Understanding Plant Nutrition: Irrigation Water Alkalinity & pH
pH And Alkalinity Are Two Different Aspects Of Water Quality
The term pH is a direct measurement of the balance between acidic hydrogen ions (H+) and basic hydroxide ions (OH-), and can be measured with a pH meter. The pH of a solution can range between 0 (very acidic) and 14 (very basic). At a pH of 7.0, the concentrations of H+ and OH- are equal, and the solution is said to be neutral. When the pH is above 7.0, the concentration of OH- is higher than H+, and the solution is said to be basic or alkaline (not to be confused with alkalinity). When the solution is below 7.0, the concentration of H+ is higher than OH-, and the solution is said to be acidic.
Alkalinity is a measure of how much acid it takes to lower the pH below a certain level, also called acid-buffering capacity. Alkalinity is not a specific ion, but rather includes the concentration of several ions that affect acid-buffering capacity.
Under most conditions, the ions that have the greatest effect on alkalinity are bicarbonates like calcium, magnesium or sodium bicarbonate and, to a lesser extent, carbonates like calcium or sodium. Several other ions (like hydroxides or sulfides) also can contribute to alkalinity, but their concentration in most irrigation water is usually so low that they can be ignored.
Units Of Measurement For Total Alkalinity
In a water sample, the concentration of all of the ions that make up the alkalinity term are combined and reported as equivalents of calcium carbonate, CaCO3, which is the main component of lime. Alkalinity can therefore be thought of as the “liming content” of the water.
The concentration of alkalinity (or any other plant nutrient) can be expressed a number of different ways:
1) Parts per million (ppm or mg/liter): A weight per weight ratio. One part per million is equivalent to one unit of something dissolved in a million units of something else. In the case of anything dissolved in water, 1 ppm is equal to 1 mg/liter.
Water Alkalinity Has A Big Effect On Substrate pH
When it comes to managing the pH of a substrate, the alkalinity concentration has a much greater effect than does water pH. Alkalinity (calcium bicarbonate, magnesium bicarbonate and sodium bicarbonate) and limestone (calcium and magnesium carbonate) react similarly to limestone when added to a container media. And just like too much limestone, the use of irrigation water containing high levels of alkalinity can cause the pH of the substrate to increase above acceptable levels for healthy plant growth.
For example, a limestone incorporation rate of 5 pounds per cubic yard will supply approximately 100 mEq of limestone per 6-inch (15-cm) pot. Applying 16 fluid ounces (0.5 liters) of water containing 250 ppm alkalinity to that 6-inch pot will supply about 2.5 mEq of lime. That does not sound like much until you consider that after 10 irrigations, you have effectively increased the limestone incorporation rate by 25 percent.
To compare the effect of water pH or alkalinity on the ability to raise pH (or neutralize acid) in a medium, 50 ppm alkalinity (which is a low alkalinity) would be similar to having a water with pH 11 (i.e. an extremely high pH). A water with a pH of 8.0 would have the same effect on substrate pH as an alkalinity concentration of only 0.05 ppm (i.e., almost nothing).
Don’t ignore water pH, though. Water pH is still important for crop management because it affects the solubility of fertilizers and the efficacy of insecticides and fungicides before you apply it to the crop (Figure 2). Generally, the higher the water pH, the lower the solubility of these materials.
Minimizing The Effects Of High Alkalinity
The common problems associated with high alkalinity result from its tendency to increase media pH. Since the solubility of micronutrients (particularly iron) decreases as media pH increases, the use of high alkalinity water often results in micronutrient deficiency in the crop (Figure 2).
The most common method for minimizing the “liming effect” of high alkalinity is to neutralize it by adding a strong mineral acid (usually sulfuric acid or phosphoric acid) directly to the irrigation water. The acid causes the water pH to decrease, which neutralizes some of the alkalinity. All of the alkalinity has been neutralized when the pH of the water reaches 4.5. For more specific recommendations on how much acid is needed to neutralize a specific amount of alkalinity, you can download the “Acid Addition Calculator” from Purdue University and North Carolina State University at www.ces.ncsu.edu/depts/hort/floriculture/software/alk.html.
Another option for alkalinity control is to use acidic fertilizers. Fertilizers high in ammoniacal nitrogen produce an acidic reaction when added to a container media, which can be used to neutralize the liming effect of water alkalinity. For example, 20-20-20 (69 percent NH4-N) has enough acidity to be used with water containing around 200 ppm alkalinity water without further acidification.
There are several drawbacks to using fertilizer for alkalinity control. Fertilizers high in ammoniacal nitrogen may cause excessive growth and are not effective when the temperature of the substrate is less than 60ËšF. In addition, you lose flexibility because you can only choose commercial fertilizers based on ammonium content. For example, fertilizers that contain more than 40 percent ammoniacal nitrogen do not contain calcium or other key nutrients.
Low Alkalinity Effects
Not everybody has irrigation water with high alkalinity. In one study from Michigan State University, about 30 percent of the irrigation water tested had alkalinity levels below 40 ppm without any acidification. Even in areas where high alkalinity is considered normal, some growers have switched to low alkalinity sources such as reverse osmosis purified water or rain water.
The primary problem associated with low alkalinity water is a tendency for substrate pH to drop over time, which can cause micronutrient toxicity problems (Figure 3). Low media pH problems are often a result of fertilizer selection. Fertilizers high in ammoniacal nitrogen are acidic, and without any alkalinity in the water to balance the reaction (resist lowering of pH), acidic fertilizers will tend to drive the substrate pH down over time.
Understanding a few technical details about water alkalinity can help you improve pH management. However, irrigation water can affect plant nutrition in more ways than just media pH. In next month’s article, we will discuss irrigation water as a nutrient source.