Treating alkalinity

Alkalinity is one of the most important chemical properties of water. It has a greater influence on root zone pH than the water pH itself. Alkalinity is like the lime in your substrate in that it buffers the pH from dropping or can even increase it. However, depending on the amount of alkalinity in your water and your crops’ requirements, you may need to modify your fertilizer choices, substrate or the amount of alkalinity in the water itself. The management actions and corrective procedures you will need to take are going to vary based on the amount of alkalinity in your water and the crops you are growing.

If you have water with alkalinity between 0 and 1.0 meq/L (0 and 50 mg/L), this may actually be lower than desirable and lead to unwanted drift in pH. You will want to use a fertilizer with a very high proportion of nitrate (NO₃-). When NO₃- is taken up, it releases hydroxide (OH-). As a result, the OH- will help maintain, or potentially raise, substrate pH over. This potential to raise pH is the fertilizer’s “potential basicity,” and this is expressed on fertilizer labels as the amount of limestone the pH-raising effect of the fertilizer is equal to.

Water with alkalinity between 1.0 and 1.5 meq/L (or 50 and 75 mg/L) is considered high-quality irrigation water. There is enough alkalinity to provide some buffering for the pH in the substrate, but not enough to cause unwanted increases in pH. For this range of alkalinity, no corrective actions need to be taken and water can be used without treatment.

As alkalinity increases to the range of 1.5 to 3.0 meq/L (75 to 150 meq), one of the first steps is to switch to a fertilizer that will acidify the substrate. The “potential acidity,” or ability of a fertilizer to lower substrate pH, increases as the proportion of ammonium (NH₄⁺) increases in fertilizers. The acidifying effect of the fertilizer will help neutralize the buffering ability of alkalinity. Another option is to adjust the limestone that is added to your substrate. By reducing the amount of limestone that is incorporated into your fertilizer, the alkalinity in your irrigation water will act like lime and compensate for reduced limestone additions to increase substrate pH to your target range.

If alkalinity is higher, between 3.0 and 8.0 meq/L (150 and 400 mg/L), you will need to acidify your water. When acid is injected into the irrigation water, it neutralizes the alkalinity and reduces buffering potential. There are a few different acids that can be used to lower alkalinity, including nitric, phosphoric and sulfuric acids, and several of these are also available in different concentrations. Whenever acids are used, they not only neutralize alkalinity, but they also contribute mineral nutrients to the irrigation water. For example, nitric, phosphoric and sulfuric acids will add nitrogen, phosphorous and sulfur to your water after alkalinity is neutralized. As a result, phosphoric acid is generally avoided for plug producers so phosphorous levels are not excessive. Sulfuric acid tends to be the most widely used acid.

If the alkalinity in your water exceeds 8.0 meq/L (400 mg/L), you will need to look at more extreme treatment options such as using a reverse osmosis system to improve water quality, then blend this in to make up irrigation water with acceptable alkalinity. This can be a costly control measure. Alternatively, new water sources may need to be explored for blending or exclusive use. Either way, the solutions get costly.

Alkalinity can cause challenges during production. Depending on the amount of alkalinity in your irrigation water, there are different corrective procedures you can take to get your alkalinity under control. By learning how to best manage your water’s alkalinity, you can reduce the potential for unwanted drifts in pH and minimize nutritional problems.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu