Water pH, Alkalinity and Buffering Capacity

Measuring the pH of the irrigation water and soil solution is of a high importance and can actually determine the success or failure of the crop. 

pH is an index of the concentration of hydrogen ions (H+) in the water. It is defined as –Log (H+). The higher the concentration of hydrogen ions in the water, the lower the pH value is.

The pH scale ranges from 0 to 14 where:

• Water with a pH lower than 7 is considered to be acidic (higher H+ concentration)
• Water with a pH higher than 7 is considered to be basic (lower H+ concentration)
• Water with a pH of 7.0 is considered to be neutral

Since pH is on a logarithmic scale, a change of one unit in the pH (e.g. from 5.0 to 6.0) means a 10 fold change in the concentration of H+ ions!

The hydrogen ions take part in most of the chemical reactions present in water and soil, which makes them extremely important. Their concentration (hence, the pH) influences the solubility of fertilizers, the ionic forms of salts (e.g. PO4-3 vs H2PO4-), the availability of nutrients to plants, stability of chelates etc.

A water or a soil solution with a pH that is too high can result in nutrient deficiencies, mainly micronutrients such as iron. Keeping the pH of the irrigation water below 7.0 is also important in order to prevent emitter clogging due to sedimentation of salts.

On the other hand, pH that is too low might result in micronutrient toxicities and damage to the plant's root system.

The desirable pH range in the root zone that is comfortable for most plants is 5.5-6.5. Therefore, many growers have to add acid to their irrigation water.

Adding acid actually means adding hydrogen ions. However, to determine the amount of acid to be added, it is not enough to know the pH of the water. Another vital parameter must be taken into consideration: the water alkalinity.

Water Alkalinity and Buffering Capacity

The alkalinity of water is related to the pH, but it is actually a different parameter. It is a measure of the capacity of the water to resist changes in pH or, in other words, it is the buffering capacity of the water. Don't confuse "Alkalinity" with "Alkaline" (which means a pH of 7.0-14.0).

The main components of the water alkalinity are

• Carbonates (CO3-2)
• Bicarbonates (HCO3-)
• Soluble hydroxides (OH-)

Alkalinity is usually expressed as ppm or mg/L of Calcium Carbonate (CaCO3).

The higher the alkalinity, the more acid can be added without considerably changing the pH. This is because the bicarbonates (HCO3-) and carbonates (CO3-2) react with the hydrogen ions (H+) contributed by the acid, preventing them from dropping the pH.

Once all the alkalinity components in the water are neutralized by the acid, the concentration of the free hydrogen ions in the water increases and there is a dramatic drop in the pH of the water. The following graph illustrates this "breaking point", where the pH drops:

Here is simple example of how buffering capacity of the water influences daily decisions: 

  

• Grower A has source water with a pH of 9.0 and alkalinity of 45 mg/L CaCO3.
• Grower B has source water with a pH of 8.0 and alkalinity of 120 mg/L CaCO3.

Both growers need the pH of their irrigation water to be  5.0, and use sulfuric acid for this purpose.

Even though Grower A's water has a higher pH (the concentration of hydrogen ions in his water is 10 times higher than Grower B's water) he actually needs to add less acid than grower B to reach the same target pH.

Less than 45 mg/L CaCO3 is considered to be a low water alkalinity, with low buffering capacity. Acid added to this water will quickly affect its pH.

Therefore, it is obvious that both pH and alkalinity are essential for finding the correct amount of acid you have to add to the irrigation water in order to reach the required pH.

Here are the equations that relate pH to alkalinity:

pH = 6.37 + log (HCO3-/H2CO3)    or    pH = 10.33 + log (CO3-2/HCO3-)