Most studies about lime requirement and the pH buffering capacity of soil h
ave either calibrated buffer solutions to soil-CaCO3 reactions or have corr
elated pH buffering with soil properties such as clay and organic matter co
ntent. A different approach is to address two fundamental questions: where
do OH- ions react when they are added to acid soils, and can we quantify th
ese reactions? The experimental hypothesis was that alkali added to a soil
(cmol/kg) could be accounted for quantitatively by summing the increase in
effective cation exchange capacity (ECEC) (cmol/kg), the decrease in exchan
geable Al (cmol/kg), and, possibly, the decrease in exchangeable Mn (cmol/k
g). It was presumed that this model would begin to fail as pH approached ne
utrality, where the precipitation of CaCO3 would start to exert an influenc
e on pH buffering capacity. Hence, this model was intended to account for t
he reaction of applied OH- in the approximate range 4 < pH(CaCl2) < 7.
Twelve soils were titrated with Ca(OH)(2) at near constant ionic strength a
nd soil:solution ratio so as to minimize changes in (H+) caused by redistri
bution between solution and adsorbed/exchangeable phases. After 16 h of rea
ction, the exchangeable cations were measured in each soil at each concentr
ation of Ca(OH)(2) addition. The data indicated that exchangeable Mn was no
t a sink for OH- under the experimental conditions. Precipitation of Ca2+ o
ccurred during the reaction period, causing an apparent loss of the applied
alkali. However, this reaction could be expected to reverse over a longer
period of time and thus was considered an experimental artifact rather than
a component of buffering capacity. The experimental artifact could be over
come by expressing pH buffering in terms of the measured slope of pH change
versus the sum of measured changes in ECEC plus exchangeable Al. It was co
ncluded that in the pH(CaCl2) range 4 to 7, the soil sink for added alkali
can be accounted for quantitatively by the increase in ECEC plus the decrea
se in exchangeable Al. Therefore, soil pH(BC) in the alkaline direction can
be estimated from changes in ECEC and exchangeable Al.