Agriculture Reference
In-Depth Information
Box 4.6
pH and Its Measurement in Soil
pH is defined as the negative logarithm of H
ion activity (measured in
solution as moles/L). For practical purposes, we equate activity to concentration in
dilute solutions. The pH scale ranges from 0 to 14, with pH 7 indicating the
neutral point (equal concentrations of H
and OH
ions).
Soil pH can be measured in the field with a universal indicator and a color
chart to an accuracy of
0.5 pH units. In the laboratory, soil pH is measured to
0.05 units in a soil suspension, using a glass electrode and a
reference calomel electrode (fig. B4.6.1). The suspension is made by shaking one
part by weight of soil with five parts by volume of distilled water. Robust glass-
calomel electrodes have also been developed for direct use on moist soils in the
field. Dilution of the soil solution with distilled water changes the soil pH. For
soils containing mainly negatively charged clays and organic matter, the measured
pH in water, which is the pH of the bulk solution, is higher than that of the
undisturbed soil.
However, if soil is shaken with a solution of 0.01M CaC1
2
, the measured soil
pH changes little with dilution, because 0.01M CaC1
2
has an ionic strength (see
box 4.4) similar to that of the average soil solution. Thus, many laboratories have
now adopted the preferred method of measuring pH in 0.01M CaCl
2
at a
soil:liquid ratio of 1:5. Because pH (CaCl
2
) is generally 0.6-0.8 units lower than
pH (H
2
O) at the same soil:liquid ratio, it is important to know, when pH values
are quoted, which method of measurement has been used.
an accuracy of
Figure B4.6.1
A portable pH meter with electrodes in a soil suspension. Photograph by the author.
See color insert.
