What is pH … and why is it important when you’re growing cannabis?


Lets keep this simple:

[pullquote right]pH is a numerical number given to how either acidic or alkaline a solution is. (in our case the feed/water  or indeed the medium we grow in) [/pullquote]


pH is measured on a scale of 1-14: 

  • 7 is neutral
  • 1 is the low end of the acidic scale
  • 14 is the top end of the alkaline





ph and cannabis grows








O.K. … whats that got to do with growing cannabis?

I’ve spent countless hours discussing with new growers about pH. I believe (and I’m not alone in this for once) that pH issues are the one of the biggest issues most growers face in the grow room.

pH imbalances can lead to

  • nutrient lock outs, which are generally misdiagnosed as nutrient deficiencies –
  • regrettably all  too often this leads to many growers pouring more nutrients into the medium.
  • As the issue isn’t lack of food but the plant’s inability to take up that food at the present pH (see below), this results in nutrient salt build ups which in turn can exacerbate the problem as the roots of the plant will then have trouble absorbing all water and nutrients.



This is not the best scenario for a bumper crop!

(I have even been guilty of this myself)

Put it like this:  every living organism relies on pH to be within a certain range to support life. Different organisms have different pH tolerances.  But why you are here reading this is because you want to know how this applies to your closet grow yeah ? Well its all about at what pH range marijuana takes up nutrients.

Below is a diagram showing what pH range you need to aim for hydroponically and what chemicals are taken up in which pH range … or more importantly, what nutrients AREN’T taken up at high / low levels of pH:

We all know how important Nitrogen, Phosphorus and Potassium are. They are the N:P:K you see listed on every grow and flower food you buy. That ratio is carefully designed to be specific to your plants needs depending on how old / what stage the plant is at.


ph and growing cannabis

But when you look at the diagram you will see that if your nutrient solution has a consistent pH of more than 6.2 your plants won’t get any potassium. If its above 7.0 they’ll start to lack phosphorus. Below 5.5 and they won’t get any nitrogen .. and that carefully designed (and expensive) NPK ratio is out of whack …

You have to think of it in this way once you flip the plant to flower then you’ve set a boulder rolling down the hill and its not going to stop until it reaches the bottom (harvest). Any problems which would lead the plant to not be growing at optimum levels (because of this predetermined time scale) is ultimately going to effect the amount of bud you are going to be able to harvest from the plant   … so it is critical when facing a problem with your plant that you diagnose it carefully and quickly to get your medium back on a even keel as soon as possible

and as always prevention is better than cure

What is the correct pH range for cannabis ?

This is going to depend on what medium you are growing with:

  • For example if you are doing a organic dirt grow the pH wants to be around 6.3
  • alternatively if you are growing hydroponically anywhere between 5.5 and 6.1
  • Personally my favorite grow medium is coco perlite and I use a “feed to waste system” – I tend to add the solution at a pH of 5.7

I am quite happy for that solution to wander up to 6.1 . As you will see in the graph above, at 6.1 I’m still able to catch all of the nute ranges within this  0.4 difference – I just catch hold of the bottom end of magnesium.

testing pH cannabis

How can I tell if I have a pH lockout?

OK this is where there’s no help for it but to put your hand in your pocket. I’ve gotten away with buying cheap pH testers from ebay when I’ve been short of cash, but they fail quickly and its often only years of growing experience that lets me see this in time. Therefore I’d say there’s no avoiding this and really it is one  area where it pays to be generous to your girls. The first thing you need to do is get yourself a digital pH pen:

Hanna Instruments Waterproof pH/Conductivity/TDS Tester




Hanna pH Pen


Hanna Eco pH pen

Or if you are in Europe, you can buy this economy pH pen for just 37€

Unlike cheap Chinese imports, the Hanna Eco PH pen is reliable and stable, and gives entry level growers (or those of us on a budget) the chance to maintain ph levels without spending a fortune.

best online grow shop europe




A digital meter is your most important tool:  it is going to tell you:

  • what the pH of your water/nutrient solution is,  going into the grow medium
  • you will be able to test the water/solution as it leaves the medium
  • it is now possible to buy pens which will give you a digital reading of both pH and the strength of the nutrient salts(cf/ec/ppm) within the solution. (see above).  This  reading  gives  you precious information you can use to keep your cannabis  plants happy.


The ability to diagnose any pH issues you may have  should be the first stop on your diagnostic journey when dealing with unhappy  plants!

If you use the pH pen as a diagnostic tool, it will help you not only get ill plants better, but also guide you as  to what’s going on in the root space  (the ppm part of the meter will help with this as well, its why I’d recommend getting the more expensive pH + EC combination pens)(many of these include temperature gauges as well – but that’s a whole other subject!)

e.g. growing in coco -perlite:

Now I’m assuming you have your pen and that periodically you’ve been pH-ing the water as it goes into the plant. You can then check the run off  for pH levels again. Now with a coco perlite grow, your solution will read 5.7  &  with at least a 25% drain you would expect the pH of the drained water to be between 5.6 and 6.1.

however if after testing the waste the reading is say 6.5 then you need to lower the overall acidity in the medium to decrease this small fluctuation. So the next time you water,  lower the pH in your nutrient solution (pre feeding) to 5.4. This time give the plants  a 100% drain off and  recheck the pH.  If its now within  the desired range then “hey no problem!” we say

…. well no actually ..

you have to ask yourself: why had the pH slipped?

Here are 2 examples:

  1. If the pH reading is above 6.3 in all likelihood the plant was taking up a disproportionately large amount of water:feed, probably because your temps are too high or humidity too low  …  a reading in the root zone can be caused by something happening in the environment … everything is related to the overall plant physiology.
  2. If your pH is acidic, i,.e 5.2- 5.3, a cause can be that your nutrient solution is too high for the plants’ requirements at this time. Flush the plant with a weaker,  slightly alkaline water until the waste comes out at the desired range for both pH & PPM.


How do I lower my pH?

The easiest way to bring down your pH  is to add a chemical called phosphoric acid. This is the main chemical contained in growshop bought “pH down” . You can also use pH adjusters designed for swimming pool use etc as well (much cheaper!). I wouldn’t bother with vinegar etc you need so much of it to make a difference.


Are there any nutrients that help maintain good pH levels?

Yes! most commercial nutrients contain agents that bring down your pH levels and maintain it to some degree. One however stands out from the crowd: Advanced Nutrients pH perfect range. I’ve recommended this to many new cannabis growers as it is quite remarkable how it keeps the pH stable within the medium. I’ve followed these grows through, and all have produced great yields as its been one less problem for an inexperienced grower to have to deal with.

Here is the link to Advanced Nutrients pH Perfect range, a 3 part nutrient range:


Advanced Nutrients pH Perfect Range 

7 thoughts on “What is pH and why is it important to a cannabis grow?

  1. Bonnie Ember says:

    O.K., now that our garden has a healthy pH, make sure that yours, is balanced as well.

    Humans, eat a very acid diet. Humans eat too much sugar, which is the food that cancer thrives on.
    Your body water, needs to be alkaline.

    Cancer flourishes in an acid pH, & is dormant at 7.4 pH, & non existent at 8.5 pH.

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