so pH isn’t exactly a component of soil. But
it’s an important attribute of soil, a measure of its
acidity. In case you’ve forgotten: The pH scale for everyday
substances ranges from 1 to 14, with 1 being extremely acidic,
7 being neutral, and 14 being extremely alkaline.
Soils have a natural tendency to acidify,
thanks to rainwater. You may have heard that acid rain is a form
of pollution—and it is—but all rain is slightly acidic,
with a pH of about 5. That’s because the hydrogen in rainwater
reacts with the carbon dioxide in air to form carbonic acid.
Extreme pollution can drop the pH of rain to 4.
Minerals in soils can also affect their
pH. A chalky soil has plenty of calcium carbonate, a famous antacid,
which buffers the pH of soil and keeps it from becoming acidic.
Why care about pH? The availability of various
plant nutrients rises and falls with pH—too high or too low
and plants will suffer. For most plants (excluding acid-lovers like
blueberries, azaleas, and rhododendrons) the optimal pH for soil
fertility is about 6.5.