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soil ph for cannabis

The perfect PH value for a cannabis plant

In the world of gardening, pH both affects and is affected by everything. Indeed, the entire process of growing plants is a study in the physical dance of pH balance.

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So, you are on your way to growing great cannabis. Your seeds have sprouted, and a small cannabis plant is now eagerly growing. You have spent good money on quality nutrients, and have made sure to properly water and feed your precious plant baby. But something is wrong; you notice your plant appears sick. The leaves are getting discoloured and growth has come to a standstill. Before you know it, your plant is withering away, and you’re stumped as to how this could’ve possibly happened.

Among fatal flaws like overwatering and overfeeding, pH imbalances are one of the most common issues in the cannabis garden. To understand why pH is so important, let us first understand the concept in and of itself.

WHAT IS PH?

pH is a measure of how acidic or alkaline something is. The pH scale ranges from 1–14, with a pH of 7 being neutral (the pH of pure water). If pH is lower than 7, a substance is considered acidic (think vinegar or lemon juice). If the pH is higher than 7, the substance is alkaline, as is the case with soaps, bleach, and ammonia.

In more scientific terms, pH level has to do with the concentration of hydrogen ions, say in the water you give to your plants. The pH scale is logarithmic to the base 10, which means that water with a pH of 6 is already 10x more acidic than water with a pH of 7.

WHY IS PH IMPORTANT WHEN GROWING CANNABIS?

As you will already know, all plants require nutrients for healthy growth. They require macronutrients nitrogen, phosphorus, and potassium, as well as micronutrients and minerals including calcium, magnesium, iron, copper, and a whole lot more. If plants cannot access these nutrients, it will lead to deficiencies and other serious health problems.

The issue with cannabis plants is that they are only able to take up nutrients within a small pH window, which ranges from about 6–7 when growing in soil. If the pH is lower or higher than that, the plant cannot take in nutrients, even if they are present—thus spurring nutrient deficiencies via “nutrient lockout”.

In those places where cannabis thrives in the wild, the soil is normally slightly acidic; therefore, homegrown cannabis plants will also prefer a slightly acidic environment. However, the way that you grow cannabis also plays a role in the optimal pH level for your plants. Cannabis grown hydroponically or without soil needs an even lower pH than a soil grow.

WHAT’S THE BEST PH FOR GROWING CANNABIS?

SOIL: 6.0–7.0 pH

If you grow in soil, the optimal pH level for the root zone is between 6.0 and 7.0. However, there is no set number within this range that is “best”. Instead, it can be good to allow for some natural fluctuation within this window to support optimal nutrient uptake. So as you adjust, try a slightly different reading each time. You can, for example, adjust your pH to 6.2 for one watering, then 6.6 the next. As long as it stays within 6.0–7.0, you should be fine. Soil is also more forgiving when it comes to pH imbalances, but it can only give so much.

If you grow purely organically—where you do not administer liquid nutrients—pH is less of an issue. If you’re using amended and composted soil with organic matter, the microorganisms within will make the nutrients more available to the roots. However, most growers using standard potting mixes and liquid nutrients will indeed have to reckon with pH.

HYDROPONICS AND SOILLESS: 5.5–6.5 pH

Hydro and soilless grows are a different beast when it comes to pH. If you grow soilless, say in coco, the optimal pH level at the root zone should be somewhat lower than in soil, between 5.5–6.5. The same goes for all methods of hydro.

With these methods, it is just as important that you allow the pH level to fluctuate across the acceptable range to support nutrient uptake. For example, in hydro, calcium and magnesium are mostly absorbed at pH levels above 6, while other nutrients like manganese prefer a slightly lower pH.

Then again, this shouldn’t be an issue since pH levels will naturally change slightly with each feeding in a hydroponic setup. You will only need to correct if the pH level exits the optimal 5.5–6.5 pH range.

When growing in coco, perlite, or hydroponically, you are in charge of administering nutrients directly to the root zone via the water, which means that huge pH fluctuations are more of a risk than in soil. The inert media used in hydro and soilless grows merely retains water and provides support for the roots of your plants. So when administering nutrients, be careful that you don’t overload your plants.

In the world of gardening, pH both affects and is affected by everything. Indeed, the entire process of growing plants is a study in the physical dance of pH balance.

Wasser und pH

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.

SOIL

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.

HYDROPONICS

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 unbalance:

  • single nutrient deficiency or multiple nutrient deficiencies causing any of the following: stunted growth, yellowing, dark blotches on leaves, small dark-blue leaves, contorted shoots, shriveled growth, leaf curl or burn, leaf drop, delayed flowering, low yield, etc.
  • higher ratio of males during sexing
  • appearance of male flowers on females
  • vulnerability to mold and fungus
  • vulnerability to pests

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:

  • water quality
  • growing medium (coco coir, rockwool, hydro correls, mixture of several mediums, other substrates, or mainly pure water such as aero-flow and bubbler)
  • nutrient products used and their buffer capacity
  • additional products or buffer agents used
  • type of watering system & watering schedule
  • EC of nutrient solution
  • size of nutrient solution tank or plant container
  • room temperature
  • size of plants and their nutrient uptake

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.

Water quality

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 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 is usually used. For germination pure water is also sufficient until the seedling appears.

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Wasser und pH 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