Alkalinity is used incorrectly by many gardeners and even garden writers. I’ll bet I’ve made this mistake. If you think alkalinity refers to a high pH, you should read this post.
What is Alkalinity?
You can describe alkalinity in two ways, which in essence mean the same thing.
a) Alkalinity is a relative measurement of the capacity of water, or soil to resist a change in pH.
b) Alkalinity is a measure of the total carbonates (CO3), bicarbonates (HCO3) and hydroxyl ions (OH) and is ususally expressed as the equivalent of CaCO3, e.g. 100 ppm CaCO3.
When acid is added to water or soil, it will react with the carbonates and be neutralized. Until all of the carbonates are neutralized, the pH won’t change. This is called buffering capacity.
Other compounds will also neutralize acids, but the carbonates exist in much higher amounts and are therefore the major ions of interest.
Why is Alkalinity Important?
High alkalinity can be toxic to plants and it usually results in high pH, which is not good for plants. Knowing the alkalinity helps to understand the suitability of soil or water for plants.
It is important to know the alkalinity if you are trying to adjust pH, since it determines the amount of acid required to cause a change in pH.
Alkalinity vs Alkaline
The two terms sound the same and are therefore mixed up in discussions. Alkaline refers to a pH over 7. A soil can be alkaline, but the soil can’t have an alkalinity of 8.5 (pH units).