The phrase “hydrogen potential” may be unfamiliar to most people, but its abbreviation “pH” should ring a bell. All things can be ranked on a scale of relative acidity and its opposite, alkalinity. It all has to do with a measure of how many free ions of hydrogen are available in a given water sample, soil profile, or even the surface of your skin.
Pure water has a neutral pH of 7.0, and the measurement is on a logarithmic scale (just like the Richter scale for earthquakes) so that a pH reading of 8.0 is 10 times less acidic, therefore more alkaline, than 7.0 pH. More hydrogen atoms equals a solution that can donate negative ions to whatever it contacts, and less hydrogen atoms means that the reaction goes the other way, with ions being in increasing demand. So what? Most plants grow in a pH range of 5.5 to 8.5, but individual species may be quite specific in their need for acid or alkaline conditions. Azaleas are notoriously acid-loving, but junipers and bougainvillea grow well in almost any soil, including those with elevated pH levels.
Plants encounter pH differences mainly through the soil in which they grow. Soils vary in pH throughout the world depending on the underlying mineral strata from which they are derived. In the more arid regions of the West, such as here on the Central and South coasts, soils tend to be slightly alkaline. The soil may be naturally rich in calcium, magnesium, and other alkaline minerals. Couple this with the lack of consistent rainfall to flush these soluble salts out, and the result is an alkaline, mineral-rich soil profile.
Since the underlying rocks in this area are primarily sandstone, they provide a source of calcium and magnesium, two elements that combine readily into alkaline compounds; the groundwater sources in this region are also tilted toward the alkaline. The consequences of these chemical reactions are important for plant growth. Extreme pH values result in other important nutrients being inactivated and, thus, not available to the plants.
There are ways to mitigate these effects. Highly acidic soil may benefit from an application of lime (calcium carbonate). To neutralize alkaline soils, emergency applications of sulfur (also known as flowers of sulfur) can temporarily remedy a deficiency. Consistent additions of organic materials from compost to mulch will eventually keep the balance in the right direction. Peat products will enhance acidity, while wood ash can lower pH. Use any such additive in moderation; it is easy to tip the balance in the opposite direction.
If you are unsure of what your soil pH may be, there are some fairly accurate home tests available from garden centers. For a more detailed analysis, submitting a sample to a soil-testing laboratory can be only slightly more expensive. The best strategy may be to grow only those plants that thrive in the particular soil type that occurs in your garden. Ginkgos will thrive, while maples may not, just as wisteria can tolerate higher pH, while dogwood struggles. Another way to enjoy those more sensitive plants is to grow them in containers where it is much easier to control the pH of the growing medium.