pH BUFFERING: THE CHEMICAL RESPONSE OF ACIDIC SOILS TO ADDED ALKALI

Most studies about lime requirement and the pH buffering capacity of soil have either calibrated buffer solutions to soil-CaCO3 reactions or have correlated pH buffering with soil properties such as clay and organic matter content. 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 these 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 exchangeable Al (cmol/kg), and, possibly, the decrease in exchangeable Mn (cmol/kg). It was presumed that this model would begin to fail as pH approached neutrality, where the precipitation of CaCO3 would start to exert an influence on pH buffering capacity. Hence, this model was intended to account for the 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 and soil:solution ratio so as to minimize changes in (H) caused by redistribution between solution and adsorbed/exchangeable phases. After 16 h of reaction, the exchangeable cations were measured in each soil at each concentration of Ca(OH)2 addition. The data indicated that exchangeable Mn was not a sink for OH under the experimental conditions. Precipitation of Ca occurred 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 overcome 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 concluded 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 decrease in exchangeable Al. Therefore, soil pHBC in the alkaline direction can be estimated from changes in ECEC and exchangeable Al.