pH is an important quantity that reflects the chemical conditions of a solution. The pH can control the availability of nutrients, biological functions, microbial activity, and the behavior of chemicals. Because of this, monitoring or controlling the pH of soil, water, and food or beverage products is important for a wide variety of applications.
Agriculture and gardening
Soil is a complex system that involves many different factors that are affected by soil pH, such as microbial activity, fungal growth, availability of nutrients, and root growth.
Under acidic conditions, many minerals in soil become soluble, releasing toxic metals such as aluminum. Some nutrients, such as phosphorus and molybdenum, become less available at lower pH values. Under alkaline (basic) conditions, the soil can become deficient in nutrients such as zinc, copper, iron, manganese, boron and phosphorus.
Most plants tend to do best in the pH range of approximately 6.0 to 7.0, which is the range over which the most nutrients are available. However, some plants prefer more acidic or basic conditions, such as blueberries (4.0-6.0) or hyacinth (6.5-7.5).
When soil pH is outside of the desired range, the pH can be altered by adding acidic (e.g., native sulfur) or basic (e.g., lime) material to the soil. To correct the pH of acidic soil by liming, an exchangeable acidity analysis must be conducted so that the required amount of lime can be calculated.
Aquaculture and aquatic ecosystems
Water that has a pH that is too low or too high can be harmful to fish and other aquatic life. At low pH, toxic metals such as aluminum can enter the water in greater concentrations, some nitrogen-bearing chemicals become more toxic, and the metabolic processes of fish can become less efficient. Water with pH below 5 can inhibit reproduction or lead to death, and young fish and other aquatic organisms are especially susceptible. Water with a pH below 6.5 can inhibit growth.
At high pH values (such as >9), most ammonium ions are converted to ammonia, which is toxic to fish. This problem gets worse with higher temperatures. Water with pH between 9 and 10 will tend to inhibit growth, and water with pH of 11 or higher will kill fish.
The pH range of 6.5-9 is acceptable for most fish. In aquariums and other closed aquatic systems, it is important that the water be sufficiently buffered (usually with bicarbonate and carbonate ions) to prevent wild swings in pH.
Whether in treating drinking water or waste water, pH is important. The pH of drinking water should be between 6.5 and 8.5. Low-pH drinking water can degrade pipes, causing toxic metals such as copper and lead to leach into the water supply. Water with a pH that is too high has an unpleasant taste, and the effectiveness of disinfectants such as chlorine is decreased.
In wastewater treatment (e.g., sewage or industrial waste), pH is controlled so that desired chemical or microbial reactions will proceed as efficiently as possible. Operators carefully monitor and adjust pH to respond to changing chemical or microbiological conditions.
Swimming pool maintenance
Swimming pools typically have pH values in the range of 7.2 to 7.8. If the pH is too high, the effectiveness of the chlorine disinfectant becomes too low, making the pool becomes susceptible to algal growth and preventing it from effectively killing viruses and bacteria. If the pH is too low, the water becomes irritating to the eyes and nose, and it may corrode plaster or metal surfaces.
In the food industry, pH is measured to test for quality, to control microbial activity, to control the taste and other properties, and to prolong the shelf life of food. In milk, pH is tested to check for impurities or infection. The pH is also affected by the souring of milk and maturation of cream, and the pH determines whether cheese will be soft or hard. The pH of cream also determines whether butter will be sour or sweet. For production of yogurt, the pH of cultured milk is kept low to maintain a desirable environment for appropriate microbial activity.
The pH of food is also used to monitor its quality. For example, a pH that is too high can indicate degraded meat.
For many foods, the pH must be kept within a narrow range so that the food can be conserved for a longer period. For example, batter for baking bread is acidified to extend the shelf life of the bread, as are sauces such as mayonnaise. When canning low-acid foods (with pH < 4.6), extra care must be taken to kill bacterial spores because they can grow when pH is greater than 4.6, potentially causing botulism.
Brewing and winemaking
Similar to other processes that involve microbial activity, pH affects many different aspects of the beer brewing process. In particular, the mash pH controls the behaviors of several enzymes used in brewing, and it should be between 5.3 and 5.8 for most mashes.
The pH of wine must be kept at a low level to prevent bacteria from degrading the wine. Lower pH wines will tend to mature more slowly and will be less susceptible to spoilage. The pH of wine also affects its taste, as more acidic wines tend to be dry. The pH values for wines usually fall within 3.0 to 4.0, and white wines tend to have lower pH values than red wines.
 Brady, Nyle C., and Ray R. Weil. 2002. The nature and properties of soils. Upper Saddle River, N.J.: Prentice Hall.