The pH is the measure of the acidity or alkalinity of a solution or substrate. A low pH shows that the solution is acidic and a high pH that it is base/alkaline. The middle value of approx. pH 6.8 is the neutral range. The pH is absolutely vital for all living organisms in various ways.
In the world of plants each plant species is perfectly adapted to a certain pH range of the soil or water. Hobby gardeners are usually familiar with the different pH requirements of various plants like rhododendron, roses, or conifers, and buy special soil mixes and regulate the pH of water appropriately.
Cannabis requires a pH range around the neutral value of 6.0-6.5 so that it can assimilate nutrients from the growing medium. In any cultivation the pH of the water must therefore be monitored and adjusted appropriately. A full point difference in pH represents a tenfold increase in either acidity or alkalinity. If you water on soil with pH 5.5 it is 10 times more acidic than pH 6.5! A pH below 6.0 can trigger a deficiency of calcium resulting in burnt root tips and black spots on leaves. A pH above 7.0 causes a deficiency in iron which results in chlorotic leaves and yellowing of veins. The assimilation of all major & secondary nutrients required by cannabis for healthy growth and flowering can be seriously affected by an incorrect pH. Most affected are phosphorous, potassium, magnesium, calcium, iron, and manganese. Read our Nutrient Problems Guide for more info on symptoms.
Photo: this is a standard colour chart for a liquid pH tester. The optimum pH range of water for cannabis cultivation in soil is a light green colour that indicates pH 6.2-6.5
Photo: electronic pH testers range from inexpensive, such as this model, to high-end. They are handy but can be less reliable than a liquid tester because you have to calibrate them regularly. For calibration you need to purchase calibration fluid for pH 4 and pH 7 and a simple thermostat.
Purchase your essential gardening kit.
SOIL: pH tester, EC meter, bottle of cheap vinegar (6° acidity), measurement cup for 10-100 ml
HYDROPONICS: pH tester, EC meter, bottle of cheap vinegar or pH UP & DOWN, measurement cup for 10-100 ml
Test the quality of your water by checking the pH and EC range. Take into account any unusual colour or smell of the water. Make a chemical analysis at a pharmacy if you think your well water or tap water is heavily contaminated.
Take appropriate action if the water quality is low: a high EC or very low/high pH are indicators (i.e. pH below 6 or above 8). A household osmosis filter is the cheapest long-term solution to improving water quality if you have no other clean water source.
Measure the volume of vinegar* required for a fixed amount of water to adjust the pH down to 6.2-6.5. Add the required amount of vinegar each time you need to water and check the pH before watering.
If the water is too acidic (i.e. below pH 6.0) you should mix or replace it with another water source that has a higher pH. This could be bottled mineral water or rain water.
If you are using tap water let it sit in a bucket for a few hours so that the chlorine evaporates.
*Vinegar is a neutral acid that is completely harmless to plants when diluted in water. It should be used to adjust the pH of the water for soil cultivation to prevent overfertilizing of plants. The commercial pH DOWN products all contain potent nitric or phosphoric acid that raise the salt level in soil and can burn your plants. Especially seedlings and young plants easily suffer from regular watering with pH Down.
Add the required nutrients to your water until you have the appropriate EC (nutrient concentration) for your plants.
Measure the pH of the nutrient solution.
If the pH is still too high add vinegar* or pH DOWN until the solution has the correct pH.
If the pH is too low add pH UP until the solution has the correct pH.
When you are using tap water let it sit in a container for a few hours so that the chlorine evaporates.
*Vinegar is a neutral acid that is completely harmless to plants when diluted in water. It can be used for Hydrofarms with hydro correls and on rockwool. Not all growing mediums may be suitable...run a test if you are using coco coir or other substrates.
Regularly check the pH and EC of your water if you are using tap water because there are sometimes larger fluctuations in water quality from municipal companies.
“Incorrect pH belongs to the most serious nutrient disorders in organic-soil gardens. Many complex biological processes occur between organic fertilizers and the soil during nutrient uptake. The pH is critical to the livelihood of these activities.” (Marijuana Horticulture, Jorge Cervantes)
Many plant problems that are attributed by the grower to lack of fertilizer or poor genetics are in actual fact caused by the wrong pH of the growing medium or water (most often of water).
Failure to adjust the pH to the desirable range will result in several negative symptoms, that will range from mild to chronic, depending on the severity and duration of the pH imbalance:
Fertilizing a plant that is suffering from a pH imbalance usually increases the cycle of problems. It may show a brief respite to symptoms, but only because the fertilizer added to the water may have changed the pH favourably for a short time. Without paying attention to the actual problem and adjusting the pH to the correct range your plants will continue to suffer and you will lose yield on a daily basis.
Due to the constant availability of nutrients in a solvent form in hydroponics there is a greater range of tolerance in pH fluctuation. Cannabis grows well hydroponically within a range of 5.5-6.5. Usually the pH is regulated to 5.8-6.0 for hydroponic systems with a growing medium that has been stabilized.
The ideal pH and pH fluctuation in hydroponics depends on several factors that you have to evaluate on an individual basis because each hydroponic system is different due to the following:
All of these factors influence how the pH should be adjusted and how it changes in the containers or tank over a period of time. A fluctuation of one full point in hydroponics can usually be tolerated by cannabis as long as the pH is stabilized to the ideal range within 24 hrs. For best results the pH should therefore be monitored daily in a system with large fluctuation.
Ideally your water quality should be good enough so that no or only minimal adjustment to the pH is required for a fresh nutrient solution.
Purchasing the correct fertilizer for your water quality helps in stabilizing the pH in your system. Several companies offer hydroponic fertilizer for either "hard" or "soft" water.
Photo: a standard pH kit for hydroponics includes a liquid pH tester, pH UP (potassium) and pH DOWN (nitric or phosphoric acid) for adjusting the pH range. Alternatively, vinegar can be used on some growing mediums to reduce pH if mineral salts are undesirable due to their effect on the EC of the nutrient solution.
Tap water and well water are two main sources that need to be checked for quality. Both can be contaminated with toxic levels of minerals. High levels of sodium (Na) are often found in well water and can cause excessive damage to plants. Saline water on the whole must be avoided.
Photo: yellowing, leaf curl, circular burnt spots on leaves, and leaf drop are typical symptoms from water with a high level of sodium.
Tap water can be “hard” from high levels of calcium (Ca) and magnesium (Mg). The pH is in this case very alkaline. Chlorine is another common additive which causes stunted growth in plants and acidifies the soil. If your tap water smells from chlorine you should fill warm water into a bucket and let it sit for a few hours so that the chlorine can evaporate.
For a general assessment of your water quality test the pH and EC range with your essential kit. Also look at the colour of the water and if it has any strong smell. If you notice anything out of the ordinary you can give a 1L sample to the pharmacy for a chemical analysis. The pharmacy sells sterile bottles for this purpose. The analysis usually costs 50-60$/Euro and provides details of common harmful contaminants. This is especially recommended for testing well water. It helps to say that you need an analysis for drinking use and watering plants so that specific contaminants are tested for. If you need to install an osmosis filter for heavily polluted water you will need this analysis to purchase the correct osmosis system and filters.
Your municipal water board can provide a free chemical analysis of the tap water in your neighbourhood if you request it because they regularly perform these tests as a standard procedure. Usually this is not necessary for you to look into unless the water is very poor quality or running through old pipes that pose a health hazard.
Photo: a typical household osmosis filter can be attached to any water faucet in your home or garden. The best buy is a 3-chamber system which contains three filters that can be replaced at relatively low cost.
Usually a household osmosis filter is sufficient to clean water from common impurities such as calcium, magnesium, and low levels of salts. The cost of 100,-$/Euro is worth the investment and cheaper than buying bottled mineral water. An osmosis filter system can last a lifetime and you only need to exchange the filters every once in a while. An osmosis filter is essential if your water has a high EC . Generally an EC above 0.7 mS/cm (or 700 µS/cm) is problematic, especially if you need to fertilize indoor or have a hydroponic system. Mixing your water approx. 50-50 with pure osmosis water solves this problem.
For germination and seedlings you should always use high quality water: “soft” mineral water with pH 6.5 and low levels of sodium is best. The pH should be adjusted to the ideal range with vinegar so that there are no salts which can inhibit germination or damage the seedlings. In hydroponics a very weak nutrient solution of EC 0.6 mS/cm (or 600 µS/cm) is usually used. For germination pure water is also sufficient until the seedling appears.
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An overview of all major nutrient problems in cannabis cultivation. Includes high quality photos.