What is electrical conductivity (EC)?
Electrical conductivity (EC) of a substance is defined as 'the ability or power to conduct or transmit electricity. Its units are milli Siemens per centimeter [mS/cm]. Electrical conductivity is considered to be a physical property since the identity of this substance isn’t changed.
Flow of charge is usually meant electric current. A metal, such as in an electric wire, contains a great number of free electrons. These electrons pass electric current from one to the next, just like a line of people forming a bucket brigade. Such a metal is called a conductor. In the same way, dissolved solids which in most cases involves inorganic salts, give the water the ability to conduct an electric current. The dissolved salts forms positively and negatively charged ions also called cations and anions. For example, one type of salt contains sodium chloride (NaCl) which forms electrolytes when dissolved in water, most of which become ions as demonstrated in the chemical equation.
Besides sodium salts, dissolved solids include potassium (K+), magnesium (Mg2+), calcium (Ca2+), sulfates (SO42-) and bicarbonates (HCO3-) to name a few. Pure or deionized water has no ions hence is not a good conductor of electricity. As we add salts to the water, the electrical conductivity of the water increases. Thus by measuring the electrical conductivity of the water, we can determine the total dissolved salts or the salinity of the water.
Also, measuring electrical conductivity cannot detect all dissolved solids. For instance, it can not measure sugars. Although sugar is soluble in water, it does not form ions, which means that it is not an electrolyte. This meanse that the electrical conductivity in the irrigation water can only measure the total dissolved salts in the water.
What is role of the EC in the irrigation water?
Plants require nutrients for their growth and development. The uptake and composition of the nutrients depends on type of plant, the growth stadia and growth conditions. Each plant has its own nutrient recipe which comprise of the nutrient compositions. Fertilizers are added to the irrigation water to provide the desired nutrient compositions for plant growth. Fertilizers are usually solids salts dissolved in water which becomes ions such as potassium ions (K+), calcium ions(Ca2+ ), and nitrates(NO3-). The plant takes up these ions as nutrients for growth and development. The following table demonstrates a typical recipe for tomatoes including the concentration and ratios. This recipe is empirically established throughout the years which gives the correlation between the nutrient concentration in form of dissolved ions and the electrical conductivity measurement.
Nutrient | Nutrient name | Typical tomatoe recipe |
NH4+ | Ammonium | 1.20 mmol/l |
K+ | Potassium ion | 9.50 mmol/l |
Ca2+ | Calcuim ion | 5.40 mmol/l |
Mg2+ | Magnesium ion | 2.40 mmol/l |
NO3- | nitrate | 16.00 mmol/l |
SO42- | Sulphate ion | 4.40 mmol/l |
PO43- | Phoaphate ion | 1.50 mmol/l |
EC | Electrical conductivity | 2.63 mS/cm |
How can we measure the EC in the irrigation water?
An EC sensor is widely used to measure the electrical conductivity of the water. An electrical current is applied between two electrodes that are a specific distance apart. Since dissolved ions in the solution carry the current, we can determine the amount of dissolved ions by the amount of current we measure. Since temperature can affect conductivity, we must measure the temperature of the water and include a compensation algorithm to ensure accurate readings.
How can we determine the EC in the irrigation water?
Growers continuously monitor the composition of their irrigation water for the optimal growing of crops. The electrical conductivity (EC) can be a reliable parameter to estimate and predict the total mixture of ions in the water as fertilizers are presented in the feed are known. Priva’s fertilizing dosing units are EC controlled. Therefore, determining the related EC for fertilizer mixtures is crucial. In addition, the electrical conductivity can be calculated from the ions concentration in an ideal aqueous solution as expressed in equation below.
Where:
EC = Electrical Conductivity,
∑ = Summation symbol, indicating that you sum over all the terms that follow.
zi = The charge number of the ion i, representing the number of charges (positive or negative) the ion carries.
ccat * i = The concentration of the cation i in the solution.
To summarize, electrical conductivity (EC) in irrigation water is crucial for plant health. Low EC can lead to nutrient deficiencies, as there aren't enough dissolved ions for plants to absorb, potentially stunting growth Conversely, high EC indicates excessive salts, which makes it difficult for plants to absorb water even when it's available.