What is pH and Why Does it Rise? A Complete and Updated Guide

If you grow cannabis indoors, you need to know about pH. pH levels are the invisible force influencing how well your plants absorb nutrients. Too acidic and nutrients reach toxic levels. Too alkaline, and your plants starve to death. It's a delicate balance, with serious ramifications.

The ideal pH for cannabis is between 6 to 6.5 in soil (5.5 to 6.5 for hydroponic). But, as many growers quickly find out, the system can drift up and down throughout the grow. Why does it tend to rise over time? And what are the best ways to adjust the pH levels, without causing even more damage to your plants?

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Hydroponic setups are likely to see pH levels rise over time, as the system becomes more alkaline. Why?

• Most nutrients are slightly acidic. As the plants absorb these nutrients through their root systems, the system slowly loses its acidity to become alkaline. Over time, the pH numbers will gradually rise until you add more nutrients.
• Temperature also affects the pH level of water. As the water warms up, it slowly releases CO2 into the atmosphere, which naturally increases pH levels.
• The strength of your nutrient solution could also affect pH drift. If the solution is too weak, pH can rapidly rise as the plants absorb the available nutrients out of the solution. On the flip side, if the solution is too rich, you'll see the pH head towards 0.

Straight from the Merriam-Webster dictionary, pH is a measure of acidity and alkalinity of a solution on a scale between zero to 14. Between 0 to 7 represents the spectrum's acidic side, while anything between 7 and 14 is the alkaline end. Seven is the neutral, a perfect balance between acidic and alkaline.

Baking soda is a good example of an extremely basic or alkaline substance, while vinegar and citrus fruits are at the spectrum's acidic side. Even water has a pH level, varying depending on the nutrients, minerals, and contaminates. Technically all substances have a base pH level. 

Vinegar and baking soda aside, why do you need to understand the pH scale when growing cannabis? Cannabis plants thrive in an environment with a specific pH level. Anything above or below this magic number impacts the success of the grow. 

What is this magic pH number for cannabis? You'll often find online grow guides suggest a pH range between 6 to 6.5 for soil-based systems, and 5.5 to 6.5 for hydroponic setups. Cannabis plants enjoy a slightly acidic environment.

An imbalanced environment makes it challenging for your plants to absorb the nutrients they need to thrive. For example, if the substrate is too acidic (pH measuring between 0 to 5), several key nutrients become excessively available to the point they are toxic. 

If the growing medium is too alkaline (between 7 to 14), the problem becomes the exact opposite. An alkaline environment prevents the absorption of iron, copper, zinc, and manganese. Eventually, this leads to serious nutrient deficiencies and starvation. At either extreme end of the pH spectrum, you could experience irreparable damage. 

Thankfully, new research published in the Cannabis Business Times has found growers don't always need to hit the exact numbers detailed in most growers forums. According to the investigation, the ideal pH range for growing cannabis is between 5.5 to 6.5. But (and this is an important discovery), the research team also had significant success growing outside this range. 

The research team from North Carolina State University tested for pH at several large scale (and successful) commercial operations. They discovered pH levels across the spectrum, and often far outside the suggested range between 6.0 to 6.5. Back in their laboratory, they were able to grow cannabis in a pH range between 5.0 to 7.5, without serious repercussions. Essentially, cannabis plants aren't so picky about pH as we once thought.

For those growing cannabis in a soilless medium, what is the recommended pH level? Each substrate will have a slightly different ideal pH level for cannabis. Here we'll cover Coco Coir, as it's by far the most popular.

Most indoor growers working with Coco Coir aim for a pH of 5.8, although as the North Carolina State University research demonstrated, a little pH swing is fine (for example, between 5.5-6.2). Intel from those working within commercial operations suggests many grow in Coco Coir with a pH between 6.0 to 6.5.

Typically, you'll see significant variation from online experts about the ideal pH for soil systems. While researching this article, we found one website stating a pH between 6.3 and 6.7, another said 5.5 to 6.5, while still another reputable source claimed 6.0 to 6.8. The study from North Carolina State University determined a more specific range of between 6.2 to 6.7 as the ideal. 

There is a lot of opinion on the best pH for growing cannabis in soil. This may be because the soil contains more variables than a soilless substrate, allowing for more room to swing up and down the pH scale.

Why do Coco Coir and soil systems have different pH targets? Soil naturally holds on to more nutrients than a soilless medium like Coco Coir. Therefore the risks of nutrient lock-up aren't as severe for soil-based growers.

If there is one constant within hydroponics, it's the shifting pH. Throughout a plant's life cycle, pH naturally shifts as you add nutrients, and plants absorb said nutrients. The best way to stabilize the system is by testing, especially if you have added new nutrients. Test frequently, even daily.

When you do find slight fluctuations, the most important step is to hold steady. Adding in pH up or pH down products at every little change will push the system to frantically up and down. Small shifts are okay, and your plants will absorb naturally. Only once new nutrients have stabilized should you consider using products to tweak the pH level. Stick with natural options should you need to increase or decrease the pH level.

For those growers using straight tap-water in their hydroponics system, it's always recommended to let your water sit, unadulterated, for 24 to 48 hours. This allows the pH levels to settle out, as the CO2 adjusts to match that of the atmosphere. There are highly variable levels of CO2 in tap water based on location, municipal water treatment, and more. While not everyone will see pH numbers decrease, some will.

If your water contains lower levels of CO2, when you allow it to sit overnight, the water absorbs CO2 from the atmosphere. This, in turn, naturally lowers the pH level. So letting water stand overnight will only lower your pH if the water doesn't have as much CO2 as the atmosphere.

If you've grown hydroponically, you've probably noticed that after you set up your reservoir and adjust your pH levels, they will slowly rise throughout the week. Why does pH rise as my water sits in the reservoir?

Most of the nutrients you'll use for growing cannabis are slightly acidic. Which means a reservoir is also an acidic place. As plants absorb nutrients, they will also absorb some of this acidity. Gradually, you'll find the pH levels rise as the reservoir becomes more basic. If you don't see levels change, you may want to assess your plants for signs of nutrient deficiency.

Temperature also affects the standing pH of your water. Straight out of the tap, you could take one reading, only to find it has subtly shifted when it comes up to room temperature. This has to do with water's ability to absorb CO2 from the atmosphere (or release it into the atmosphere), which shifts the pH levels. pH levels begin to rise as carbonic acid turns into CO2. As the water warms up, the CO2 evaporates from the reservoir. This natural process increases the pH level. This shift in pH is an inevitable aspect of hydroponic systems.

Still, there are ways to reduce the effects of pH shift in a freshly filled reservoir? Have a reservoir sit for a few days before you add any nutrients or pH balance chemicals. This will allow the pH balance to level out. Test the pH level before and after adding nutrients. 

Even in a perfectly-balanced hydroponic system, you can expect pH levels to rise over time. Gradually increases are manageable, but keep an eye out for alarming jumps in pH level. Rapid shifts in pH levels can harm your plants and can be challenging to get under control without additives, which can further shock the system.

There are two methods of testing pH levels: ph testing kits and Digital pH Meters (also called pH Pens). There are slightly different methods for hydroponics versus soil-based systems.

If you are using a testing kit in a soil-based system, you first have to make a slurry. Here is how you test with pH paper strips:

1. Dig a sample from a few inches down in the container. You'll need about ½ cup or more.
2. Mix with distilled water to form a slurry, about equal parts water to soil.
3. Allow this mixture to sit at room temperature for at least 30 minutes.
4. Pour the mixture through a coffee filter to remove the solids.
5. Test the liquid with the pH test strips provided in the testing kit. 
6. Compare the color of the strips with a ph scale.

What about hydroponic systems? In this scenario, there is no slurry required, and you'll test the reservoir.

1. Test the nutrient solution (at room temperature) with the pH test strips provided in the testing kit. 
2. Compare the color of the strips with a ph scale.

Testing kits that use the paper strips is an affordable method of testing soil and soilless environments, but they are not the most accurate. Digital pH Meters are the preferred method for measuring pH by most experienced growers. They are more accurate and easier to read than testing strips. If you are using a pH Pen to test soil, it also starts with the same slurry required for the paper-testing kits:

1. Dig a sample from a few inches down in the container. You'll need about ½ cup or more.
2. Mix with distilled water to form a slurry, about equal parts water to soil.
3. Pour the mixture through a coffee filter to remove the solids.
4. Calibrate your pH pen following the package instructions.
5. Dip the tip of the Digital pH meter into the liquid as per device instructions, or until a number appears.
6. Compare the pH reading with the pH scale.

Testing the pH of a hydroponic set up with a pH Pen is perhaps the easiest process of all:

1. Calibrate your pH pen following the package instructions.
2. Insert into the nutrient solution until a pH reading appears.
3. Compare the number against a pH scale.

First things first, don't make any sudden movements. Sudden and aggressive changes in pH levels can do more harm than the wrong pH level. Any changes you introduce into the system must be slow and steady.

You will also want to test pH levels over several days to confirm the functionality of your pH meter and take a running average of pH. pH spikes are not common, but they do happen. 

Before we get into the details of adjusting pH levels up or down, always test the pH of your water before getting started. This could have implications for what additives you introduce into the system. 

If you have tested your system's pH levels and discovered they are running high, what's the best way to lower them? Once again, start slow and steady. Do a flush of the system with nutrient-free water, ideally with a pH between 5.5 to 6.5. With automatic irrigation, a system flush is a simple task. This will help clear out any built-up salts within the soil before adjustment.

If you are growing in soil, try adding a splash of vinegar or lemon juices to a gallon of water. You can also add phosphoric acid during the flowering stage or nitric acid during the vegetative stage. A little goes a long way.

For hydroponic growers, you'll want to avoid the lemon and vinegar routes commonly used among soil-based growers. Instead, stick with nitric, phosphoric, or citric acids to lower pH. Small daily adjustments are vital to reducing the risk of pH spikes.

The final option is a pH down product. These are additives specifically designed to reduce the pH levels for cannabis. Apply ph Down following the package instructions.

What happens when your system is too acidic? How can you adjust the pH level back to cannabis' preferred pH equilibrium? Generally speaking, super-acidic soil or reservoirs are a far less common problem than super-alkaline systems. Yet, they still occur, and you should know how to address them.

For soil-based growers needing to bump up their growing medium's acidity, the easiest and safest option is powdered dolomite lime. A tablespoon mixed into a gallon of water is enough to help improve the pH score. Other options for soil-based systems include baking soda, hydrated lime, potassium silicate, and potassium hydroxide.

Hydroponics systems are a bit more finicky to adjust, but powdered dolomite lime, potassium silicate and potassium hydroxide are good options.

As a last resort, the pH up and pH down products available at most hydroponics stores are a final go-to. But again, most experienced growers steer clear of these unless absolutely necessary. These products damage the many beneficial microorganisms within your system, which is an especially unappealing idea for organic growers.

Adjusting pH in a Deep Water Culture (DWC) system is in many ways similar to other hydroponics systems, but there are a few essential points you may want to note. First, DWC needs daily checks to monitor nutrients and pH. If levels suddenly go haywire (and they do), there is no buffer before your plants feel the shock. Invest in a Digital pH Pen, and make testing routine.

Like in other hydroponic systems, within a DWC, a small amount of pH drift is fine. You'll add nutrients, and the plants will absorb nutrients, which will all impact the final pH. It's a moving target to hit the sweet spot of 5.8. Do not continually correct up and down, as they will damage your plants more than a slow natural drift will.

If you notice an alarming pH drop, this is an indication your plants have gone through an intense feed and absorbed nutrients more rapidly than expected. Because all nutrients sit somewhere on the pH scale, the type of nutrients your plants choose to absorb can also set the whole system into imbalance. More N and K and no P is one such example. You may want to consider increasing the reservoirs' size for next grow and tinkering with EC.

No matter how critical the pH is for growing cannabis, it's one of the most frequently overlooked aspects. pH is an invisible influence over your plants, but a critical one.

Because pH influences how much or how little nutrients the roots absorb, the signs of pH imbalance almost always appear as issues with nutrients. Before you treat any suspected nutrient deficiency, it's always best to test for pH first.

Growers big and small should test the pH of their system frequently and adjust if needed. Keep in mind that soil naturally provides more of a buffer for pH drift, and hydroponics growers will likely see their pH levels rise over time. If you need to adjust, do so gradually using organic options whenever possible.