The use of gypsum or phosphogypsum (PG) as an ameliorant of subsoil acidity has been reported. However, the role of these amendments in aluminum (Al) speciation and solubility is not clearly understood. We studied the root growth of soybean [Glycine max(L.) Merr.] cv. Lee in dilute nutrient solution containing 2.0 g L−1PG without the addition of Al or in solutions containing 40 μM Al without any amendments or with the addition of PG, CaSO4. 2H2O, or CaSO4. 2H2O plus varying concentrations of P and F. The prediction of Al speciation by using the GEOCHEM computer program revealed that in solutions containing F∼, either added as NaF or as impurities in the PG, Al was entirely complexed with F∼. The addition of CaSO4. 2H2O (2.07 g L−1) to a solution containing 40 μMAl decreased the predicted activity of Al3+from 36 to 11% and increased that of AlSO4+from 33 to 80%, while the total root length of soybean increased by about fourfold. Addition of PG (2 g L−1) to a solution containing no Al decreased the total root length by 14%. Root growth was very similar in PG-amended solutions with or without the addition of 40 μMAl. The addition of 400 μMF∼ as NaF to Al solutions amended with CaSO4. 2H2O seemed to decrease root growth as compared to that in solutions amended with CaSO4. 2H2O without the addition of F∼. Root growth was very poorly related to the sum of predicted activities of Al monomers. Aluminum measured by the modified aluminon technique (without preacidification and preheating of the solution; 30-min reaction time) failed to exclude Al complexed with SO42-and F∼, which are less phyto-toxic. Therefore, this measure of Al showed a poor relationship with root growth response. On the contrary, Al measured by the modified 8-hydroxyquinoline technique (15 s reaction) excluded about 92% of Al complexed with F∼, although it failed to exclude Al complexed with SO42-. Thus, both of the Al assay techniques used in the present study are poor indicators of Al toxicity when the SO42-ligand plays a major role in the alleviation of Al toxicity. However, Al measured by the modified 8-hydroxyquinoline technique was a better indicator than that measured by the modified aluminon technique when the F∼ligand was responsible for the alleviation of Al phytotoxicity.