SOIL ACIDITY IN RELATION TO SOIL PROPERTIES AND LIME REQUIREMENT

Abstract

The nature and components of acidity in a group of 20 Saskatchewan soils (pH 5.0–5.8) were investigated. KCl-exchangeable acidity was very low (0.3–2.2 mmol(+)∙kg⁻¹) in all soils. Titratable acidity, determined as the difference between effective CEC and buffered (at pH 8.1) CEC, ranged from 9 to 191 mmol(+)∙kg⁻¹ and constituted about 99% of the total acidity in the soils examined. Titratable acidity was highly correlated with organic carbon (r = 0.83) and with Al extracted using citrate-dithionite-bicarbonate (r = 0.95), potassium pyrophosphate (r = 0.92) and pH 4.8 ammonium acetate (r = 0.79). The combination of organic carbon and citrate-dithionite-bicarbonate-extractable Al accounted for 96% of the variation in titratable acidity. Although lime requirement determined by the Shoemaker-Mc-Lean-Pratt procedure was very highly correlated with titratable acidity (r = 0.98), the data show that the estimated amount of lime to raise soil pH to 6.8 actually exceeded the total (titratable plus exchangeable) acidity in 19 of the 20 soils. Key words: Extractable Al, organic matter, SMP buffer, cation exchange capacity