Agricultural crops differ in pH preference, and buffermethods allow
liming rates necessary to attain specific soil pHs. In pond aquaculture,
there is no known reason to attain a specific pH in the bottom soil.
The objective is to increase soil pH and allow greater total alkalinity in
the water (Boyd, 1995). The PB method was designed to neutralize a
large amount of exchangeable acidity and lower base unsaturation as
much as possible. This approach has a considerable advantage because
a graphical relationship between base unsaturation and pH of soils is
not needed. Theweight of pond soils is typically less than that of agricultural
soils; the average dryweight of the 1 ha × 15 cmlayer of pond soil
is about 1,500,000 kg ha−1 (Boyd and Cuenco, 1980; Boyd et al., 2010).
The basis for the newlime requirementmethod proposed and tested
in the present study involves the use of a potassiumchloride-potassium
bicarbonate solution rather than the buffer solution containing pnitrophenol
of the AE (and PB) methods. The concentration of potassium
ion in the solution for the proposed method (henceforth called the
K-bicarbonate method) is almost twice that of the buffer solution for
the PB method to displace as much exchangeable aluminum ion from
the soil sample as possible by exchange for potassium. The solution for
the K-bicarbonate method is 0.04 N in bicarbonate (equal to 2 mg mL−1
total alkalinity as CaCO3 equivalent). The principle of the method is to
displace aluminumion by potassiumion (as in the AE and PB methods).
However, in the K-bicarbonate method, hydrogen ion from hydrolysis
of aluminum ion displaced from soil will neutralize bicarbonate, and
the decrease in the total alkalinity of the solution—that can bemeasured
by acidimetry—allows a direct estimate of the amount of liming material
(as CaCO3 equivalent) necessary to neutralize soil acidity and lower
base unsaturation as much as possible. The pH of pond bottom soils
has a very low base unsaturation that is typically 7.0 to 7.5 (Pillai and
Boyd, 1985).