pH Control in Hydroponics


Following on from our article about controlling nutrient solution strength in your hydroponics system, this part will deal with the other main aspect of nutrient solution creation and maintenance i.e. pH.

pH is simply the measurement of how acidic or how alkaline a liquid is. A pH below 7 is an acid. A pH above 7 is an alkali. A pH of exactly 7 is described as being neutral.

The reason that this is important is because plants can only absorb certain nutrients at low pH ranges and they can only absorb certain other nutrients at higher pH ranges.

A quality hydroponics nutrient contains everything that a plant needs. However, the pH of the nutrient solution (or the range that it drifts through) will dictate whether all the elements in the nutrient solution will actually be available to the plant, or not, as the case may be:

Availability of different nutrients at different pH ranges in hydroponics:


The chart above illustrates how the availability of some of the most important elements changes with respect to the pH of the nutrient solution in a hydroponics system. It can be seen that there is a “sweet-spot” in the pH range of 5.5 to 5.8 where all the elements are available to a sufficient degree.

There is a principle that governs maximum plant growth speed with regard to the amounts of the nutrients that are available to it. It is called “Liebig’s law of the minimum”. This principle basically says that plant growth is not mainly restricted by the total amount of resources available. Instead, plant growth is set only by the single scarcest resource, which is called the “limiting factor”. For example there may be ample amounts of every necessary element available to a plant except, say, zinc. “Liebig’s law of the minimum” says that in this situation, the growth speed of the plant will be solely attributable to the availability of zinc.

What this actually means is that for optimum plant growth speed, we MUST make sure that our plants have sufficient access to EVERY nutrient (Macro, Secondary and Micro) with absolutely no exceptions. If there is even a slight deficiency in just one of the elements, then the whole growth speed of the plant will be limited by that one element that it can’t get sufficient amounts of. From the above chart we can see that this means keeping the pH within the range of 5.5 – 5.8 and then the plant will have all the nutrients available to it

Well that’s great news, isn’t it? We can make up our nutrient solution and then carefully add pH Down until we get to between 5.5 and 5.8 and then we can forget about it, right?

Unfortunately, it is not quite as simple as this, as I shall attempt to explain.

pH Drift:

After a fresh batch of nutrient solution has been adjusted to a pH of 5.5 to 5.8, it will almost straight away begin to drift back upwards. It is not unusual for it to have drifted back up to 6.2 within the first day. Sometimes the pH can drift even more.

There is a multitude of reasons for why this happens and to what extent:

1)      The grow-medium, such as Hydroton (clay pebbles), is not pH neutral. Even if they are soaked water that has been pH adjusted to between 5.5 – 5.8 water for 2-3 days before use, they will still alter the pH of the nutrient solution to some extent.

2)      Unless you use distilled, de-ionised or reverse osmosis water to make up your nutrient solution then your water will contain impurities. These impurities in the water can bind themselves to the acid in pH Down, causing the pH to drift upwards.

3)      Different nutrients from different manufacturers have different effects on the amount of pH drift. This is because certain manufacturers use ingredients which have more effect on the pH drift than the ingredients used in other manufacturers’ products. Also, some manufacturers, most notably Advanced Nutrients, add “pH Buffers” to their hydroponics nutrients which help to keep the pH at or close to the “ideal” 5.5 – 5.8

4)      The stronger the nutrient solution, the more the pH will drift. This means that in the first few weeks of veg, which is when the strength is low, the drift will be slow. At the peak of flowering or fruiting which is when the nutrient solution is at its strongest, the pH drift will be faster and greater.

Example of pH Drift:

pH Swing

Nutrient solutions should ideally be completely refreshed once per week. After the nutrient solution has just been changed and pH adjusted, there will normally be a rapid rise in pH. Let’s say the grower checks and adjusts his nutrient solution once per day. On the first day after a nutrient solution change out, he will see that the pH has drifted up by quite a bit so he will re-adjust it back down. On the second day after the nutrient solution change out, the pH will have drifted less than on the first day after, and less adjustment is required. On the third day, the pH will have drifted even less than on the second day. When the nutrient solution is into the second half of the week after a change out, the pH will normally have become relatively stable. However, a wise grower will still continue to check and adjust the pH as necessary, on a daily basis, right the way through to the next nutrient solution change out.

pH Measurement and adjustment

Now we have established that pH measurement and adjustment is absolutely essential to optimising the growth speed of plants, we can now discuss what you will need and how to use it. For most people, a pH measurement pen, some concentrated phosphoric acid and a pipette is sufficient.

First of all, take a measurement of you pH with your pH pen:


Next, add some pH down. This stuff is usually very concentrated so be add it very slowly, usually a few drops at a time. Bear in mind this stuff is highly caustic so don’t get it on your skin, clothes and especially your eyes:


Give the solution a good stir – a long clean spoon or the bluelab truncheon EC/CF meter is ideal for this:


And finally recheck the pH again:


Repeat this process until the pH reads 5.5. It is best to add the acid very gradually so that you don’t “overshoot” and go below 5.5. It is always best to get it right with just acid rather than go too low and have to start adding pH Up to correct the pH if you can help it.

And that’s it! Just rinse off your meter in water running from the tap. Remember to rinse out your pipette or beaker that has had acid in it.

Re-check and adjust your pH daily for best results and best plant growth.

Taking care of your pH meter:

When you first get your meter, you will usually get a small sachet of calibration fluid. Follow the instructions to calibrate your meter before first use.


Calibration fluids are available in pH 4 and pH 7. Most need to be calibrated with one of these or both. To keep your meter accurate you should recalibrate your meter regularly (i.e. at least once every week or two).

pH Meters contain a fibrous sensor which can become contaminated with salt.


This makes them inaccurate over time. For this reason, meters tend to need to be replaced every 6 to 12 months. You can help to keep your meter working properly for as long as possible by cleaning it properly. After every use, rinse the end under running tap water and then shake it out like you are shaking out a lit match. Repeat this 3 times at least. As well as this, it can be helpful to use a pH probe cleaning solution once a month.