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 treatment. Soils with initial pH w    7.4, incubated with H 2 SO 4 were 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 susceptible to acidification than buffer curves derived from liming acid soils might predict. A new measure of soil susceptibility to acidification, the acidification resistance index (ARI), was derived from buffer curves. It is defined as the amount of acid (cmol (p + ) kg −1 ) required to reduce soil pH from its initial level to pH 5.0. Best-fitted multiple regression equations between ARI and soil properties routinely measured in soil test laboratories determined that 79% of the variation in ARI could be explained by a combination of extractable cations and initial soil pH. This regression model could be used to provide an inexpensive measure of soil susceptibility to acidification for orchards where acidifying fertilizers are applied through drip irrigation systems. Key words: Soil acidification, pH buffer curves, Malus domestica Borkh, drip irrigation