Several laboratory methods for estimating lime requirements of acid soils were compared using 24 soils from Alberta and northeastern British Columbia and 15 from elsewhere in Canada. The Peech, Schofield, Woodruff and SMP (Shoemaker et al. 1971) buffer methods were equally well correlated with lime requirements for raising soil pH to 5.5 or 6, which in turn were highly correlated with the amounts of soluble and exchangeable Al and organic matter in the soils. The SMP buffer method is recommended for use as the diagnostic index of lime requirement to achieve pH 5.5 or 6 because of its speed and simplicity. A refinement is suggested for Alberta and northeastern B.C. soils on the basis that lime need not be added to achieve pH 5.5 but should be added to reduce Al below the toxic level for sensitive crops. The lime requirements to reduce Al in those soils were highly correlated with the amounts of 0.02 M CaCl2-soluble Al they contained and it is recommended that the 0.02 M CaCl2-soluble AI be used as the diagnostic index of lime requirement. Lime requirements related to SMP (pH) and 0.02 M CaCl2-soluble Al are presented.