actually a measure of salinity. Excessively high salinity can affect plants in several ways:
- Specific toxicity of a particular ion (such as Sodium)
- Higher osmotic pressure around the roots prevents an efficient water absorption by the plant
- Some plants are more susceptible to salinity effects than others.
What the EC Reading Really Means:
While water EC is a good indicator of total salinity, it still doesn’t provide any information about the ion composition of the water. The same EC values can be measured in low quality water (e.g. water rich with Sodium, Boron and Fluorides) as well as in high quality irrigation water (e.g. adequately fertilised water with appropriate nutrient concentrations and ratios).
Tips On Electrical Conductivity Of Irrigation Water:
- Most EC meters have the following information indicated on them, or in their manuals:
- The measurement units – different meters may use different units, e.g. ds/m or ¼s/cm.
The range of the measurement:
- Whether the instrument automatically compensates for temperature. If it doesn’t, the measurement of the water EC should be done at 25 ° C.
- Calibrate your EC meter regularly.
- Never immerse the meter totally, but only up to the level indicated in the instructions.