Understanding the Significance of pH in Cannabis Cultivation

Soil meter is used on loam for planting, Measure soil acidity.
Photo: kram-9 / Shutterstock

The world’s greatest athletes and trainers focus immense energy on perfecting the fundamentals of their activities to reach peak performance. As the cannabis industry continues to learn more about what it takes to consistently produce the best possible flower at scale, it’s become quite apparent that commercial farms have a lot more to focus on than THC content. Few factors are more fundamental on the farm than pH (power of hydrogen) or hydrogen ion concentration, which ranges from zero to 14.0 and allows us to ensure the plant’s organic matter can interact properly with its nutrients and environment.

The soil’s pH value, which determines whether a growing medium is more acidic or basic, is measured logarithmically (a.k.a., pH levels of 5.0 are 10 times less acidic than levels at 4.0).

Advertisement

This often overlooked yet significant detail means even minor changes in cannabis soil pH can have a large impact. Other examples of similar logarithmic scales include the Richter Scale which is used to measure the magnitude of earthquakes.

This measurement is important to understand as an operator or consumer. In many instances, it’s the pH level that’s monitored closely and used to determine how plants are treated throughout the growing process. The pH can have a major effect on the overall quality of the final product if farms are struggling to maintain balance, adding or removing nutrients regularly to address the volatility issue.

Understanding cannabis pH values

Putting cannabis aside for a moment, let’s talk about the power of hydrogen (pH), which is used to measure a wide variety of chemical compounds.

In general, a pH range will move from acidic-based to alkaline-based.

Compounds with the highest acidity have a pH of 0, compounds with the highest alkaline levels have a pH of 14.0, and neutral compounds perfectly balanced in the middle have a pH of 7.0. Pure water is the most common example of a substance with a neutral pH of 7.

As we’ve mentioned, the pH scale is logarithmic, so when measuring pH levels you’ll need to take into account that a 14.0 pH level is not just twice as acidic as a compound at 13.0 pH; it’s 10 times more acidic.

Keeping that in mind, a battery is 10 times more acidic than human stomach acid, which is 10 times more acidic than a lemon, and so on. Drain cleaner will contain 40 times more alkaline than stomach tablets, and stomach tablets will contain 20 times more alkaline than blood.

The Science pH Scale illustration
Chart: BlueRingMedia / Shutterstock

This is imperative to understand, especially when growing plants. A low pH can cause nutrient solubility (like calcium and magnesium) to increase, which can cause toxicity. Equal risks of nutrient deficiencies or nutrient lockout are presented with high pH levels, which is why it’s ultimately important to keep your plant’s pH at the optimal range to ensure successful growth and development. Any major deviation in pH requires correction, which can ultimately have a negative impact on the final product.

The perfect pH for cannabis

According to Wisconsin-based commercial cultivator, Kurt Kinneman, different genetics and growing environments will require every cultivator to develop the “perfect pH” for the plants they grow. However, there are a few best practices to follow. In general, a pH greater than 6.0 and less than 6.5 is considered standard, but cannabis root zone pH levels can live anywhere from a pH of 4.0 to 7.0.

Of course, there are a few factors to consider when determining which pH zone will be the most beneficial for your plants.

For hydroponic growing the pH should be between 5.8 and 6.3, and for soil, the pH should be between 6.3 and 7.0 because the plants have the soil as a buffer — the pH can be closer to 7 without plants struggling to uptake nutrients. Also, if the pH is closer to 7 when growing in soil, it can provide a healthier environment for the microbes present. More microbes mean there is more organic matter being broken down into usable nutrients for the plants. 

Growing in soil

If you are growing plants in soil, the ideal pH level will live between 6.3 and 7.0. It’s okay to allow for natural fluctuation within this range depending on the nutrient levels, as long as you don’t stray too far below 6.3 or above 7.0.

“Because the plants have the soil as a buffer, the pH can be closer to 7 without plants struggling to uptake nutrients,” said Kinneman. If the pH is closer to 7 when growing in soil, it can provide a healthier environment for the microbes present, and microbes mean there is more organic matter being broken down into usable nutrients for the plants.”

In the same vein, your nutrient administration will determine how closely you need to pay attention to pH in the first place. If you’re growing entirely organically — without the use of any mineral nutrients — pH levels won’t be as significant. 

Hydroponics or soilless growing

If you grow hydroponically or soilless, your optimal pH range will be a little more acidic — about 5.8 to 6.3. Just like growing in soil, it’s okay to allow the pH levels to fluctuate slightly depending on nutrient intake, but just make sure it stays safely within that range at all times.

How to test pH

“The pH should be measured whenever you are watering your plants or feeding them nutrients. The pH can change as water sits too, so even if you check your reservoir one day and the pH is good, the next day the pH could change. It is best to keep a log and track your pH constantly throughout the grow cycles,” said Kinneman.

Many growers use drops or a digital pH meter to take regular measurements:

  • Drops

With this method, you’ll prepare your fertilizer, stirring it gently and being careful not to over-oxygenate it, as that may throw your reading off. You’ll fill half of a test tube with fertilizer, adding three drops of testing liquid, gently shaking, and using a color chart that should come with your kit to measure the plant’s level.

  • Digital pH meter

With a digital pH meter, testing is incredibly simple. You’ll calibrate the device, stick it into the fertilizer, runoff, and soil, and come away with an accurate reading instantly. While many old-school growers swear by drops and testing strips, a digital pH meter is a far more effective tool for commercial operators who want to take soil samples from multiple plants in different areas of a grow room quickly.

How to adjust pH for cannabis

If your reading reveals your plant’s pH levels are out of whack, it’s time to bring in some pH adjusters.

When the pH is out of range, nutrient lockout can occur,” said Kinneman. “This means that plants are not able to uptake nutrients from the growing medium. Having a pH that is out of range will cause leaves to start yellowing and show signs of deficiencies. If the pH is not corrected, necrosis and death of the plant tissue can occur.”

If your pH reading begins to stray too far from your target level, utilize one of the following methods to make the appropriate adjustment:

Natural methods

The most straightforward and simple way to adjust your soil’s pH is with the use of natural products. Effective mediums include manure, worm castings, compost, compost teas, lemon juice, vinegar, lime, limestone, pine needles, or wood shavings.

Natural methods like living soil tend to take longer to impact plants than products on the market designed to affect pH, so if your soil’s pH is wildly out of balance, you’ll want to purchase a pH down or up solution to quickly take care of the issue.

However, if you have more time to finesse the soil and prefer to take the natural route, these are great options to establish a healthy and lasting microbial life around the plant’s roots, which will ultimately encourage continued healthy growth as well as protection against pests and pathogens. 

pH down

You’re more likely to need to lower your pH than raise it. In order to adjust your pH to the acidic side, you can purchase a pH-down solution from your regular nutrients supplier, local hydroponics store, and major gardening retailers.

pH up

In the rare instance that you need to raise your pH levels, you can source some pH-up solution, which does the opposite of what pH-down solution does.

Advertisement