Exchangeable Al and 0.01M CaCl2‐soluble Al were measured in 40 acid surface soils by Clark's procedure for corrected lime potential and exchangeable cations, and by a modified procedure which was developed here. In the modified procedure, 0.01M CaCl2‐soluble Al was extracted by using 16 hr of shaking instead of 5 days, and exchangeable Al was extracted with 1N KCl for 2 min instead of with 2N NaCl for 16 hr. Good agreement was given with the theoretical relationship between corrected lime potential and percent base saturation of the permanent charge sites by both the original and modified procedures. Aluminum extracted by the two procedures and also that extracted by three other methods were then assessed for their availability to plants grown on the soils in a greenhouse study.Criteria of plant‐available Al were the yield responses to liming by barley (Hordeum vulgare L.), turnip rape (Brassica campestris L.), and alfalfa (Medicago sativa L.), and the Al concentration in alfalfa grown on the unlimed soils. Exchangeable Al and 0.01M CaCl2‐soluble Al measured by Clark's and the modified procedures were all closely correlated with the criteria of Al availability. Aluminum extracted by 1N NH4OAc (pH 4.8), 1N NH4OAc (pH 3), or 0.002N HCl methods were not so closely correlated with the criteria.The use of extractable soil Al for the diagnosis of the need for liming is discussed.