pH is a numerical scale used to test the acidity or alkalinity of a given substance. The scale ranges from numbers 1 through 14. A pH of 7.0 is totally neutral, 1.0 to 6.9 is acidic and 7.1 to 14.0 is alkaline. For several reasons the pH has a profound effect on the cannabis plants. As also happens in humans, plants have pH fluctuations in their metabolism, which mainly depend on the pH range and stability. Thus, cannabis plants have a better or worse development and overall health state depending on the pH, being more or less sensitive to pests and diseases.
The pH in the growing medium of any plant has a massive effect on the availability of the nutrients in the medium at the plant’s roots. Marijuana thrives best in a slightly acidic growing medium. The reason for this is to do with the natural environments where marijuana first evolved. In fact, with a few exceptions, most plants prefer a slightly acidic pH.
The optimum pH for marijuana in soil is around 6.0 – 6.8
The optimum pH for marijuana in soilless or hydro is around 5.5 – 6.5
In this range the plants nutrients are water soluble. This means that the nutrients are available to the marijuana plants. When the pH level is outside of this range, the nutrients become less available because the nutrients won’t dissolve as well.
We have to adjust the pH of the nutrient solution in order to achieve the best possible nutrient uptake by the leafs. If we don’t adjust the pH value, fertilizing our plants is almost useless since they can’t absorb nustrients correctly. This means that the plant may begin to show symptoms of deficiency in a particular nutrient, even though that nutrient may be physically present at the plant’s roots. If you are unable to monitor and understand the pH of your growing medium you can end up in the position (and it happens A LOT) where you are feeding a plant more and more nutrients to fix a deficiency and the plant simply cannot take them on board. If unchecked this situation can lead to a build up of salts in the growing medium which block up the plant’s roots. This is nutrient lock out. Your plants can literally die of starvation despite how much you are feeding them.
pH in Soil
Naturally, soil has a capacity to control and mantain the pH level, which is called buffer effect. This capacity helps to keep the rootball healthy and protected thanks to a pH range suitable for the growth of cannabis plants. In nature, when it rains the soil becomes more acidic, freeing up some of the nutrients that were previously unavailable. The plant gradually sucks up these nutrients, wicking the moisture out of the soil and consequently raising the pH. During this time the plant has access to a whole range of nutrients and minerals that were locked up in the soil. The same thing happens when you grow marijuana in soil in containers. When you add water the pH of the soil changes and the whole range of nutrients become available to your plant as the soil slowly dries out again. This means that some nutrients that are otherwise dormant in the soil become available when it is wet.
Water your soil grown plants with clean, uncontaminated water with a pH of around 6.0 to 6.8. If you are using tapwater, let it sit in a bucket or reservoir for a few days to allow it to dechlorinate, then check the pH to make sure it is within the required range.
pH in Hydro
In hydroponic crops, the pH value is as important as the amount of nutrients given to the plant. The pH value must be adjusted for each stage of the plant so it can properly absorb nutrients in every irrigation. During the growth stage of seedlings and young cuttings a pH of 5.5 is recommended. We can raise this value up to 5.8 as the plants grow. During the stretch (pre-flowering stage) the plants start to demand more and different nutrients; we can raise to 5.9 now. In this way, plants have all the necessary nutrients to grow and start flowering without a problem. During the bloom period we should use a pH range from 6 to 6.2 for a more efficient nutrient uptake. During the last weeks of this stage, we can raise the pH value to 6.3-6.4.
When making up the nutrient solution for your hydro set up, always mix the separate components in the water. Never mix them together directly as this can cause them to chemically react with each other and may change their desired properties.
There is only one accurate way to adjust the pH. This is using pH test papers or a pH meter. When the pH level is outside the range, the nutrients are less available to the marijuana plant. This means that the roots don’t have access to them and now the plant indicates deficiencies, even though the nutrients may be present. If the pH level is outside the proper pH range, marijuana plants have small dark-green leafs and grow very slowly, when growing in water or soil. Check the pH (by using test strips or a pH meter) before you plant them in soil or planting mix.
Digital Meter – This is by far the easiest, most popular and most accurate method for checking your pH levels. Digital pH meters are easy to work, just insert the probe and read the pH levels off of the digital read out.
pH Strips – Cheaper to buy initially, but more expensive in the long run, and more hassle. pH strips turn a specific color depending on the pH. You then compare the color to an index and that gives you the pH value. If you are trying to measure the pH of your soil you will need to make up a soil solution in water.
You should measure pH periodically as part of your plant maintenance program. With experience you will need to measure less often as you get your set up dialled in. Special care should be taken to measure pH when you seriously change the nutrient regime you are following