pH
pH refers to the concentration
of hydrogen ions (H+) in a substrate such as soil. pH is measured on a
logarithmic scale such that each numerical increase in pH corresponds to
a decrease in H+ by a factor of ten. A pH of 1 is considered acidic because
it has a very high H+ concentration and a pH of 13 is considered very alkaline
because it has almost no free H+. pH dramatically effects the CEC of soil
by limiting the available exchange sites at low pH. H+ bind to soil particles
tighter than other cations, thus, any metal bound to a soil particle will
get booted off in the presence of excess H+ (Garcia-Miragaya and Page,
1978). At low pH (<6), H+ is in excess and replaces all other cations
on the micelle, thus making them bioavailable. At high pH (>7), cations
are less bioavailable because they have less competition from H+ for available
binding sites. Many cations bind to free hydroxyl groups (OH-) and form
insoluble hydrous metal oxides which are unavailable for uptake, such as
CdCO3 (Salisbury and Ross, 1992; Ramachandran, 1999). Evergreens in alkaline
soils turn yellow due to lack of available iron (Fe), similarly, many plants
experience aluminum (Al) toxicity in low pH soils.
Decreasing the pH of soil will
increase Cd bioavailability, and will usually increase plant uptake of
Cd unless the Cd elicits a toxic response in the plant. Optimum Cd mobility
is achieved at pH = 4.5 - 5.5 (Bingham, 1980). Therefore, altering soil
pH by adding an acid is a viable method for increasing Cd bioavailability,
a strategy already utilized by many plants that release organic acids for
nutrient scavenging (Salisbury and Ross, 1992).
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