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How Much Acid Is Needed to Reduce Water Alkalinity?

Thursday, September 7, 2023 | Troy Buechel

To limit the upward climb of your growing medium's pH, you may need to inject acid into your water to reduce excess alkalinity. Water alkalinity is a measure of the carbonates and bicarbonates in the water and acts just like limestone in the growing medium.

The higher the alkalinity, the more “limestone” that is applied at each irrigation and the faster the pH of your growing medium increases. So, at what point is it necessary to inject acid into a water source to reduce alkalinity?

Why is The Growing Medium's pH Rising?

As plants uptake fertilizer, the roots release acids (hydrogen ions) and bases (hydroxyl ions) to maintain a neutral charge within the plant's roots. This is why fertilizers are labelled with a potential acidity or basicity that predicts how the plant will react with the fertilizer.

When the plant cannot generate enough acid from fertilizer uptake to neutralize the water alkalinity in the growing medium, this causes the growing medium's pH to climb. Therefore, acid injection is needed to neutralize excess water alkalinity or “limestone” in the water.

Sulfuric acid is a popular choice to inject into a water source to reduce water alkalinity.  Source: Premier Tech 

The point at which acid injection is required will vary depending on the stage of plant development, the fertilizer analysis and the quantity of fertilizer the plant uses. For example, a heavy feeder, like a petunia, uses a lot of fertilizer during its active growth phase and can produce a lot of acid to drop the growing medium's pH. However, a small petunia seedling uses very little fertilizer and therefore produces little acid. For a grower rooting cuttings or producing plugs, the alkalinity of the water must be lower than for a grower transplanting plugs or rooted cuttings into larger pot sizes. So, what are the recommended thresholds at which acid injection is required?

General Guidelines for Acid Injection

Table 1 shows the general recommendations that can be used to determine when acid should be injected based on the container size and water alkalinity. These are general guidelines, but the type of fertilizer used is not considered in these guidelines. Table 2 provides more specific guidelines as it considers the fertilizer applied. Remember that a potentially basic fertilizer, when used by the plant, will generate alkaline materials and thus increase the pH of the growing medium.

Table 1. General guidelines used for injecting acid in greenhouse irrigation water based on container size.

Table 2. General guidelines used for injecting acid in greenhouse irrigation water based on container size and fertilizer applied. 

As seen in Table 2, the higher the potential acidity of the fertilizer, the more water alkalinity that can be handled by a crop. In other words, a water-soluble fertilizer can help manage the pH of the growing medium. If the pH of your growing medium tends to increase, perhaps the first option is to switch to a fertilizer with a higher potential acidity. If the alkalinity of your water exceeds those listed for the crop grown, then acid can be injected.

Iron deficiency as seen in these pansies can be caused by excess water alkalinity, which causes the pH of the growing medium to rise. Growers use acid to reduce water alkalinity and avoid iron deficiency. Source: Premier Tech 

Reduce, Not Eliminate

Notice in Tables 1 and 2, the goal is not to eliminate all the water alkalinity, but to reduce it to manageable levels. Neutralizing the alkalinity to 0 ppm CaCO3 will reduce the water's pH to the point it can damage roots near the surface of the growing medium and interfere with seed germination. It can also create fluctuations in the growing medium's pH and problems with pH buffering after the lime charge has been used up.

Note: Since Premier Tech does not manufacture acidifiers, we invite you to consult your manufacturer for specific information concerning the use of their products. 

If you have further questions, please contact your Premier Tech Grower Services Representative or your Regional Sales Representative.

Ed Bloodnick Horticulture Director South Eastern US, Japan and Overseas	Nathan Wallace-Springer Horticulture Specialist South Eastern US	Lance Lawson Horticulture Specialist Western US and Western Canada	Victor Brantly Horticulture Specialist Central US
Troy Buechel Horticulture Specialist Eastern and North Eastern US	Susan Parent Horticulture Specialist Eastern Canada and North Eastern US	Jose Chen Lopez Horticulture Specialist Mexico and South Western US