Why Does My Reservoir pH Slowly Drift Up?

Growing hydroponically offers a great deal of advantages when it comes to increased yield, less space, no need for buying soil, etc., but it comes with one significant disadvantage as well: you have to deal with chemistry.

Water chemistry, involving the uptake of nutrients from an aqueous reservoir by plant roots, involves several factors—the two most important being total dissolved solids (TDS) and pH. People sometimes express TDS as electrical conductivity (EC), simply because electrical conductivity is the physical measurement your device takes, and TDS can be inferred from this measurement because all dissolved salts contribute to the electrical conductivity of water.

Although TDS, or EC, is simple enough in that it measures the amount of “stuff” you add to water, plus the “stuff” that’s already present in the water (calcium, salt, magnesium, etc.), pH is a little more complicated. Without going into too much of the chemistry behind pH, we’ll attempt to explain the pesky, but misunderstood, problem of people noticing the pH of tap water slowly increasing over time.

We all know that things are more soluble in hot water, like salt or sugar for example. But did you know the trend is the opposite for gases?

This means gases are more soluble in cold water than hot water. There are only two soluble gases important to a grower: oxygen and carbon dioxide. Roots need some dissolved oxygen to stay alive, which means you shouldn’t let the water get too hot.

How does dissolved carbon dioxide affect your reservoir?

After CO₂ dissolves in water, it forms carbonic acid. This reaction is reversible, meaning CO₂ basically goes in and out of water as it pleases. In this case, the temperature of the water is the main influencing factor we need to consider.

CO₂ dissolves in tap water, in often copious amounts, due to bubbling and frothing in the plumbing. It comes out quite cold, usually at around 50°F. As it slowly warms, carbonic acid turns into carbon dioxide which evaporates along with other gases. Less acid in solution means the pH goes up. It is often a good idea to let tap water sit for some time to let chlorine dissipate (some, not all; read here for more info on how to really get rid of it), but few realize how much that can also change their pH.

If you tap water’s pH started out at around 7 right out of the faucet, once it has had time to fully equilibrate with its surroundings, it may start to show its true colors with a pH of around 8. It may be useful to do your own experiment to try and figure out how much the pH changes after your water adjusts to room temperature. Take a pH measurement as soon as it comes out, then again after an hour or more. Leave the container open to the air as it sits.

The pH of your nutrient solution will also drift mildly once it comes in contact with your plants. As the roots uptake nutrients, they shift the pH up and down, and you might also have some effects due to carbon dioxide exchange with the plants and the air.

As long as the pH never gets below 5.5 or above 6.8, you should be alright.

Don’t chase after the ideal pH everyday by constantly adding “pH Up” or “pH Down.” This will cause excess TDS, which may prevent roots from absorbing anything at all. Always remember that quickly adjusting pH while your plants are in the reservoir is never good; abrupt shifts in pH can kill your plants.

(Photo credit: Jena Anne)

Related: Air-Prune to Stop Plants from Getting Root-Bound

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