Acidification rate of limed soil in a semiarid environment

Abstract

To evaluate the benefits of liming acid soils, a method is needed to predict the longevity of its effect on soil pH. We coupled a simple index of soil buffering with estimates of the proton budget to predict long-term pH changes in a limed soil (Dark Brown Chernozem) in Saskatchewan. Analysis of published data for Saskatchewan soils showed that acceptable estimates of soil buffering can be obtained from organic matter and clay content. Buffer capacities of organic matter and clay were estimated at 487 and 26 mmol(±) kg−1 (pH unit)−1, respectively. Soil pH, titratable acidity, and effective cation exchange capacity (CEC) were monitored for 18 yr after lime application [Ca(OH)2 at rates of 0, 4.5 and 6.7 t ha−1] to field plots in a wheat (Triticum aestivum L.)-fallow rotation. In limed plots, there was a tendency for pH, exchangeable Ca and effective CEC to decrease with time in the 0–7.5 cm layer and to increase in the 7.5–15 cm layer. This was attributed to mixing of the two layers during cultivation. In the 0–15 cm layer as a whole, there was no discernible change in acidity, Ca, or CEC during the monitoring period. Negligible re-acidification in limed soil was consistent with the estimated H+ budget. External acidification sources were negligible (no N fertilizer was applied). Acidification due to leaching of nitrate and export of cations in grain over 18 yr was estimated at 6–7 kmol(H+) ha−1. This amount of acidity would lower soil pH by less than 0.1 units [buffer capacity of the top 15 cm of soil was ≈70 kmol(±) ha−1 (pH unit)–1], an amount too small to be detectable against background variability. Key words: Buffer capacity, organic matter, proton budget, titratable acidity