Water alkalinity and pH: What they mean in regards to water quality

Understanding the difference between alkalinity and pH and how alkaline water can affect plants is important in determining what to do to remedy possible nutritional problems.

When thinking about water quality, alkalinity is much more important than pH. pH tells you whether the water is acidic, neutral or basic, but not the buffering capacity of the water. Buffering capacity is the ability of water (or compound) to resist a change in pH. Alkalinity tells you the buffering capacity in the basic pH range of the water.

You can have a high (or low) pH water with very little buffering capacity, meaning you can easily and quickly change the pH of the water; this also means the water is unlikely to change the pH of soils or potting mixes. You can also have water with a pH above 7 with a high alkalinity, meaning it will be difficult to change the pH of the water. This also means the water, if the alkalinity is high enough or a lot of water is applied, can increase the pH of soils or potting mixes.

As water alkalinity increases, there is a greater resistance to a change in water pH or, more importantly, a greater ability of the water to change the pH of something else like soils or potting mixes. It is much more likely to change the pH of potting mixes since pots are watered much more frequently than plants in the ground and there is a limited volume of potting mix in a pot. Neither pH nor alkalinity in the ranges typically seen in untreated water cause direct damage to plants; the damage is indirect by causing a change in pH of soils or potting mixes that make some nutrients unavailable. If the water is very alkaline, it can cause plant damage by slowly creating deposits over the leaves—we do see this with some water in Michigan and if a large amount of alkaline water is frequently applied overhead to plants. Most municipal water treatment processes reduce alkalinity.

Reports of nutrient disorders in landscape plants as the season progresses may be due to water with high alkalinity, but are usually the result of some other issue. The native pH of most Michigan soils is in the range of 5.5 to 7.7, appropriate for good nutrition for most plants. However, there are some plants (most conifers, some maples, others) that will show nutrient deficiency symptoms in soil pH above 6, sometimes lower. These symptoms may not appear early in the year, but develop as there is more growth and vegetation on the plants.

In either case, whether the water is alkaline or the soil pH is relatively high, correcting the soil pH is the easiest and longest lasting solution for homeowners and field-grown crop producers. Commercial producers of container ornamentals have other options available from modifying cultural practices to injecting acid into water.

I have received questions about reverse osmosis water with a high pH. Reverse osmosis water is created by forcing water through ultra-fine membranes resulting in almost all of the ions being removed. Reverse osmosis water is typically neutral to slightly acidic, but can be basic (not alkaline) depending on the source water and how it was treated. Also, because of the low ion content, it is very difficult to accurately measure reverse osmosis water pH—minor residues on sensors or collection cups can cause a large error. 

Some municipalities use sodium hydroxide (NaOH, often used in drinking water treatment) or a similar chemical (one that generates OH- ions) as part of water treatment that could result in higher pH after the reverse osmosis process. The reverse osmosis membranes will exclude the sodium ions but not the hydroxide ions; the hydroxide ions will increase the pH but not the alkalinity. Again, it is easy to change the pH of high pH, low alkaline water. High alkalinity in the source (feed) water to the reverse osmosis system can cause scaling on the membrane because calcium carbonate (primary cause of alkalinity in water) precipitates on the membranes.

In summary, the main problem that arises from high pH water is when it is alkaline. Alkaline water can raise the pH of soils or potting mixes. Nutrient deficiencies will occur if the pH of soils or potting mixes is raised above the ideal range for nutrient solubility. The Michigan State University Soil and Plant Nutrient Laboratory will test water for alkalinity (for a fee) and there are private laboratories that will do this as well. The Michigan Department of Environmental Quality has a list of certified labs that will test for alkalinity and water potability, although alkalinity does not affect potability.

For commercial growers and pesticide applicators, alkaline water can also affect mixing of some fertilizers and pesticides by causing them to precipitate out of solution. There are relatively inexpensive alkalinity test kits to determine water alkalinity. In addition to MSU’s Soil and Plant Nutrient Laboratory, there are also several commercial labs that will analyze water for agricultural quality parameters.

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