pH buffering: The chemical response of acidic soils to added alkali

Citation
Mk. Conyers et al., pH buffering: The chemical response of acidic soils to added alkali, SOIL SCI, 165(7), 2000, pp. 560-566
Citations number
19
Categorie Soggetti
Environment/Ecology
Journal title
SOIL SCIENCE
ISSN journal
0038075X → ACNP
Volume
165
Issue
7
Year of publication
2000
Pages
560 - 566
Database
ISI
SICI code
0038-075X(200007)165:7<560:PBTCRO>2.0.ZU;2-6
Abstract
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.