PH and Growing Cannabis in Soil


Okay lets get the technical bit out of the way. Ph is measured on a scale of 1.0 to 14.0. Pure water has a Ph of 7.0 and is deemed Ph neutral. Ph below 7.0 is deemed to be acidic and Ph higher than 7.0 is deemed to be alkaline.


A product that decreases Ph (Ph-down) is called an acid while a product that increases Ph (Ph-up) is called a base. A product that helps nutrient solutions resist changes in Ph is referred to as a buffer.

A Ph difference of 1.0 is equal to a ten times increase or decrease in Ph. That is, a nutrient solution with a Ph of 6.0 is ten times as acidic as a nutrient solution with a Ph of 7.0. A Ph difference of 2.0 is equal to a hundred times increase or decrease in Ph.

It is essential to keep the Ph levels within certain limits when growing cannabis. The Ph level of your soil will ascertain how well your cannabis plants are able to take up the nutrients. If the Ph level is out of the correct range, the growth rate of the plants can stall or even stop.

Ideal Ph Levels for Growing Cannabis

When growing cannabis in soil the Ph should usually be around the 6.5 – 7.0 mark. When growing in pots, a single Ph reading for each pot is advised. When growing outdoors it is best to take two or three Ph measurements from different areas of the grow area. If you have a large grow area, you may have to adjust the Ph in various parts to different levels. Check the Ph every couple of weeks.

Most nutrients will cause a Ph change in the soil. Adding fertiliser to the soil often results in a lower, more acidic Ph. As time goes by, the amount of salts produced by the breakdown of nutrients in the soil causes the soil to become increasingly acidic and eventually the concentration of these salts in the soil will hurt the plant, effecting production and destroying the leaves.

As the plant gets older its roots also become less efficient in supplying food to the leaves. To help avoid the build-up of these salts in your soil and to ensure that your plant is getting all of the nutrients it needs, you can foliage feed your plants at the age of about 6 weeks.
Checking the Ph levels Of your soil.

There are various ways of checking the Ph level of your soil, the most common ones being the PH metre, Ph test kit, Soil Ph metre and the soil test kit.

Ph Meter: used to calculate the Ph level of water, nutrient solutions and soil.
A digital Ph meter is long lasting, and in general they give more accurate results than other methods of measuring Ph. They have probes and batteries that eventually will need to be replaced. This is the option that we recommend in most cases.

Ph Test Kit: used to determine the Ph of liquids like water or nutrient solutions.
Simple Ph test kit to check Ph levels. They are low cost, simple to use, and can be used many times. However, you will eventually run out of Ph test liquid and have to buy a new kit and they can be a pain in the proverbial.

Soil Ph Meter: used to check the Ph of soil.
Soil growers could also get a soil Ph meter to measure the Ph level of the soil. They work by inserting the probes of the unit directly into the soil you are
growing in, and taking a reading. Follow the manufacturers instructions included with the soil Ph meter you have and you will get accurate measurements.

Soil Test Kit: used to calculate various elements or just the Ph, for example: Nitrogen, Phosphorus and/or Potassium levels
Another option for soil growers is a soil test kit. These are simple to use and dependable kits that contain separate tests for Ph, nitrogen, Phosphorus, and potassium. They give instant outcomes on the soil conditions in your garden.

Adjusting the Ph Levels

A good way to stabilize the soil for growing cannabis is to use dolomite lime (calcium-magnesium carbonate). Dolomite lime acts slowly and continuously, so soil will remain Ph stable for a few months. It is important to use the finest grades of dolomite lime. Dolomite lime has been used by gardeners as a Ph
stabilizer for many years. It has a Ph that is neutral (7.0). When added to soil in the correct proportions, it will stabilize soil around that 7.0 mark.

When growing in pots, add one cup of fine dolomite lime to each cubic foot of soil, lightly water it and wait for a day or two before checking the Ph.

If you use soil specifically aimed at cannabis growing then it is already going to have the right quantities of many things and should stabilise itself.

Lowering Ph: small amounts of composted leaves, cottonseed meal, or peat moss will lower the Ph of soil. We always use ‘PH Down’ a product specifically made for this purpose and available in a bio version.

Raising Ph: small amounts of hardwood ashes or crushed egg shells will help to raise the soil Ph. Hydrated lime can also be used to raise the Ph of soil..

Paz’s Conclusion

Straightforward but very important. My general ph routine is 6.3 as seedlings climbing up to 6.5 throughout.