Water alkalinity versus pH

Water alkalinity and pH are not the same. Water alkalinity is a measure of the carbonate and bicarbonate levels in water, whereas pH measures the amount of hydrogen (acid ions) in the water. Think of carbonates and bicarbonates as dissolved limestone. The higher the alkalinity of the water, the more dissolved lime it contains, therefore, the more rapidly the water can cause the pH of the growing medium to rise. The pH of the water does not have any influence on the pH of the growing medium; however, alkalinity does.

Here’s a great example:  The chart below shows the starting pH of two water sources and the amount of acid required for each to reach a pH of 5.0. As indicated, the water with the higher pH did not require as much as acid as the one with the lower starting pH. At first look, this may not make sense.



Now take the same water sources above and include the alkalinity (see table below). The water with the lower alkalinity did not require as much acid to reach a pH of 5.0. The alkalinity actually determines how much acid to use, not the pH of the water.



How does water alkalinity influence the pH of the growing medium?  In Figure 1 below, vinca plugs were grown for 49 days and constant fed at 125 ppm with 13-2-13 (a potentially basic, plug fertilizer). The plugs were irrigated with one of three water sources with varying alkalinities. The pH rose in all three media samples, not only due to the 13-2-13 and limestone in the growing medium, but also from the alkalinity of the water source. In this example, the higher the alkalinity, the higher the pH of the growing medium rose.

Figure 1:

Figure 1: This graph shows the influence of water alkalinity on the pH of vinca plugs after 49 days. Notice the higher the water alkalinity, the higher the pH of the growing medium rose after 49 days. Data taken from: D. Bailey & P. V. Nelson, Substrate pH and Water Quality. 1996 Ohio Short Course

It is clear that the pH of the water and the alkalinity of the water are not the same. In fact, the pH of the water does not dictate the pH of the growing medium; it is that alkalinity that influences the pH of the growing medium. This is important to remember and monitor, since alkalinity has an impact of what type(s) of fertilizer should be used or if acid injection is required.