ACIDIFICATION RATE OF LIMED SOIL IN A SEMIARID ENVIRONMENT

To evaluate the benefits of liming acid soils, a method is needed to p redict the longevity of its effect on soil pH. We coupled a simple ind ex of soil buffering with estimates of the proton budget to predict lo ng-term pH changes in a limed soil (Dark Brown Chemozem) in Saskatchew an. Analysis of published data for Saskatchewan soils showed that acce ptable estimates of soil buffering can be obtained from organic matter and clay content. Buffer capacities of organic matter and clay were e stimated at 487 and 26 mmol(+/-) kg(-1) (pH unit)(-1), respectively. S oil pH, titratable acidity, and effective cation exchange capacity (CE C) 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 aesti vum 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 c m 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 source s were negligible (no N fertilizer was applied). Acidification due to leaching of nitrate and export of cations in grain over 18 yr was esti mated at 6-7 kmol(HC) ha(-1) This amount of acidity would lower soil p H by less than 0.1 units [buffer capacity of the top 15 cm of soil was approximate to 70 kmol(+/-) hai (pH unit)(-1)], an amount too small t o be detectable against background variability.