The acidity or alkalinity of a liquid is measured as pH. A low pH indicates high acidity, while a high pH indicates high alkalinity.
More concretely, pH—short for “potential of hydrogen”—reflects the concentration of hydrogen ions. When acids dissolve in water, they release H+ ions, lowering the pH. Bases release OH– ions, raising the pH. The pH scale ranges from 0 to 14, with 7 being neutral (Figure 1).
Figure 1. pH scale. Acidic solutions have a low pH. Alkaline solutions have a high pH. Neutral pH is 7. [1]
Maintaining the proper pH is crucial in hydroponic systems, as it not only directly influences nutrient availability, but also impacts plant growth and productivity.
The acidity or alkalinity of a liquid is measured as pH. A low pH indicates high acidity, while a high pH indicates high alkalinity.
More concretely, pH—short for “potential of hydrogen”—reflects the concentration of hydrogen ions. When acids dissolve in water, they release H+ ions, lowering the pH. Bases release OH– ions, raising the pH. The pH scale ranges from 0 to 14, with 7 being neutral (Figure 1).
Maintaining the proper pH is crucial in hydroponic systems, as it not only directly influences nutrient availability, but also impacts plant growth and productivity.
Figure 1. pH scale. Acidic solutions have a low pH. Alkaline solutions have a high pH. Neutral pH is 7. [1]
Why pH matters
One of the most important reasons pH matters is that nutrient availability depends on it. If pH is too low or too high, certain nutrients may become unavailable, leading to deficiencies or toxicities (Figure 2).
Figure 2. Availability of essential nutrients at different pHs.
The pH should be kept slightly acidic, as calcium, iron, magnesium and manganese may precipitate in alkaline conditions, making them unavailable for absorption. Potassium and phosphorus also become less available when pH is too high.[2] Moreover, high sodium uptake, often linked to alkaline conditions, can contribute to leaf chlorosis and disrupt stomatal closure, leading to water loss.[3]
Figure 2. Availability of essential nutrients at different pHs.
The pH should be kept slightly acidic, as calcium, iron, magnesium and manganese may precipitate in alkaline conditions, making them unavailable for absorption. Potassium and phosphorus also become less available when pH is too high.[2] Moreover, high sodium uptake, often linked to alkaline conditions, can contribute to leaf chlorosis and disrupt stomatal closure, leading to water loss.[3]
However, if pH is too acidic, aluminum, manganese and other minerals become more soluble. In high concentrations, they can result in toxicities.[4]
In addition to nutrient availability, pH also impacts:
- The rhizosphere: Plant roots release anions and cations to absorb nutrients. Unlike soil, hydroponic growing media lack buffering capacity, often leading to imbalanced ion exchange and pH fluctuations.[5] If the pH of the rhizosphere drops too low, plants become more susceptible to abiotic stress—either directly from root injury due to acidity or indirectly from restricted phosphorus availability.[6]
- Beneficial microbes: An alkaline pH disrupts plant-growth promoting rhizobacteria and other beneficial microorganisms by breaking molecular bonds and allowing for lipid hydrolysis, or breakdown of lipids.[7] Conversely, an acidic pH can impair microbial processes such as nutrient cycling and organic matter decomposition.[8]
- Germination: Research shows pH significantly influences germination rates. One study found that a pH of 5.0–7.0 resulted in higher germination rates compared to more acidic or alkaline conditions (Figure 3).
Figure 3. Effect of pH on germination of A. artemisiifolia. Image source: Gentili, Rodolfo, Roberto Ambrosini, Chiara Montagnani, Sarah Caronni, and Sandra Citterio. 2018. “Effect of Soil pH on the Growth, Reproductive Investment and Pollen Allergenicity of Ambrosia artemisiifolia L.” Frontiers in Plant Science 9:1335. https://doi.org/10.3389/fpls.2018.01335.
Figure 3. Effect of pH on germination of A. artemisiifolia. Image source: Gentili, Rodolfo, Roberto Ambrosini, Chiara Montagnani, Sarah Caronni, and Sandra Citterio. 2018. “Effect of Soil pH on the Growth, Reproductive Investment and Pollen Allergenicity of Ambrosia artemisiifolia L.” Frontiers in Plant Science 9:1335. https://doi.org/10.3389/fpls.2018.01335.
- Plant messaging and signaling: pH functions as both a signal and a messenger, helping regulate responses to hypoxia, harmful microorganisms, drought and changes in light intensity. It also links environmental cues to physiological changes, such as stomatal movements.[9]
- Plant characteristics: pH shapes various plant characteristics, from vegetative growth to flowering, including height, lateral spread, biomass, pollen production and flower size and number (Figure 4).
Figure 4. Generalized logistic growth curves of vegetative traits: plant height (A), lateral spread (B), leaf length (C) and leaf width (D) of A. artemisiifolia at different pH levels (pH 5, pH 6 and pH 7, displayed by different colors) at 25°C. Image source: Gentili, Rodolfo, Roberto Ambrosini, Chiara Montagnani, Sarah Caronni, and Sandra Citterio. 2018. “Effect of Soil pH on the Growth, Reproductive Investment and Pollen Allergenicity of Ambrosia artemisiifolia L.” Frontiers in Plant Science 9:1335. https://doi.org/10.3389/fpls.2018.01335.
Figure 4. Generalized logistic growth curves of vegetative traits: plant height (A), lateral spread (B), leaf length (C) and leaf width (D) of A. artemisiifolia at different pH levels (pH 5, pH 6 and pH 7, displayed by different colors) at 25°C. Image source: Gentili, Rodolfo, Roberto Ambrosini, Chiara Montagnani, Sarah Caronni, and Sandra Citterio. 2018. “Effect of Soil pH on the Growth, Reproductive Investment and Pollen Allergenicity of Ambrosia artemisiifolia L.” Frontiers in Plant Science 9:1335. https://doi.org/10.3389/fpls.2018.01335.
Maintaining the ideal pH for cannabis
While the ideal pH range for the root zone of most hydroponically grown crops is 5.5–6.5,[10] the optimal pH range, or sweet spot, for cannabis is slightly narrower at 5.8–6.3. This range ensures maximum nutrient availability while minimizing the risk of deficiencies or toxicities (Figure 5).
The exact pH should be adjusted to the plant’s growth stage. Seedlings require a slightly higher pH (around 6.0) to support root development, while mature plants benefit from a slightly lower pH (around 5.8) for optimized nutrient uptake.
Figure 5. The ideal pH range for hydroponically cultivated cannabis.
Maintaining the ideal pH requires regular monitoring and adjustments, as needed:
- Using a pH test kit or meter, routinely check the pH of the growing medium, irrigation water, nutrient solution and, if using a recirculating system, the runoff.
- Ideally, the pH should be tested daily at the same time to track fluctuations.[11]
- Before adding nutrients, the irrigation water should be tested to ensure its pH is within 6.0–7.0, as highly alkaline water can resist pH adjustments.
Figure 5. The ideal pH range for hydroponically cultivated cannabis.
Maintaining the ideal pH requires regular monitoring and adjustments, as needed:
- Using a pH test kit or meter, routinely check the pH of the growing medium, irrigation water, nutrient solution and, if using a recirculating system, the runoff.
- Ideally, the pH should be tested daily at the same time to track fluctuations.[11]
- Before adding nutrients, the irrigation water should be tested to ensure its pH is within 6.0–7.0, as highly alkaline water can resist pH adjustments.
If the pH strays outside the ideal range, a pH-lowering or pH-raising solution can be used:
- Lower pH: Add an acid, such as phosphoric acid or citric acid.
- Raise pH: Add a base, such as potassium hydroxide or calcium carbonate.
Adjustments should be made gradually in small increments, as rapid changes can harm root systems and destabilize nutrient availability. Emerald Harvest’s pH Up and pH Down provide an easy-to-use solution—simply add the required amount and test periodically until the desired pH is reached.
If pH fluctuates too frequently, the growing medium may have a low buffering capacity. In this case, a buffering agent can help stabilize pH levels.
Emerald Harvest Team
[1] brgfx, Freepik. 2021. “The pH Scale diagram on white background.” Freepik. https://www.freepik.com/free-vector/ph-scale-diagram-white-background_17563930.htm#fromView=search&page=1&position=3&uuid=b29a16a5-bb0f-4bd4-a8a5-69371dc9e44f&query=%40brgfx+ph
[2] Singh, Hardeep, Bruce Dunn, and Mark Payton. 2019. “Hydroponic pH Modifiers Affect Plant Growth and Nutrient Content in Leafy Greens.” Journal of Horticultural Research 27 (1): 31-36. https://doi.org/10.2478/johr-2019-0004.
[3] Msimbira, Levini A., and Donald L. Smith. 2020. “The Roles of Plant Growth Promoting Microbes in Enhancing Plant Tolerance to Acidity and Alkalinity Stress.” Frontiers in Sustainable Food Systems 4: 106. https://doi.org/10.3389/fsusfs.2020.00106.
[4] Foy, Charles D. 1984. “Physiological Effects of Hydrogen, Aluminum, and Manganese Toxicities in Acid Soil.” In Soil Acidity and Liming Volume 12, Second Edition, edited by Fred Adams. American Society of Agronomy, Inc., Crop Science Society of America, Inc., and Soil Science Society of America, Inc. https://doi.org/10.2134/agronmonogr12.2ed.c2.
[5] Fathidarehnijeh, Elham, Muhammed Nadeem, Mumtaz Cheema, Raymond Thomas, Mano Krishnapillai, and Lakshman Galagedara. 2023. “Current Perspective on Nutrient Solution Management Strategies to Improve the Nutrient and Water Use Efficiency in Hydroponic Systems.” Canadian Journal of Plant Science 104 (2): 88-102. https://doi.org/10.1139/cjps-2023-0034.
[6] Alexopoulos, Alexios A., Efstathios Marandos, Anna Assimakopoulou, Nikolina Vidalis, Spyridon A. Petropoulos, and Ioannis C. Karapanos. 2021. “Effect of Nutrient Solution pH on the Growth, Yield and Quality of Taraxacum officinale and Reichardia picroides in a Floating Hydroponic System.” Agronomy 11 (6): 1118. https://doi.org/10.3390/agronomy11061118.
[7] Msimbira, Levini A., and Donald L. Smith. 2020. “The Roles of Plant Growth Promoting Microbes in Enhancing Plant Tolerance to Acidity and Alkalinity Stress.” Frontiers in Sustainable Food Systems 4: 106. https://doi.org/10.3389/fsusfs.2020.00106.
[8] Gazey, Chris, and Gaus Azam. 2018. “Effects of Soil Acidity.” Last updated September 17, at 3:27 (UTC). https://www.agric.wa.gov.au/soil-acidity/effects-soil-acidity.
[9] Felle, H.H. 2001. “pH: Signal and Messenger in Plant Cells.” Plant Biology 3 (6): 577-591. https://doi.org/10.1055/s-2001-19372.
[10] Alexopoulos, Alexios A., Efstathios Marandos, Anna Assimakopoulou, Nikolina Vidalis, Spyridon A. Petropoulos, and Ioannis C. Karapanos. 2021. “Effect of Nutrient Solution pH on the Growth, Yield and Quality of Taraxacum officinale and Reichardia picroides in a Floating Hydroponic System.” Agronomy 11 (6): 1118. https://doi.org/10.3390/agronomy11061118.
[11] Dunn, Bruce, and Hardeep Singh. 2017. “Electrical Conductivity and pH Guide for Hydroponics.” Published in April. https://extension.okstate.edu/fact-sheets/electrical-conductivity-and-ph-guide-for-hydroponics.html.
