Acidify irrigation water to lower soil pH

Proper pH is one of the most important factors influencing the suitability for soils for growing turfgrass. Most turfgrasses prefer neutral to slightly acid pH. The good news is that you can maintain proper pH and correct improper pH with simple practices.

In many regions of the United States, the primary concern is pH that is too low. That's why liming, which raises pH, is common in these areas. However, in other regions, particularly the arid West, soils tend to have high pH. In this case, you must acidify soil to lower pH to a more desirable level.

What is pH? Soil pH isn't as mysterious or complicated as it may seem. It is simply a measure of the concentration of hydrogen ions (H+) in the soil solution. Low pH is referred to as "acidic" and high pH is termed "basic" or "alkaline."

The factors that influence H+ concentration in soil are complex and beyond the scope of this article. But here's how it translates to pH: * Low H+ concentration = high soil pH (above 7.0) * Medium H+ concentration = neutral pH (around 7.0) * High H+ concentration = low soil pH (below 7.0)

Knowing this, you can see that if your soil pH is too high, you can correct the problem with an amendment that adds hydrogen ions to the soil. By contrast, adding lime increases the soil pH by removing hydrogen ions.

Correcting high soil pH By definition, an acid is any material that, when added to a solution, will increase the H+ concentration of that solution (in our case, the soil solution). An example is sulfuric acid, H2SO4. When you add it to the soil solution, sulfuric acid separates into its components-two hydrogen (H+) ions and one sulfate (SO4-2) ion.

You can add sulfuric acid in one of two ways: either dry application of granular sulfur to the soil or adding sulfuric acid to the irrigation water before applying it to the soil through the irrigation system. There are advantages and disadvantages to each method.

* Granular sulfur. When you apply granular sulfur, soil bacteria quickly convert it to sulfuric acid. An advantage of granular application is that you can apply enough sulfur in a short time to quickly lower the pH of a soil in which the pH is substantially too high. Plus, you have better control of the uniformity of the application.

Granular sulfur's ability to rapidly lower pH is also its main disadvantage. If you are dealing with a soil that has a low buffering capacity, such as a golf green or high-sand ball-field root zone, a slight over-application of sulfur can lower the pH to levels fatal to the turf.

Nevertheless, in many instances granular sulfur application is the most appropriate way to go. It is simple and inexpensive, and can do the job if your irrigation system is not easily adaptable to the use of acid injection.

* Sulfuric acid. Adding sulfuric acid to irrigation water is a somewhat "gentler" method. By lowering the pH of irrigation water to slightly below 7 (such as 6.5), there is little danger of excessively lowering the soil pH to a degree that could harm the turf. Another advantage is that it eliminates the labor needed for granular applications.

Unfortunately, even the best irrigation systems are not completely uniform in their coverage. As a result, some areas will receive greater acidification than others. Also, storing and handling concentrated sulfuric acid presents a serious safety concern.

A final disadvantage, particularly with older equipment, is that a malfunction could release too much acid into the water, or worse, into the pump station, with dire results.

Acidifying irrigation water There are two primary ways to acidify irrigation water: acidifying the reservoir and acidifying the wetwell.

* Acidifying the reservoir. With this method, you add acid to the irrigation reservoir to lower the pH of the entire body of water. This method is relatively safe in that if you accidentally add too much acid, the size of the reservoir will serve as a buffer and prevent the pH from dropping excessively.

Also, the stored acid and equipment may be kept some distance from the pumps or other sensitive equipment.

Water is pumped from the lake into a mixing chamber. Acid is then dripped or pumped into the chamber to mix with the incoming water. The mixture then flows back into the lake, lowering the pH of the entire body of water, which will eventually be applied to the soil through the irrigation system.

An automatic controller often manages the rate of acid injection. A sensor monitors the pH of the lake water and sends this information to an electronic programmer, which regulates the acid flow. The technology for this type of system may seem rather simple, but it is quite effective.

* Mixing in the pump wetwell. With this method, water is pumped from the wetwell while a sensor monitors pH in the water stream (see illustration, below). The stream then enters a mixing chamber into which acid is pumped. The acid and water mix in the chamber and the mixture is discharged back into the wetwell. On the way back, sensors again measure the pH of the water stream. The two sensors are connected to an programmer that regulates the amount of acid pumped into the mixing chamber to achieve the desired pH.

This method has a potential downside though. If a malfunction occurs and an overdose of acid is added, the small volume of the wetwell would offer little buffering ability and the water pH could drop to levels that could damage equipment. On several occasions, I have heard pump-system vendors blame deteriorated pump parts on the acid dispensed into the wetwell. I'm not sure that this was always true, but it is a difficult argument to refute.

Additionally, the acid storage and handling equipment is contained within the pump station, a location that is not ideal from a safety standpoint. I have asked vendors why they prefer this arrangement, and it's usually because of the convenience of installing the entire system on the pump skid without additional trenching, plumbing or extending the power service.

I should note that some newer acid-handling equipment is safer and more reliable than that offered 5 years ago.

Sulfur burners Sulfur burners have been used in agriculture for many years to lower the pH of irrigation water. More recently, they have become available for turf use and are enjoying rapid acceptance.

This method is similar to acidifying reservoir water with acid. The unit pumps in water, acidifies it and then returns it to the reservoir. However, no acid is stored on hand. Instead, the unit burns granular sulfur (stored in an attached hopper) to form sulfur dioxide (SO2). The sulfur dioxide is then mixed with the water stream and ultimately forms sulfuric acid. The acid lowers the pH of the water, which is then returned to the reservoir.

Sulfur burners offer significant advantages over acid injection. There's no need for storing acid, which is an improvement in safety. These units are not prone to mechanical malfunctions that could lower water pH to damaging levels. Plus, they are compact-an entire unit may be little larger than a 55-gallon drum.

The burners produce some exhaust, but it is not problematic, or even noticeable, to facility patrons. Overall the burner system is a trouble-free, effective method of acidifying water.Achieving optimal soil pH is one of the keys to successful turfgrass culture, particularly in the arid West where soil pH often is high. Fortunately, the options available to superintendents and make pH control a fairly easy task. Be sure to have your soil tested by a professional laboratory before deciding on steps to alter your soil's pH.

Dr. Harold F. "Chip" Howard is a Certified Professional Agronomist, Certified Professional Crop Scientist and Certified Golf Course Superintendent. Howard owns TurfScience Inc., a consulting firm serving the turfgrass industry (Phoenix, Ariz.).

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