MEASUREMENT OF THE SENSITIVITY OF ORCHARD SOILS TO ACIDIFICATION

The pH buffer curves for 45 surface soils from apple (Malus domestica Borkh.) orchards in southern British Columbia were determined. Buffer curve shape differed according to the initial soil pH and incubation t reatment. Soils with initial pH(w) < 7.4 and incubated with either H2S O4 or CaCO3 had typically sigmoid or modified sigmoid buffer curves. N on-calcareous soils with initial pH(w) > 7.4, incubated with H2SO4 Wer e far less buffered above pH 6.5-7.0 than soils with initial pH < 7.4. Thus, non-calcareous neutral to alkaline soils may be more susceptibl e to acidification than buffer curves derived from liming acid soils m ight predict. A new measure of soil susceptibility to acidification, t he acidification resistance index (ARI), was derived from buffer curve s. It is defined as the amount of acid (cmol (p(+)) kg(-1)) required t o reduce soil pH from its initial level to pH 5.0. Best-fitted multipl e regression equations between ARI and soil properties routinely measu red in soil test laboratories determined that 79% of the variation in ARI could be explained by a combination of extractable cations and ini tial soil pH. This regression model could be used to provide an inexpe nsive measure of soil susceptibility to acidification for orchards whe re acidifying fertilizers are applied through drip irrigation systems.