It's not about pH, it's about the affect of alkalinity on water and medium pH. Plain and simple, not enough cannabis growers know this stuff. The books, magazines and FAQs have failed on the issue of water quality in the growroom.
RO water has little to no alkalinity, i.e. capacity to buffer acids.
Read on:
Understanding Water Quality
The first of a two-part series will focus on water pH, alkalinity and control of media pH.
- Bill Argo and Paul Fisher
Differences Between Water pH and Alkalinity
There is a great deal of confusion when it comes to understanding the difference between water pH and water alkalinity, and why they are important to the management of your crop.
The term “pH” is a direct measurement of the concentration of acidic hydrogen ions (H+) contained in a solution. The pH of a solution can be directly measured with a pH meter, and can range between 0 (very acidic) and 14 (very basic). At a pH of 7.0, the solution is said to be neutral. When the pH is above 7.0, and the solution is said to be basic or alkaline (not to be confused with alkalinity). When the solution is below 7.0, the solution is said to be acidic.
Alkalinity (also called acid-buffering capacity) is a measure of all the chemical bases contained in a water. Alkalinity is not a specific ion, but rather includes the concentration of several ions that affect acid-buffering capacity. Under most conditions, the ions that have the greatest effect on alkalinity are bicarbonates like calcium, magnesium or sodium bicarbonate and, to a lesser extent, carbonates like calcium or sodium.
In a water sample, the concentration of all of the ions that makes up the alkalinity term are combined and reported as equivalents of calcium carbonate (CaCO3, the main component of lime), or total alkalinity. Alkalinity can therefore be thought of as the “liming content” of the water. The units used to report alkalinity can be parts per million (ppm), mg/liter or milliequivalents (mEq).
Alkalinity can’t be measured with a pH meter. Instead, you have to use a meter or test kit specifically designed to measure alkalinity. The cheapest and easiest to use are colormetric test kits where an indicator dye is added to the water, then a dilute acid is added until a color change occurs at a specific pH (usually at a pH of 4.5). The alkalinity concentration correlates with the amount of acid in the water.
Water Alkalinity Affects Media pH
When it comes to managing the pH of a container media, the alkalinity concentration has a much greater effect than does water pH. Alkalinity (calcium bicarbonate, magnesium bicarbonate and sodium bicarbonate) and limestone (calcium and magnesium carbonate) react very similarly when added to a root medium. And just like too much limestone, the use of irrigation water containing high levels of alkalinity can cause the pH of the root media to increase above acceptable levels for plant growth.
To compare the effect of water pH or alkalinity on the ability to raise pH (or neutralize acid) in a medium, 50-ppm alkalinity (which is a low alkalinity) would be similar to using a water with pH 11. Irrigation water with a pH of 8.0 would have the same effect on media pH as an alkalinity concentration of only 0.05 ppm.
My tap water has low alkalinity, around 45 ppm....It's basically perfect for growing. I never use pH up or down. After mixing nutrients, the pH always settles right around 5.8 to 6.0, despite that during the Summer and Fall, the pH out of the tap is over 9.0 (and it's 7 during the Winter and Spring). I've read that it goes up to 9.0 during the warmer months because of organisms in the water supply....The reservoir is literally around the corner from my house.
