What is pH? pH is a logarithmic scale which measures of the acidity or basicity of a solution. Specifically, pH is the measure of the weight of hydrogen ions (H+) within a solution. What is the pH scale? pH is measured on a scale ranging 0 – 14, with 0 being the most acidic (example: battery acid) and 14 being the most basic (example: Lye). On the pH scale 7 is considered neutral. A neutral solution contains equal amounts of hydrogen and hydroxide ions. What is Alkalinity? Water alkalinity is a measure of the capacity of water to neutralize acid… in other words, the amount of acid it takes to lower pH below a certain level. Water alkalinity measures the carbonate and bicarbonate (think limestone) levels within the water. This is why it important to know a few things about your local water supply. For example, knowing how much limestone is in your regional water will help you prepare for your grow or know what is causing your pH imbalance. If your water contains a high level of limestone, your growing medium may be too basic (pH greater than 7); which means you'll need to add acid to reduce the pH levels. Water alkalinity vs. Water pH Water alkalinity and pH do not measure the same thing. Water alkalinity is specific to the carbonate and bicarbonate levels. Water alkalinity directly affects the pH of your growing medium. Water pH measures the amount of hydrogen ions in your water. The pH levels of your water do not directly affect the pH levels of your growingmedium. Water pH will affect the solubility of fertilizers, fungicides, and insecticides. The higher your water's pH the less soluble these substances become. Ideal pH levels for Soil Cannabis plant's ideal pH range for soil based medium is 6.2 - 6.7. Ideal pH levels for Soilless (including peat moss mediums) Cannabis plant's ideal pH range in soilless medium is 5.6 - 6.2. Why is pH a big deal? The pH of your growing medium affects the availability of many critical nutrients your plant needs to grow. As your growing medium becomes more acidic (pH less than 7) an overabundance of nutrients may be found, resulting in nutrient lockout if the pH imbalance is not corrected. As your growing medium becomes too basic (pH greater than 7) your plant may experience nutrient deficiencies such as iron or manganese deficiencies. If pH imbalances are not corrected they may ultimately affect the quality and quantity of your plant's yield. Signs of pH imbalance Signs of pH imbalance vary depending on the growth stage of your cannabis plant. During the early stages of growth, if the pH levels are imbalanced your cannabis plant may start to develop small dark-green leaves that grow very, very slowly. During the later growth stages you may notices existing leaves yellowing, and eventually dying and falling off. If this is happening to your plant you may need to take a closer look at your pH levels. My experience Your choice of growing medium – either indoors or outdoors can make a difference in your pH levels. Personally, I have had a good experience using PRO-MIX HP. I like this growing medium for a couple of reasons, namely because it includes Dolomite and Calcitic limestone (which helps adjusts the pH). Dolomite lime breaks down to calcium and magnesium, which is just one less thing to have to add! Since PRO-MIX HP is primarily composed of peat moss (considered soilless), it is important to remember the ideal pH levels for soilless mediums should be more acidic than soil-based mediums. I have had better luck keeping my pH levels within a 5.8 - 6.2 range.