In “Don’t Trust Your TDS Meter“, I explained the problems with using them for obtaining absolute values of fertilizer and additive concentrations. At best, they are nothing more than an estimating device, and at worst, usually grossly inaccurate. That is why electrical conductivity meters are the preferred instrument. Let’s first look at at the basics of electrical conductivity (EC) in solutions.
In a pure water molecule, H2O, the hydrogen and oxygen atoms share electrons in a “covalent” bond, so there is no conductivity. However, once anything dissolves in that water – and I do mean “anything”, including gases from the air – some of those bonds are broken, resulting in positively-, and negatively-charged ions being present in the solution.
It is the travel of those charged particles through the solution when a voltage potential is applied that gives us a measure of electrical conductivity.
There are two things that affect the conductivity of an ion in solution, its charge and physical size. As you can imagine, an ion with a greater charge will move faster than one of the same size with a lesser charge. Conversely, for two ions with identical charges, the smaller one travels faster than the larger. A third factor, concentration, also comes into play as that’s simply changing the number of ions present.
Every chemical that dissolves in the water to make a solution has its very own dissociation products – the segments of the molecules that break apart and become ions – and they are of different sizes and charges. When combined, they result in the net EC of the solution, so it’s easy to see how a slight change in chemistry will give a difference in conductivity. Fertilizers tend to be fairly complex blends of nutrient chemicals, and here are some examples of the variability in the EC.
50 ppm N
100 ppm N
150 ppm N
I highly recommend that if you are intent on really controlling your feeding regimen to “tune” your culture for better plants, you invest in a decent EC meter. I use a model COM-100 from HM Digital, and they are readily available online for about $50-$60. They are simple to use, automatically adjust for solution temperature, and are quire accurate.