In order to understand irrigation water EC (Electrical Conductivity), we first have to understand the meaning of TDS.
TDS stands for Total Dissolved Solids, i.e the total amount of solids dissolved in water, mainly mineral salts.
TDS is measured in ppm (parts per million) or in mg/l.
What Does TDS Has To do With 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 directly related to the concentration of salts dissolved in water, though to TDS. This is because the salts dissolve into positively charged ions, as well as negatively charged ions, that conduct electricity. This is why distilled water have EC of 0, and will not conduct electricity.
Nevertheless, when the salt concentration reaches beyond a certain level, EC is no longer directly related to this concentration, because ion-pairs are formed. Ion pairs weaken each other's charge, so that above a this level, higher TDS will not result in equally higher EC.
Electrical conductivity of water is also temperature dependent: the higher the temperature, the higher the conductivity. Electrical conductivity of water increases by 2-3% for an increase of 1 degree Celsius of water temperature. Many EC meters nowadays automatically standardize the readings to a 25oC.
Why You Should Measure The Electrical Conductivity
We can say that EC is actually a measure of salinity. Excessively high salinity can affect plants in several ways:
1. Specific toxicity of a particular ion (such as Sodium)
2. 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 Does The EC Reading Really Mean?
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 fertilized water with appropriate nutrient concentrations and ratios).
The commonly used units for measuring water EC are:
S/cm (microSiemens/cm)
or
dS/m (deciSiemens/m)
Where: 1000 s/cm = 1 dS/m
Converting EC to TDS can be easily done using the following calculation:
TDS (ppm) = 0.67 X EC (S/cm) = 670 X EC (dS/m)
Tips On Electrical Conductivity Of Irrigation Water
Here are some tips to make the measurements of electrical conductivity reliable:
1. Most EC meters have the following information indicated on them, or in their manuals:
2. Calibrate your EC meter regularly.
3. Never immerse the meter totally, but only up to the level indicated in the instructions.
Smart! software automatically predicts the EC value of your irrigation value, as you add fertilizers.
Guy Sela is an experienced agronomist and a water specialist. In 2005 Guy developed "Smart! Fertilizer Management". Smart! is a unique software that helps growers achieve optimal plant nutrition, in order to maximize yields and quality of their crop.
Visit Guy's website at:
http://www.smart-fertilizer.com
