Professional Marijuana Grower


Marijuana pH and Water

Posted January 25th, 2017 by Robin Nichols in

For marijuana growers, whether using soil, soilless or hydro methods, pH levels can make a huge difference to your success and yields. In this article we will take a closer look at pH values and discuss why you need to know about them and how that knowledge can help you become a better grower.

What is pH?

So, let’s start at the beginning. pH is the measure of the acidity or alkalinity of a given substance. Technically it is about the concentration of hydrogen ions. A pH of 7.0 is totally neutral, 1.0 to 6.9 is acidic and 7.1 to 14.0 is alkaline. When using a pH scale it is important to know that the decimal points really count. The pH scale is a logarithmic scale which means that for every one point of pH, the concentration changes by a factor of ten. For example, an increase in pH from 7.0 to 8.0 is actually a tenfold increase, so be aware!

Why is pH Important in Growing marijuana?

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 ideal pH for growing marijuana should be considered as a range rather than a specific number. This is because different nutrients become available to the plant at slightly different pH levels. By having your pH sitting within a range rather than at a specific point, you make more nutrients available.

Take a look at the diagram above. The pH range is marked along the bottom. Just follow the lines up to see the uptake and availability of different nutrients at different pH values. Note that in soil there are much smoother lines. This shows how soils act as a buffer. There is more room for maneuver when growing in soils as opposed to hydroponics. If you are growing with properly amended or composted soils then there is a much lower chance of running into pH problems.

Optimum pH

The optimum pH for marijuana in soil is around 6.0 – 6.8 and the optimum pH for marijuana in soilless or hydro is around 5.5 – 6.5. If you grow in a soilless medium like coco, but with added organic matter (worm castings for example), you should consider aiming for a pH value somewhere between soil and hydro. Remember though, you are looking for a pH range rather than an actual number.

If the pH of your growing medium veers outside of these ranges, certain nutrients and trace elements will no longer be available to the plant. 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 absorb them. If unchecked this situation can lead to a build-up of salts in the growing medium which block 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

When growing marijuana in soil you are less likely to run into pH problems, especially if you are using specifically mixed soils that feed the plant throughout its life, without having to add any liquid nutrients. The soil acts as a buffer, meaning it helps slow the change of pH values as opposed to hydro systems where changes in pH take effect much more quickly.

It helps to look at how marijuana grows outdoors in its natural environment. The pH of rainwater is normally around 5.5 – 6.0. Of course, this can vary a lot from place to place, and even from time to time. 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.

The biggest concern is the pH of the root zone, as this is where all the nutrient action takes place. A more useful way to gauge what is actually going on in the soil is to test the pH of your run-off rather than the pH of the water you put in. The run-off will contain any salts that might be building up in the soil and alert you to possible problems before they occur.

Dolomite Lime & Water

The addition of Dolomite Lime to soils at about 1 – 2 tablespoons per gallon of soil is a good way to help control the pH of overly acidic soils. It is worth mentioning that using rainwater for growing marijuana is a controversial subject. It creates problems for some growers while other growers will use nothing else. pH values can vary wildly from place to place, and some locations have high levels of pollution. If you are going to use rainwater, be sure to measure its pH before you do. 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. It is always worth considering a reverse osmosis water filter (RO) to make sure your water is free from any contaminants. However, you should note that the almost pure water provided by RO machines has no natural pH buffer at all and its pH levels can swing up or down with ease.

pH in Hydro

If you grow marijuana in a hydroponics system then pH management is a much more important issue. Without soil to act as a buffer changes in your pH values take effect much more quickly. With hydro grows allowing your pH to fluctuate within a prescribed range is important. This allows all of the nutrients in the solution to become available to the plant in turn. Luckily, this happens naturally as the pH of the solution in your hydroponic reservoir will drift over time.

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. Mix the nutrients gently. Overly vigorous mixing adds oxygen to the solution and will temporarily raise pH levels. Some growers like to shake their solution vigorously to add oxygen which is good for the roots. If you like to do this, do it after you have checked and adjusted the pH.

Make your mix in a clean reservoir and check the pH. Let the mix stand for an hour and check the pH again. The pH of nutrient solutions often changes quickly within the first hour so you should recheck and adjust it as necessary. If you are topping up your solution, or adjusting the pH in the reservoir, try not to subject the plants to drastic changes in pH which will stress them. Make changes slowly. Do not mix nutrients or pH regulators directly into the plants’ reservoir. Make a mix in a separate container first then add that to your reservoir so that changes take place slowly. Remember that pH drift is not only normal, it’s desirable. Allow the pH in your reservoir to change gradually, but make sure you keep within the range of 5.5 to 6.5.

Checking pH

Although some soil growers don’t bother, some kind of pH tester is a crucial piece of the kit for the serious marijuana grower. There are a couple of options that are available to you.

Digital Meter

This is by far the easiest, most popular and most accurate method for checking 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

These are cheaper to buy initially, but are 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 dialed in. Special care should be taken to measure pH when you seriously change the nutrient regime you are following, when flipping to 12/12 for example.

Adjusting pH

The best way to adjust pH is to buy proprietary pH Up and pH Down solutions. There are lots of forum posts by people who add vinegar or baking powder to adjust pH. While there is some convincing evidence of this working, we recommend using proprietary solutions for reliable results. pH Up is a strong alkali formula for raising pH and pH Down is an acid based formula for lowering pH.

As explained above, adjust the pH of your solution a little at a time. Try to use only either Up or Down. If you overshoot with one and then have to readjust with the other you can end up unnecessarily stressing your plants. Mix a little of the required pH adjuster in a separate jug and add a little at a time to your reservoir. Allow time for the whole reservoir to even out and settle. It is better to get it right with 3 slight adjustments than have it wildly swinging up and down.


The pH levels at the root zone of your cannabis plant play a massive role in how well your marijuana takes on nutrients and minerals and how easily they are made available to it.

While pH levels are important to all marijuana growers, hydro growers need to be more in control of pH than soil growers.

Monitoring pH levels should become part of your regular plant maintenance routine.

It is easy to adjust pH levels with pH Up and pH Down formulas. All adjustments should be done slowly and gently to avoid large pH fluctuations which can stress your plant.

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