Kraft lignins (KLs) modified with ozone were saponified with 0.1 M NaOH at room temperature overnight and the abilities of the modified KLs in reducing aluminum toxicity in acidic soil were assayed by planting experiments with radish (Raphanus sativa var. radicula Pers.). The abilities of ozone-treated KLs in reducing aluminum toxicity and accelerating root elongation increased markedly by saponification. Furthermore, KL treated with 0.1 M NaOH at room temperature overnight also effectively reduced aluminum toxicity and accelerated root elongation. These results suggest that KL contains chemical structures that contribute toward these favorable effects. Saponified ozone-treated KLs reduced aluminum toxicity at lower dosages than the alkaline-treated KL.