What is PH?

PH or the PH scale is the measurement that measures how acidic or alkaline something is. This PH scale starts from 0 being the most acidic and ends at 14 being the most alkaline. The PH of 7 is neutral and for each number above or below that the alkaline or acidic increases 10 times. This measurement doesn’t measure the exact level of the PH but rather the intensity of alkalinity or acidity.

How to measure PH?

ph tester stripsdigital PH testerPH measurement can be done in two ways. The first and the cheapest way is to buy one time use PH paper tester strips. They are cheap but you will need a lot of them if you need to measure your water each time you are watering your plants. The second and the most effective way is to buy an electronic PH tester which will cost you form 20$ to 50$. I myself use a tester witch is at the cheap end of the scale but it works perfectly!

How PH affects autoflowers?PH scale cannabis

Autoflowering cannabis like any other plant needs a certain PH range at which it can grow and absorb all nutrients that is necessary for healthy growth. The ideal PH for autoflowering plants is in the middle of 6.2 to 6.5 for soil and 5.5 to 6.1 when grown in hydroponics, but a little variance won’t affect the plant that much. All the things that plant will come in contact with need to be at this exact PH range and you must check every component with the PH meter to ensure everything is OK. Usually, new growers measure the PH of the water and the PH of the growing medium but after watering they forget to measure the runoff water PH which should be in the same range!

If the PH of water and soil becomes too acidic then plants start suffering because the salts will start chemically binding together with nutrients like (P, Mg, N, Ca) and they become unavailable to the plant’s roots to absorb.

When the PH becomes too alkaline then elements like (Mn, Fe, B) start becoming unavailable to the autoflower plant.

What can change the PH?

The biggest thing that usually changes the PH of the soil and the water is the added nutrients (fertilizer). This change usually is to the acidic side and you always need to measure your water after adding fertilizer. The nutrient solution that is watered all over the growing medium penetrates the ground and can start to induce salt buildup in the ground. Some other thing that changes the PH is the minerals, salts, and nutrients that are used in the mixing of the soil/soilless mix.

PH up and PH down

How autoflower cannabis shows PH problems?

Cannabis plants start showing some signs of PH problems when some or all of the nutrients are unavailable to the plant to be absorbed.

The first changes to the plant usually are in leaves. Leafs can start yellowing or curling and even show signs of burn (brown leaf tips). These leaves can also start drying out and turning into straw-like shapes but the same signs or damage can be seen in a number of conditions that are not related to PH.

How to Raise PH?

The first thing to consider when you want to raise PH is Dolomite lime which is PH – 7 (neutral) and will regulate the PH of your soil. The dolomite lime can be added before planting your autoflowering seeds, but when your plant is already growing then it is much harder to do and you will most likely use some other methods.

Second and maybe the most common way to raise PH is to buy “PH UP” – a commercial product which is dissolved in water or nutrient solution. There are also some household products that can raise the PH and they are:

  • Hydrate lime,
  • Baking soda,
  • Calcium carbonate,
  • Potassium silicate.

You just need to experiment with these items and see how they work and how strong or weak they are.

How to lower PH?

Again the first possibility is the Dolomite lime which is neutral and will neutralize the PH.

The second solution is again a commercial product “PH DOWN” which you can buy at any Home Dept or any other gardening store.

Then there are also some household items that lowers PH and they are:

  • Hydrochloric acid,
  • Nitric acid,
  • Lemon juice,
  • Phosphoric acid.

How to fix PH problems?

If you see that your plant suffers and you determine that you have problems with PH then use some of the methods to either lower or increase the PH.

If you have a salt buildup in your soil then the problem is not so easily detectable and many inexperienced growers will confuse the symptoms of some nutrient deficiency and will start to give more fertilizer which will only increase the problem. The only way you can fix a salt buildup in the soil is to flush it by pouring 3 times the volume of the pot through the soil and let that water flush down the bottom with these excess salts.


  1. Hi I found your page by mistake when i searched AOL for this matter, I need to point out your website is totally valuable I also like the style, it is wonderful!

  2. Yo what about if water has limescale? I dont use it but which way is it..up or down needed?

      • mashpotatoes44 on

        hey im an novice grower theres so much information what to do an what not to do im basically teaching myself how with the experience of other growers an not on my own! It can tend to get a little confusing trying to filter out the unneeded aspects of the game wish i had an hands on experienced teacher i need a little help.

      • Bryan Wall on

        I believe limescale is a scaly build up of the mineral lime( which is alkaline). It ruins plumbing ( especially copper pipes and fixtures such as faucets and shower heads and such.

  3. just need help on

    new to this need help from you
    PH of the growing medium ? what does that mean and how do i test it?? and how do i test the run off water??

  4. Great site. I’ve read the whole guide, but I realized that I had to stop reading and just plant the seeds. I started my two autoflowers one week ago. Acidity of soil is about 7.5, but seedlings seem to be okay. Four leaves on each stem super small, stem super long (lights weren’t low enough). How do I know how much baking soda to use to get the pH between 6.2 and 6.5? Should I just start with a small amount and keep testing the pH of the soil until I get right?

    • You are correct, start with a small amount and test the correct amount. Good luck on the grow!

  5. Ted crane on

    My auto flowers all burned up I used distilled water when should I nute them I messed up and nuted them to early I guess but I need a good feeding schedule for 18 gallon dwc I use general organic grow pack

    • Hi Ted,
      I am primarily a soil grower so I don’t have a great DWC feeding schedule for autos, and even if I did then it would greatly vary depending on your exact nutrients and your strain. But What I can tell you is that autos in general require less nutrients than photo-sensitive strains and that may be why you have burned your plants with too much nutrients at the start of their life.

  6. K nuggers on

    Protip: great way to lower the PH is to use Acscorbic Acid, aka vitamin C. You can get a lifetime supply on amazon for like $20.

    My tap water PH is 8.0. I only need about 1 3/4 tsp to bring my PH into the 6 range. Its a bitch if you go too far and have to dilute the water, so add and re test little by little.

    Vitamin C / Acscorbic Acid also neutralizes Chlorinate which is harmful for plants.

    You can also let your water sit overnight with activated alumina pebbles to absorb the fluoride.

    I do this on my no til set up every water and my plants thrive

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