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What is pH and How To Adjust It

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Added 06 October 2020

Measure And Adjust pH For Healthy Cannabis Plants

If you've ever grown plants before you will probably have come across the term pH. The measurement of pH is important in many scientific and biological practices, including the cultivation of cannabis. Understanding and maintaining pH can help any farmer optimize the efficiency of their grow operation to produce even better crops.

Many of the root problems or deficiencies associated with cannabis have to do with fluctuations in pH. Fortunately, pH is easily corrected with some affordable techniques that are easy to perform. This article contains everything you need to know to keep your plants in the perfect pH range.

What is pH?

The pH, or 'potential of hydrogen' is the scale we use to measure the acidity or alkalinity (basicity) of a water based solution. Measuring the pH essentially tells us the concentration of hydrogen ions present. The more acidic a solution is, the lower the pH level. The more alkaline a solution is, the higher the pH level. A solution with a pH of 6 is ten times less acidic than one with a pH of 5.

The pH Scale

A pH scale ranges from 0 (acidic) to 14 (basic), with 7 being neutral. Pure water has a pH of 7 (most tap water), which means it is neither acid nor alkaline. The interesting thing about this is that temperature changes the neutral point of a solution. So if the temperature is higher than 25°C (room temperature) then pH neutral is lower than 7.

Super strong acids can extend below pH 0 and super strong bases can be higher than pH 14. For cannabis we only need to focus on a pH of around 4 - 8 since it is rare that tap water falls out of this range.

Soil Condition pH Effect
Ultra Acidic < 3.5 Toxic/Root Burn
Extremely Acidic 3.5 - 4.4 Nutrient Lock Out
Very strongly Acidic 4.5 - 5.0 Poor Nutrient Uptake
Strongly Acidic 5.1 - 5.5 OK
Moderately Acidic 5.6 - 6.0 Acceptable 
Slightly Acidic 6.1 - 6.5 Optimal ph Level
Neutral 6.6 - 7.2 Acceptable
Slightly Alkaline 7.3 - 7.8 Too basic
Moderately Alkaline 7.9 - 8.4 Poor Nutrient Uptake
Strongly Alkaline 8.5 - 9.0 Root Burn
Very strongly Alkaline > 9.0 Toxic

Why is pH Important?

For most plants, the correct pH allows roots to make the most effective use of the available nutrients dissolved in the water. Roots begin to lose efficiency when the pH is outside the desired level, leading to deficiencies, toxicities and an overall decline in plant health. 

pH Levels For Proper Nutrient Absorption In Soil And Hydroponics

Whether you grow cannabis in soil or hydroponics, pH needs to be consciously maintained for proper growth and yields. Fluctuations in pH are actually beneficial for cannabis plants because of how minerals are absorbed at certain levels.

PH is also important to consider because higher concentrations of some nutrients are required at different stages of growth. In soil, cannabis plants uptake nutrients best at a pH range of 6 - 7. In hydroponics, the ideal pH level is between 5.5 - 6.5. Either way, both methods benefit from a slightly acidic solution.

Measuring pH level

Best pH For Soil And Hydroponics

Digital pH meters can be purchased from most grow shops at an affordable price which make measuring the pH extremely easy. Ph drop kits can be used but are not usually as accurate as a meter. Taking a pH reading is slightly different in soil to hydroponics, so let's discuss how you would measure each:


Taking a pH reading of soil can be achieved by watering with pH neutral water and then using a digital meter to measure the runoff. If the pH of the runoff is lower than when you watered, a slightly more alkaline watering may be necessary to achieve the right level. For example, if the runoff measures 6 you can water with a less acidic solution with a pH of 6.5 which will adjust the soil pH to around 6.2 - 6.3.


In hydroponics, the way to measure pH is easier than it is in soil. Growers can get a much more accurate reading when the water is measured directly. However, everything happens faster in hydroponics, meaning roots are affected quicker by improper pH. Depending on the hydroponics setup, a pH reading can be taken straight from the reservoir, preferably at least once per day. Alternatively, some water can be drained from the res and measured in a bucket before adjusting.

Tip: pH meters need to be calibrated every so often to ensure the reading is as accurate as possible.

Healthy Special Kush #1 grow by EnergyAlchemist from GrowDiaries

How to Adjust pH level

When the pH falls out of the correct range, it is very important that it is fixed. PH adjusters are used to increase or decrease the pH level of water or a solution. These acids and bases can be bought in almost all agriculture stores and are very useful for any grow room. PH adjusters come in two separate bottles, labelled as pH up (+) or pH down (-). PH up makes a solution more basic and pH down makes a solution more acidic.

Because tap water normally has a high pH, most growers use pH down liquids to adjust. Adding nutrients to water first will bring down the pH, however further adjustments may need to be made. So, once nutrients have been added to your water, wait 10 - 15 minutes for the pH to stabilize. Ph up or down can then be used to further correct the pH.

Digital pH Meter

Here's an example of how one would adjust the pH of a fresh watering solution:

  1. Fill a bucket with 10 gallons of tap water
  2. Let water sit for 24 hours to allow chlorine to evaporate
  3. Add nutrients at half strength
  4. Wait 10 - 15 minutes
  5. Check pH level using digital pH meter
  6. Adjust using 1ml of pH up/down per gallon of solution*
  7. Wait 10 minutes and check/adjust again.

*Only tiny amounts of pH adjuster are needed to change the pH of water or a solution. Again, this depends on the quality of the tap water you are using. Hard water needs larger doses of pH up or down than soft water does to get the pH to the correct level.

pH Up And Down Products Can Be Used To Raise Or Lower pH

Problems Associated With pH

Let's go over some of the difficulties you might face when dealing with pH levels. Fortunately most pH problems can be corrected quickly if they are not left unattended for too long. In this sense, soil is much more forgiving than hydro is.

  • PH Adjusters Are Corrosive - pH up and down products are harmful if they come into contact with skin. Just in case, wear goggles and gloves to protect your hands and eyes. Spills should be cleaned immediately with water.
  • Grow Medium - Depending on which medium you are growing in, pH will be more or less of a problem. In soil, pH fluctuations happen slower than they do in hydro so it is easier to maintain but less accurate. Coco or soilless mediums need to be checked more frequently as they are susceptible to faster changes.
  • Nutrient Deficiencies - The most common issue with bad pH is failed uptake of nutrient due to lock out. Roots cannot function properly when the solution is too acidic, and vice versa.
  • Nutrient Burn - Keeping the right pH is great, but if plants absorb too many nutrients they will get 'burnt'. This is also why a variety of pH levels can work well; so plants don't focus on the uptake of only a few nutrients and receive a balanced diet.

Tip: If you are maintaining the correct pH but are noticing other issues with your plants, check the environment. There are other factors which affect nutrient absorption and pH level such as temperature.


Keeping the right pH for your cannabis plants takes a bit of practice in the beginning. Just make sure you check the pH daily, make any adjustments and see how your plants respond.

Once you know the quality of your tap water and how nutrients change its pH, watering and feeding will become much easier.

Checking and adjusting regularly leaves less risk of mineral problems and gives your plants a higher potential to produce to the best quality they can. So, stay consistent within the proper ranges for soil and hydro and you should not have any problems with pH.

External References

International pH Scales and Certification of pH. Analytical Chemistry. - Kristensen, Hans & Salomon, Arne & Kokholm, Gert. (2008)

Automated Monitoring And Controlling pH Levels For Hydroponics Cultivation Technique. Indonesian Journal of Electrical Engineering and Computer Science. - Saaid, Mohammad & Yassin, Ahmad & Tahir, Noorita. (2020).

This article was updated September 2020


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