Why can’t our pH probes read accurately in it?
What can you add into RO water to increase the EC so that our probes will work in it?
RO (reverse osmosis) water is an excellent type of water to use to irrigate your crops with or as a base to dissolve your nutrient mixes in. This method uses a semi-permeable membrane to remove ions, molecules and larger particles from water that could disrupt carefully calculated nutrient ratios and/or negatively impact the health of your crops. For example, bore/well water can often contain heavy metal elements that have leached into the water from the surrounding ground and can stress plants and cause toxicity.
Deionisation and distillation processes will also remove ionic compounds from solution and will also prevent the measurement of an EC value.
Note that reverse osmosis will not remove microorganisms present.
Challenges measuring RO water:
As RO water only contains pure water (H2O) it is a very poor conductor of electricity (negligible). Measuring the EC is measuring the electrical conductivity of a solution and as the ionic compounds that are able to conduct electricity have been removed we would expect the EC value to be zero.
As reverse osmosis removes all ionic and solid components present there should only be water present. The water molecules are made up of equal ratios of Hydrogen (H+) and Hydroxide (OH-) ions and so the pH should be neutral (7). However, as these ions are bound up in a water molecule (H2O) they cannot interact with the pH probe so you will not obtain a reading. High sensitivity probes can be used, but are a lot more expensive as they use a different technology, but these are not necessary for agricultural measurements.
It is popular to add a small amount of Calcium Magnesium (calmag) to RO water to be able to obtain a pH reading as our probes can measure pH once there are some ions present (Cl- and Mg2+). However, the addition of calmag doesn’t change the pH of the solution at all as it doesn’t contain Hydrogen or hydroxide ions which are the only ions that can change the pH. Even though you may now receive a reading, it has not changed from before you added the calmag, so what’s the point?
The quantity added of calmag added should be so small that it will not negatively impact the plants as in natural conditions seedlings would have a variety of nutrient ions present in their root zone and grow successfully.
The only thing that could mean your pH is not 7 is if a small quantity of CO2 gas is naturally present from the atmosphere or has passed through the semi-permeable membrane. As there are no ions present in RO water the water is not able to chemically stabilise its pH and so a small quantity of CO2 will create the acidic carbonic acid and lower the pH value significantly as the effects of the acid are not stabilised at all.