The pH of an aqueous solution is expressed in pH units — decimal numbers that range from zero to 14. A solution is said to be acidic if its pH is less that 7 and basic if its pH exceeds 7. As just one example, consider lemon juice, an acidic liquid; it has a pH of 2.3. A neutral liquid — neither acidic nor basic — would have a pH close to 7.0.
Most pH measurements are made with water samples. It is also possible to measure the pH of a water-saturated soil paste.
Natural waters, such as streams, rivers, lakes, and groundwater, seldom have a pH lower than about five or above nine, unless the water is severely polluted. Therefore, if the reported pH of an irrigation water is outside that range, further investigation is necessary. It may be that the very high or very low pH is the result of unusual geochemical conditions or some kind of pollution process. Or it may be that the extreme pH provides a hint that the water contains constituents for which analyses were not carried out.
For irrigation of landscape plants, the ideal pH of water is 6.5 to 7. Most water sources available for irrigation are of pH 6.5 to 8.4, however, so a slight degree of basicity, relative to the ideal, is not uncommon. The pH of water, by itself, is seldom a problem for plants commonly used in landscaping. More likely, the pH can become a problem if it affects the chemistry of the soil to which the water is applied.
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