Understanding & Maintaining pH in Hydroponics

By: Chris | Last Updated: November 29, 2020

Watching your hydroponic garden thrive is a wonderful feeling. On the other hand, watching plants wither or produce poorly is an anxiety-inducing experience. One of the most common causes of poor plant performance is mismanaged pH in hydroponics systems.

The chemistry behind pH and nutrient interaction is complex, but understanding the basics is really simple. This article will help you master the pH levels in your hydroponic garden, resulting in better plant performance and more smiles on faces!

Looking for pH & EC information for a specific plant? This list of optimal pH & EC ranges for hydroponic vegetables and herbs has you covered!

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What is pH?

The level of acidity or basicity (alkalinity) of a water-based solution is notated as pH. This notation stands for ‘power of hydrogen’ or ‘potential of hydrogen’. I prefer to think of it as the ‘power of hydrogen’ for one simple reason.

pH is notated as a base 10 logarithm meaning that a solution with a pH of 5 is ten times more acidic than a solution with a pH of 6. This is important to keep in mind when testing the pH in hydroponics solutions. 

pH Scale

The pH of a liquid is noted on a scale from 0 to 14. A solution with a pH below 7 is considered acidic, while a solution with a pH above 7 is considered basic or alkaline. A pH of 7 is neutral.

Why is pH important for hydroponics?

The pH of a hydroponic nutrient solution is directly related to a plant’s ability to absorb essential nutrients. If the solution is too acidic, micronutrients are absorbed in toxic levels while macronutrients are lacking. Conversely, if the solution is too alkaline, plants will not absorb micronutrients.

Plants rely on 16 elements to thrive and produce maximum yields. These elements are grouped into macronutrients, secondary nutrients, micronutrients, and nutrients provided by air & water.

Macronutrients: Nitrogen, Phosphorus, Potassium
Secondary Nutrients: Sulfer, Calcium, Magnesium
Micronutrients: Iron, Manganese, Boron, Copper, & Zinc
Air & Water: Hydrogen, Oxygen, Carbon

pH in Hydroponics Solution Effect on Nutrient Availability

The chart above helps provide a visual understanding of how nutrients are absorbed in different pH levels. Pair this with an understanding of the nutrient requirements of a specific plant throughout its life-cycle and you can begin to tailor pH levels at specific growth phases to optimize the absorption rate of individual nutrients.

This is especially important for leafy green plants because they rely heavily on micronutrients for proper leaf development. It is common for plants suffering from poor micronutrient uptake to have stunted, underdeveloped leaves.

pH in Hydroponics: Case Study

The chart above gives a good visual representation of nutrient absorption, but it doesn’t tell the whole story about how different nutrients interact with each other.

Let’s look at tomatoes as an example.

Nitrogen (N), Calcium (Ca), and Magnesium (Mg) encourage heavy vegetative growth which is good in the initial growing phase.

With that in mind, it seems like a pH of 7.0 would optimize all three nutrients, but it is more complicated than that. Excessively high levels of N, Ca, & Mg can be detrimental to reproductive growth and can induce Potassium (K) and Iron (Fe) deficiencies. A pH of 6.0 allows for balanced vegetative nutrient absorption, maximizing the effect of your growth phase fertilizer.

As the tomato plant enters into the reproductive, flowering/fruiting, phase it will rely much more heavily on Potassium (K). Potassium is incredibly important for the development, and flavor, of tomatoes. During this phase, it is ideal to slightly lower the plants’ absorption of Nitrogen, Calcium, Magnesium, and Phosphorus. A pH closer to 5.5 will lower the absorption of growth phase nutrients, and focus more absorption on the flowering/fruiting phase nutrients Potassium and Iron.

It is important that your nutrient solution be formulated for each phase of life-cycle.

Example Nutrient Solution:
Growth phase K:N nutrient ratio – 1.2:1
Fruiting phase K:N nutrient ratio – 2:1
Heavy Fruiting phase K:N nutrient ratio – 2.5:1

The pH level of a hydroponic nutrient solution helps set the stage for absorption, and the nutrient makeup of the fertilizer determines what level of each nutrient is available in the solution to be absorbed. Adjusting pH won’t magically make nutrients appear in the solution, so make sure you are providing adequate nutrients at the right time for your plant.

Optimal pH for Hydroponic Plants

Plants will have varying pH requirements, but in general, a pH range of 5.5 – 6.5 is considered optimal for hydroponic gardening. The majority of plants produce best in slightly acidic conditions. 

If you use rockwool starter cubes, it’s important to be aware that rockwool is slightly alkaline with an average pH of 7.8. Before using a rockwool cube, soak it in pH 5.5 water for at least 15 to 20 seconds to bring down the pH level. Larger cubes will need to soak for up to 20 or 30 minutes to effectively lower their pH level.

Looking for optimal pH ranges for a specific plant? Check out the Growing section.

When to Adjust pH in Hydroponics

The hydroponic nutrient solution needs to be pH adjusted after you’ve mixed the nutrients with the water. Mixing nutrients with pH balanced water will shift the pH of the final solution, which means you would need to adjust again. Save yourself some time, and wait until you have your nutrient solution ready to go before adjusting the pH level.

Any time you top off the reservoir, check the pH of the reservoir. If it has shifted, adjust it. It’s also a good idea to check the pH of the reservoir on a weekly basis as nutrient absorption can effect pH levels.

How to Adjust pH in Hydroponics Solution

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The most reliable way to adjust pH levels in a hydroponic reservoir is by using commercial pH adjusting chemicals. There are numerous brands available, at varying price points. Which one you use is totally up to you. Personally, I use pH Up and pH Down by General Hydroponics.

Adjusting pH Level

There are other methods that can raise or lower the pH of your solution, but they often leave undesirable elements in the solution that become part of the TDS of the solution. The end result is a reduction in the amount of nutrients suspended in the hydroponic solution. With that in mind, stick with purpose-made adjusting chemicals like pH Up & Down.

Common pH Testing Methods

In order to know the pH of a solution or liquid, you need to test it. The method used to test the liquid is up to you. When I first started hydroponic gardening, I bought a liquid pH test set from General Hydroponics. I learned a lot using this method, and I still use it today if I suspect my pH meter is losing calibration.

Testing pH in a Kratky jar with electronic pH meter
Testing levels in a Kratky jar with an electronic pH meter.

pH Meters

One of the more popular testing methods, a pH meter is an electronic device that outputs the pH level of the liquid it is placed in. Using an electronic pH meter is simple. Turn it on, stick it in the liquid, read the screen. It doesn’t get any easier than that.

Electronic pH meters have to be calibrated before being used the first time, and will need to be calibrated regularly to maintain accuracy. Buffer solutions are used to calibrate a pH meter. A buffer solution is a known pH level substance that is mixed with distilled water.

Most pH meters have a “Cal” or calibrate button. To calibrate the meter, mix the buffer powder or solution, turn on the meter, place the meter in the buffer solution, and hold the calibration button.

It’s worth noting that a lot of “cheap” pH meters only come with a single-use buffer powder. That’s a bummer, but it’s not the end of the world. Any standardized buffer powder or solution brand will work to calibrate a pH meter. Make sure to buy additional buffer powder when you buy the meter so you can keep it calibrated.

Litmus Test Strips

Litmus test strips are by far the least expensive way to measure pH. Litmus test kits come with pH paper and a color scale. Using a dropper or cotton swab, wet a paper test strip. The test strip will quickly activate and change color based on the acidity or alkalinity of the liquid. Compare the test strip to the color chart to determine pH level.

Litmus test strips are cheap, and many people swear by them. One word of advice, don’t buy cheap imported litmus strips on Amazon. Spend the extra money to buy quality litmus strips, a liquid pH test kit, or an electronic pH meter.

Liquid pH Test

If you’ve ever tested pool water, a liquid pH test kit is very similar. They normally come with a small dropper, a small container with lid, a color changing solution, and a color chart. These kits are very accurate, an I still use mine when I suspect my electronic pH meter is losing calibration.

To use a liquid pH test kit, add hydroponic solution to the test container with a dropper. Add 3 to 4 drops of the color changing solution, and secure the lid on the container. Lightly tap the container to mix the solutions. Once the color change has stabilized, match the color in the container to the pH level color indicator that came with the kit.

How Often to Test pH Level

If you’re new to hydroponic gardening, check pH levels daily until you get accustomed to how your system fluctuates. This may seem like overkill, but you will quickly start to learn 

Once you’re confident your levels are stable, check pH levels weekly or whenever you add or replace nutrient solution to the reservoir.

FAQ

How do you adjust pH in hydroponics?

The easiest way to adjust pH in hydroponics is by using commercially available pH adjusting chemicals. One chemical is acidic, and the other is alkaline. For example; General Hydroponics makes pH adjusting chemicals called pH Up and pH Down.

pH Up is used to raise the pH level of a solution, and pH Down is used to lower it. It is recommended to wear protective glasses when using this type of product, as a chemical reaction does occur. Add these chemicals to the hydroponic solution slowly to prevent any kind of chaotic chemical reaction blowing up in your face!

In small scale hydroponic reservoirs less than 10 gallons, apply the needed chemical to the reservoir with a dropper. General Hydroponics recommends using not more than 1 milliliter of chemical per gallon of water. Start with 1mL per gallon and wait 15-20 minutes to allow the chemical time to disperse, then check the pH level again.

Can you use vinegar to lower pH in hydroponics?

Short answer, no. Vinegar, or acetic acid, is a very weak acid and will not effectively lower pH in hydroponics over a long period of time. The amount of vinegar needed would be as expensive as a commercial pH adjusting product.

Additionally, the extra carbon and hydrogen will add to the overall TDS (EC) of the solution. Which limits the nutrient density of the solution. By comparison, commercial pH lowering products use phosphate compounds that lower the pH while also providing usable nutrients to plants.

What is the best pH for hydroponic tomatoes?

The optimal pH range for hydroponic tomatoes is 5.5-6.0, but the plant will perform well at a wider range of 5.5-6.5. The vegetative growth phase should perform well at 6.0 pH, and the flowering/fruiting phase will be better served with a more acidic pH closer to the 5.5 range.

The reason for the change in pH is related to the changing nutrient dependence as the tomato plant develops.

Why does pH change in hydroponics?

In an otherwise “balanced” hydroponic system, pH naturally shifts for several reasons.

Most nutrients are slightly acidic. As they are absorbed by the plant, the solution becomes more alkaline.

As water evaporates, or is absorbed by plants, pH shifts as nutrients become more concentrated. This is a key reason to keep a close eye on water levels in a hydroponic reservoir. Reservoirs should be topped off regularly to prevent widely shifting pH levels.

Why is my hydroponic nutrient solution pH dropping?

If the pH of your hydroponic solution continues to drop, there are a few likely causes.

The nutrient solution is either too strong, or the hydroponic system needs maintenance/cleaning.

Contaminated hydroponic systems are the most common cause of unstable low-end pH levels. Not replacing nutrient solution regularly can result in an increased amount of microbial activity, which causes pH levels to continuously drop. If you haven’t completely replaced the nutrient solution in your reservoir lately, drain it, clean the reservoir, and refill it with fresh solution.

Using an over strength nutrient solution will cause pH levels to fall because most nutrients are slightly acidic. If your hydroponic system has been cleaned and flushed recently but the pH levels are still dropping, the nutrient solution is probably too strong. Test the EC, TDS, or PPM of the solution. If it is out of range, dump the solution and add a new batch to the reservoir.

Decomposing roots release acidic bacteria into the hydroponic solution that can lower pH levels. Inspect the root systems of your plants to ensure they are healthy.

Why does my nutrient solution pH keep going up?

Temperature, time, and nutrient solution strength are primary reasons why pH continues to rise in a hydroponic system.

As temperatures rise in a reservoir, water releases CO2 which slowly increases the pH in the reservoir. Keep reservoirs out of direct light, and preferably in the coolest part of the grow area.

Time as a factor, relates to how long the nutrient solution is in use. Most nutrients are slightly acidic. As the nutrients in the solution are absorbed by the growing plants, the solution naturally becomes more alkaline.

Weak nutrient solutions will result in a faster pH shift because the nutrients are absorbed more quickly by the plants. Test the TDS/EC/PPM of the solution to ensure it is within the optimal range for the plants you’re growing.

Is a pH between 6 and 7 optimal for nutrient retention and plant growth?

No, in hydroponic solutions, a pH of 6.0-7.0 is not optimal.

Instead, a pH range of 5.5-6.5 is considered more optimal as this range results in a greater absorption availability of micronutrients by a wider variety of plants.

Why is the pH of the nutrient solution so important in hydroponics?

Managing optimal pH in hydroponics is important because it directly influences how plants absorb nutrients. Not only does pH effect how many nutrients are absorbed, it can also be used to increase, or decrease, the absorption of specific nutrients.

This is especially important when growing plants that have vegetative, flowering, and fruiting phases.

Chris Cook started Happy Hydro Farm to share his passion for hydroponic gardening! Growing your own food is incredibly rewarding both physically and mentally. His mantra - "Take excellent care of your plants, and your plants will take excellent care of you."