pH Value

It is very important to monitor the pH levels of your grow medium, as well as the water you give your cannabis plants. It effects how you plants grow and could potentially need adjusting at any time throughout the entire life cycle of your marijuana.

What is pH?

pH is the measurement of a how acidic or basic a substance is. It goes through a scale, ranging from 1 for very acidic all the way up to 14 for very alkaline. The middle ground, 7, is neutral. Anything below 7 is considered acidic and anything above 7 is considered basic.

The ideal pH levels are 6.5-7.0 for a soil based grow and 5.5-6.0 for a hydroponic grow.

Why is pH important?

pH directly affects how well your cannabis can absorb nutrients from your grow medium. If the pH of your grow medium strays outside of the safe limits then the nutrients your cannabis sorely needs become unavailable. For example, let's say you are growing in soil and it becomes to acidic; in this situation acidic salts will bind nutrients together, making them completely non-absorbent to your cannabis plants. This is sometimes referred to as nutrient lockout.

When your pH is out of balance you will begin to notice that your plants are showing signs of stress. Novice cultivators often mistake this as a sign of underfeeding and add more nutrients to try to combat it. This makes things even worse. As nutrients cannot be absorbed it will cause toxic levels of salt to build up within the soil, eventually causing your plants to stop being able to absorb water as well. As a result, it is highly advised that when you notice something is wrong with your plant, check the pH first. Do not change how often or how much nutrients you give your cannabis until you are sure the pH of your medium is correct. This lead onto our next question.

How do you measure the pH levels of your setup?

There are two easy ways to measure the pH levels of your setup. Either buy a cheap electronic pH tester or buy some pH testing paper. Both can quickly tell you the pH levels of anything you may use, (an electronic tester is going to give you way more precise results than pH testing paper).

Electronic pH testers are often called pH meter sticks. These are basically an LCD display on top of a rod. This rod detects the pH level of a substance; it is simply placed into the substance in question to obtain a precise and clear numerical reading – often to two decimal places! You would be looking at a rough investment of around 20-40 euros for a decent electronic tester.

pH testing paper usually comes in packs of little strips. You place the strip into a substance you are testing and it changes color depending on the pH level. You compare this color to a chart to get a rough reading. This is not very precise, but can give you a general idea of your pH levels and costs very little to buy.

It should be noted that both these methods are designed to work in wet substances and will not give you an accurate reading if you try to measure dry soil.

What causes my pH levels to change?

In an indoor grow room it will depend on what growing setup you are using. Should you be using a soil based medium, then the main cause of pH change is going to be through the decomposition of fertilizer within the soil. As fertilizer decomposes it leaves salts behind, these are acidic and will cause the pH level of your soil to decrease. A second cause is by mixing your own, by doing this it is also possible to leave pockets of soil with a different pH to everything around it; if you do mix your own it is very important to mix your soil as thoroughly as possible to stop this from happening.

Should you are using a hydroponic setup then the nutrient solution you use can very easily cause fluctuations within the reservoirs.

If you are growing outdoors then pH is largely going to be effected by where about in the world you are. In drier climates, pH of natural water tends to be above 7, which means your soil is going to become steadily more alkaline. In wetter climates natural water is often pH 6 or below, this will cause your soil to become more acidic.

How do you raise/lower you pH levels?

When you are using soil, the best way to regulate your pH levels is with the use of Dolomite Lime. It has a pH level of 7, so it is able to treat soil that is both too acidic and to alkaline. Simply put one cup per square foot of soil into the pot and water in with the next watering cycle, wait a couple of days and then measure your pH levels. If you are using it outdoors then it is best to follow the manufactures instructions as there are a lot more unaccountable variables in an outdoor grow.

If the pH balance of you hydroponic set up is incorrect then it is possible to buy pH-up and pH-down on the market. These are chemical solutions that are specifically designed to do just as their name suggests. You can usually buy them from your local gardening center. More often than not they come in 1L bottles of concentrate which you add to your water. It should be noted that these can also be used for soil grows if things get out of control.

It is important to precisely follow the manufacture's directions when using pH-up or pH-down. They can be quite strong, you do not want to over do it. The best way is to measure out the exact amount needed with a syringe and mix it with the required amount of water, then apply to your plants. This way you get no rough measurements.

In an extreme situations you can perform a soil flush (please see our article on flushing your cannabis). This will remove all nutrients and salts from the soil, but can place your plant under a lot of stress if it is still growing - possibly forcing it into shock. The only time you should really want to flush your plant is pre-harvest when your plant has finished. Doing so beforehand should only be done as a last resort.

What are the signs my cannabis will exhibit if the pH is wrong?

Usually the signs of a pH imbalance will be that the leaves of your cannabis plants start to go yellow or brown. It can also cause stunted growth and wilting. If you see these signs it is important not to jump to conclusions. These are also symptoms of other problems. Start by measuring the pH of your grow medium and go from there.

As an extra tip, we would highly recommend testing the pH of your grow at least once a week, the pH of your water every time you water your plants, the pH of the drainage after you water your plants, and the pH of your grow medium 2 days after a feed. It may seem like a lot, but the best treatment is prevention – by running a tight operation and being constantly aware of pH levels, you will ensure your cannabis plants never suffer pH related woes.