Cannabis Science


How pH Affects Cannabis Plant Growth

If you talk to any seasoned grower about the importance of pH when growing marijuana, you will probably be directed to pull up a chair and have a seat. PH influences just about everything when it comes to growing cannabis.

From soil biology to chemical processes and physical properties, pH affects cannabis plant growth. And ultimately, the result in terms of quality and yield.

There are many factors to consider when growing a sumptuous cannabis cultivar. Decisions on growing mediums, environmental controls, and nutrient formulas form a plan but learning how pH plays a role in these choices elevates a grower’s success. Read on to find out why pH is so important and how it affects cannabis plant growth.

How pH is Measured

Simply put, pH is a measurement of how many hydrogen ions are present in a solution. Let’s delve into chemistry for a moment. Ions form when the number of protons in an atom is not equal to the number of electrons. With more protons present, the ion is positively charged, known as a cation. If an ion has more electrons, it is negatively charged and is called an anion.

Ions are busy little transporters, slewing electrons and protons here and there as atoms form. They are attracted to opposite charges but repel like ones. Hydrogen ions present a significant role in aqueous solutions, especially when nutrients enter the mix. Nutrient atoms also have electrical charges. When mixed with water, hydrogen ions go into overdrive.

Hydrogen ion concentrations determine whether a solution is acidic or basic, and pH is the measurement of the acidity or basicity of a solution. The scale ranges from 1 – 14, with seven being neutral. Acidic conditions rise as the scale trends downward, whereas basic, or alkaline, situations increase in number on the scale.

Furthermore, pH measurements are logarithmic, meaning each number increases ten-fold. For instance, a pH of 4 is ten times more acidic than a reading of five – there is a wide range between 4.1 and 4.9. This concept is imperative to realize as pH can vary widely with nutrient inputs.

Why is pH so Important to Cannabis Plant Growth?

From the substrate a cannabis plant is grown into the water and nutrients applied, pH affects how the plant will grow. Additionally, enzymatic activity, cellular mobility, and other biological actions depend upon proper pH balancing. Learning to find that sweet spot for optimal plant growth is challenging because conditions change quickly with pH imbalances.

Cannabis plants grow best in slightly acidic soil with a pH between 6.0 -7.0. Hydroponic growers, on the other hand, need to drop the range to 5.5 – 6.5. These parameters define the ability of marijuana plants to uptake soluble nutrients.
Solubility, or the ability to dissolve chemical compounds, determines pH.

As seen on the nutrient uptake chart, the big macronutrients, Nitrogen, Potassium, and Phosphorus have a wide berth on the pH scale. Looking at the microelements and some trace minerals toward the bottom of the chart reveals different circumstances.

While iron and manganese are essential elements in cannabis plant development, they become toxic to plants if pH levels drop below 5.0 as they become more soluble. Molybdenum and silica, on the other side, are taken up more readily in alkaline conditions. As you can tell, there is a lot more going on than just feeding a marijuana plant a good diet.

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How pH Affects Nutrient Uptake

A cannabis plant receiving proper nutrition may start looking forlorn, showing signs of sickly, discoloured leaves and slow growth. Even though it is receiving a proper diet, if the pH is not within the appropriate range, the marijuana plant cannot absorb the necessary nutrients.

Moreover, if the pH is out of whack, elements become insoluble and precipitate out. The result is the formation of solids in the root zone, which the plant cannot take in. Too much build-up, especially with chemical fertilizers, overloads the plant’s root system with excess salt accumulation.

Nutrient lockout shows up quickly in cannabis plants. One day everything is fine, and the next, droopy or discoloured leaves may appear. Over time, the plant will show signs of stunted growth with immature formation. Keeping pH levels within optimal parameters balances nutrient uptake allowing the plant to flourish.

Understanding Substrate pH

Marijuana plants grown in organic-rich soils are not as fussy about pH. Microorganisms working below ground are busy breaking down larger molecules into soluble nutrients the plants can easily absorb. Microbial activity is influenced by pH in the soil, performing well in the same range as plants. However, overly acidic or alkaline soils reduce their ability to work synergistically with the plant.

Enzymatic activity is also reduced in extreme situations, preventing the formulation of essential compounds. Some microbes adapt to overly acidic or alkali changes within the soil, such as fungi thriving in acidic conditions. Rogue bacteria, detrimental to marijuana plant growth, is often found in these extreme conditions. Maintaining optimal levels of 6.0 – 7.0 keeps everything under the soil, moving in a positive direction.

Hydroponic and soilless cannabis growers, on the other hand, have to be on top of the game, monitoring pH levels to ensure plant growth success. Dialling in pH can get a little tricky when nutrients are added to a solution as hydrogen ions come into play.

Nutrient compounds also have unique charges contributing to the mix. For example, nitrogen applied to a cannabis plant as the nitrate form (NO₃)⁻  holds a negative charge. On the flip side, the ammonium formula (NH₄)⁺ is positive. When nitrate is applied, plants release ions to maintain electrical neutrality, increasing pH. Conversely, ions released to balance NH₄ will decrease pH.

That is just nitrogen; imagine the effect on a mixture of various elements formulated in a nutrient recipe. This combination is an ionic exchange in hyper-mode. Naturally, this concoction affects pH depending on the inputs and their chemical composition.

Keeping pH in an Optimal Range

Commercial nutrient products contain different sources of chemical compounds in their formulas. Nitrogen, once again, is a good example. Some products may have nitrate, ammonium, or urea forms, and as we have learned, there are differences.

Just when a cannabis grower figures out the beneficial pH range in the vegetative state, nutrient requirements change as the cannabis plant matures into the flowering phase. Keeping a close eye on growth changes, knowing what compounds are in the product, and documenting pH levels help narrow down optimal growth parameters.

While cannabis plants prefer a range between 6.0 and 7.0 in soil and 5.5 – 6.0 in hydroponic or soilless systems, each unique cannabis strain presents an individual pH sweet spot. A thick-leafed indica cultivar may prefer 6.6 – 7.1 in soil whereas, a lanky Sativa dominant variety in hydro performs better in a more acidic range of 5.3 – 5.7. Finding the optimal range takes some time and variation but dialling in pH results in healthy plant growth.

pH affects cannabis plant growth

Taking pH Measurements

While there are various methods a cannabis grower can use to read pH, by far, the money spent on a digital pH meter is well worth it. They provide accurate readings down to tenths, which helps a grower determine incremental changes if necessary. As long as they are routinely calibrated and not subject to random crashes on the floor, pH meters are simple and easy to use.

Other options include pH strips and drops that turn colour when dipped into a solution. The disadvantage of this situation is that everyone interprets colour differently. What may look green to one person may be blue to another. However, long before digital meters were even dreamt of, old-school marijuana growers could pinpoint a solution’s pH using strips and drops. Talk to one of them today, and they’ll tell you ‘drops never lie; equipment can malfunction.

Reading pH Data

One of the best ways to figure out how a marijuana plant responds to a nutrient formula is through pH measurements. To do this, first record the pH level of the aqueous solution going into the plant. After feeding, measure the pH run-off coming from the bottom of the pot using a catch basin.

The results from a pH happy plant should show slightly elevated readings. This data means the plant is taking up nutrients in a pleasing manner, leaving a small reserve. On the other hand, if measurements are substantially higher or lower, adjustments need to be made quickly. The cannabis plant is overloaded with nutrient compounds, unable to uptake them up through the root system.

Flushing the marijuana plant with water until pH levels are within range helps rid the plant of excess salt build-up. Keep taking daily pH readings throughout the flushing process to see when to begin feeding again. You don’t want to flush all the nutrients out of the plant. Adjusting nutrient recipes will help the cannabis plant get back on track, but diligent pH monitoring during the plant’s whole lifecycle elevates its success.

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There is an art to balancing pH in cannabis plants. Sometimes it is uneventful, and the plant will thrive. Other times, it can be a constant battle trying to figure out what the poor, sickly thing is trying to tell you. However, it is usually the hard-to-grow plants that teach the most valuable lessons. And then, you apply those learning techniques to the next marijuana plant you cultivate.

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Post author
Charle Thibodeau
Charle’ Thibodeau is a freelance writer with almost a decade´s experience, specializing in cannabis content for the past two years. A strong motivation to educate, inform, and promote the culture surrounding this miraculous plant is her earnest mission.
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