The pH levels in small hydroponic systems can often be overlooked if a grower is focusing more on monitoring a solution’s EC or TDS, balancing nutrients, providing beneficial additives and avoiding algae and pathogen problems. However, overlooking pH control can be perilous for plants, particularly those that rely on water supplies with high alkalinity.

The pH of the nutrient solution is a major factor in determining the uptake rate of many essential nutrient ions.

Run pH too high and the dreaded nutrient lockout looms. Often, the first sign that pH has drifted out of range is a slight paling or yellowing of the younger foliage as the plants struggle to take up certain essential nutrients.

Many inexperienced growers tend to misinterpret this so a quick check of pH is always worthwhile when troubleshooting growth problems.

What is pH?

In simplest terms, pH is a measure of the acidity or alkalinity of a solution. The pH scale is logarithmic, which means a pH of 4 is 10 times more acidic than a pH of 5 and 100 times more acidic than a pH of 6. With a pH of 7 being neutral, such as with pure water, values below 7 are acidic and those above are alkaline (or basic).

Plants grown in hydroponics have a different optimal pH level than those grown in soil, so soilless gardeners need to be careful not to apply the pH recommendations for soil-grown crops to those they produce in hydroponics.

For most commonly grown hydroponic crops, an optimal pH range is between 5.5 and 6.5. Commercial growers often use a narrower range of 5.8 to 6 for most crops when they are using automatic controllers that regularly dose acid into recirculating systems to maintain this precise level.

The optimal acidic pH range for hydroponic crops is important as it affects the solubility, availability and uptake of several of the essential plant nutrients.

If the pH drifts too high (past 7), plant uptake of some nutrients becomes less efficient. For example, plants can become iron deficient, even if sufficient iron is present in the nutrient solution.

Calcium is also affected by high pH, forming insoluble salts that precipitate out of the nutrient solution to form whitish deposits on reservoir walls, channels and equipment—otherwise known as scale.

This type of nutrient lockout will typically show up as iron deficiency on new foliage (yellow, interveinal chlorosis on the foliage) and as tipburn and leaf cupping, which are symptoms of a reduction in calcium availability and uptake.

Inexperienced growers may misinterpret these nutrient deficiency symptoms as a problem with the nutrient formulation or product itself, rather than an issue with pH rising above optimal levels for iron and calcium uptake.

What Causes pH Fluctuations in a Garden?

In a healthy, well-run hydroponic system, pH fluctuations are normal and in some instances, such as recirculating nutrient film technique with a large crop of mature plants and small nutrient volume, pH changes can be quite rapid and require frequent adjustments to stay within a narrow range for optimal nutrient uptake.

As plants remove nutrient ions from the solution, the solution’s pH drifts up or down. If left uncontrolled, the pH will often drift downwards for several days after planting a new crop, after which the pH will steadily increase.

This is due to the differential uptake of ions from the solution, with the release of hydrogen (H+) or hydroxyl (OH-) ions from the root system.

As positive ions such as cations Ca2+, K+, Mg2+ are removed from the solution, hydrogen ions (H+) are released from the root system to equalize the ratio of anions to cations in the root zone and this lowers the pH of the solution.

When the crop begins an active growth phase, anions such as NO3 are taken up, which increases the pH through the release of hydroxyl ions (OH-) into solution.

Once plants are well established, most hydroponic systems tend to see a gradual and continual increase in pH over time, which is countered with doses of diluted acid.

Water Supply and Water Treatment for pH

Many of the pH problems that growers encounter originate from the water supply. Many source waters, including the city supply, can have issues with alkalinity and while that is not a problem for human consumption, it can cause problems in a hydroponic system.

The alkalinity of a water supply describes the strength of a high pH so a water supply with a high starting pH and a high alkalinity takes a far greater volume of acid to bring it down to optimal levels than a water supply with a low alkalinity and of the same pH.

Alkalinity is usually provided on water analysis reports from city supplies and is worth checking if continual pH rises requiring large doses of acid is a problem.

High alkalinity is considered greater than 300 mg/L of calcium carbonate and low alkalinity is less than 100 mg/L. When high-alkaline water is first added to the nutrient reservoir, it can take large volumes of acid (pH down solution) added over a number of days to finally bring down and stabilize the pH in the 5.8 to 6 range.

However, adding large amounts of acid is not only time consuming in terms of monitoring, adjusting and readjusting, but it can also create imbalances in the nutrient ratio as acids also add in nutrient ions.

Nitric, phosphoric and sulphuric acid all add N, P or S to the carefully balanced nutrient solution, so accumulation can occur. Growers with a water supply of high alkalinity can prevent this issue by pre-acidifying the source water down to a pH of 6 before using it to make up nutrient solutions or adding as top-up water to a nutrient reservoir.

Once the water supply has been stabilized at a pH of 6 and the alkalinity countered to the point, it remains at that pH for 24 hours and can be used in the hydroponic system. Much less acid will then be required for pH control.

Hydroponic Nutrient Formulation and pH

Nutrient formulations or products all have different starting pH values because individual salts become more or less acidic when dissolved into water. Salts such as monopotassium phosphate lower the pH more than salts such as calcium nitrate. Most formulations will result in an initial pH of around 5.5 to 6, which is ideal for the growth of hydroponic crops.

In hydroponic solutions, some salts can be used to influence the pH control of the nutrient solution, reducing the requirement for acids during growth development phases of the crop.

Ammonium nitrate is one salt used for this purpose, and the optimal amount is that which provides 10 to 15% of the total nitrogen of the formula in the ammonium form.

Ammonium in nutrient solutions tends to be acidifying because, unlike nitrate, it is a positive ion, and when taken up by plants it is replaced by hydrogen ions, reducing pH in the root zone. In addition, ammonium forms ammonium hydroxide and hydrogen ions that produce a mild acidifying effect when in solution.

Many of the nutrient products on the market designed for hard or alkaline water sources use a certain percentage of ammonium nitrate to help counter pH increase.

While this is effective in keeping pH levels down, the ammonium form of nitrogen can cause growth issues and even toxicity if the percentage used is too high and under certain growing conditions.

Ammonium competes for calcium uptake, so only small levels—10 to 15% of total nitrogen—of ammonium as a nitrogen source are recommended for most crops and well-formulated nutrient products usually contain less than this.

Testing and Adjustment pH Levels

The pH of a nutrient solution can be easily adjusted with the use of specifically designed pH up and down products that are diluted and ready to use.

Commercial growers usually make up their own pH adjustment solutions of 10% nitric or phosphoric acid or potassium hydroxide for increasing pH.

By using a ratio of 50% nitric and 50% phosphoric acid, the solution can be kept in better balance as both N and P are added, but when a lot of acid is being used, the nutrient formation should be adjusted for these additional N and P sources, as they are macronutrients.

There are organic acids such as citric acid and white vinegar (acetic acid) that are sometimes used in small systems to bring pH down, but these are weak acids and only give a short-term pH reduction.

Organic acids also add carbon to the nutrient solution, which microbes (both good and bad) love to feed on, encouraging an unwanted reduction in dissolved oxygen in the solution.

There is a wide range of pH testing equipment available, from inexpensive liquid test kits and strips sold by aquarium/swimming pool suppliers to high-tech, electronic meters.

Test kits and strips should only be used for the lower pH range—many of these measure pH 5 to 7.

Growers with electronic meters should also invest a few dollars in a cheap liquid pH test kit or strips just to double check the accuracy of their electronic meter from time to time. Meter probes tend to drift over time and need to be calibrated on a weekly basis, cleaned and replaced annually, and many growers have been caught out by pH meter inaccuracies gradually occurring.

A quick check against a pH test strip will soon show up any irregularities. Calibration of pH meters is essential and often overlooked by many growers—electronic pH meters usually come with buffers.

A pH 4 and a pH 7 solution can be used to check and adjust the meter’s readout.

Additionally, buffer solutions for this purpose are usually available through hydroponics retailers and are a good investment.

Always follow the instructions for your pH meter. Some require the probe to be stored wet, others don’t. Cleaning the sensitive probe tip is also important for accuracy.

Different Hydroponic Systems

With a recirculating system such as nutrient film technique, checking pH is simple as the solution in the reservoir is what is in direct contact with the roots.

For media-based systems, which are often drip irrigated and in which the nutrient solution may not recirculated, pH control is slightly more complex.

For these types of systems, pH (and EC/TDS) needs to be measured both at the nutrient supply reservoir and in the solution that drains from the base of the grow beds, bags or slabs containing the plants, called the leachate solution.

With large, rapidly growing plants, there will usually be slight differences in pH between the feed nutrient and the leachate solution—pH changes as the solution flows past the roots and has ions absorbed/released and the grow medium may influence pH as well.

Adjustment of pH at the nutrient supply reservoir should be based on the pH measured in the leachate draining from the plant’s root system.

For example, if the pH is 6.9 in the leachate and 6.2 in the feed solution, the pH of the feed solution should be acidified down to the point where the drainage solution starts to come down to 5.8 to 6 as this is the pH that the root system will be experiencing.

Summing It Up

Measuring and adjusting pH in a nutrient solution is usually quick and simple. There is a wide range of pH testing tools and adjustment products on the market to choose from and retailers can provide valuable advice on what is most suitable for different types of systems.

Keeping a garden’s pH within the correct range means your plants will have access to the full menu of essential nutrient ions for optimal growth and productivity.