A closer look at acid choices for reducing water alkalinity

Some water sources cause the pH of a growing medium to rise rapidly over time. This has nothing to do with the pH of the water, but rather with the alkalinity in the water. Alkalinity is a measure of the bicarbonates and carbonates in the water. Another way to understand water alkalinity is that it is a measure of the limestone content in the water. The higher the alkalinity, the more limestone that is being applied, causing the pH of the growing medium to climb. To offset this pH rise, acids are injected into the irrigation water to neutralize some of the alkalinity and reduce the limestone content in the water.

Selecting an acid
The most common acids used to reduce water alkalinity include sulfuric, nitric, phosphoric and citric acids (Table 1). When deciding which acid to use, there are several factors to consider including safety, additional plant nutrients provided, cost, and availability. Table 1 provides a summary of these factors.

Table 1. Sulfuric, nitric, phosphoric and citric acids: how much is required to drop the alkalinity of the water by 50 ppm CaCO3, the nutrients provided, relative safety, and cost.
*Relative safety: 1 = fairly safe; 5 = very dangerous **Cost: $ = least expensive; $$$ = most expensive

Nutrients provided by acid
Table 1 shows the parts per million (ppm) of the element supplied by each acid. Citric acid does not provide any significant levels of elements. Nitric and phosphoric acids provide needed plant nutrients that are often provided solely by fertilizer. When using a higher rate of an acid, it can provide a significant level of a plant nutrient as seen in Table 1, so a fertilizer with a lower ratio of this element may need to be used.

In the case of nitric acid, this may mean reducing the application rate of an existing fertilizer. If this does not work, then use a fertilizer with a lower ratio of nitrogen to potassium, such as 15-2-20, 15-5-25, 17-5-24, 20-5-30, etc. Remember that using a new fertilizer may have a different influence on the pH of the growing medium.

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