Field trials were carried out in order to examine the role of some geochemical indices (S:Al; Mg:Al) and partition coefficient (Kd) for evaluating response of crops to aluminum toxicity under acid soil conditions. They were established in 2007/2008 at Gluszyna Lesna (52014, N and 16056, E), a 300-hectare agricultural farm near Poznan. Two different crop plants were tested: (i) winter oilseed rape, variety Cabriolet, (ii) maize, variety Anamur. The source of magnesium and sulphur was kieserite (MgSO4⋅H2O) applied at four Mgrates: 0, 25, 50, 100 kg Mg ha–1 in the first decade of November 2007. The results revealed that the values of partition coefficient for magnesium (KdMg) decreased along with a rise in pH, although a reverse trend was observed for the partition coefficient of aluminum KdAl. Changes in S:Al indices observed at both sites along with increasing kieserite (Mg) rates suggest that S-SO4 concentrations in soil may reduce Al toxicity. The introductionof S-SO4 to soil may be intended to meet plant’s nutritional requirements and, simultaneously, react with exchangeable aluminum (Alex) in order to mitigate its phytotoxicity. On the other hand, the incorporated magnesium (Mg2+) was expected to exchange with Al3+ ions in the soil cation exchange complex (CEC). The values of Mg:Al indices decreased with raising kieserite rates at the oilseed rape site. Changes of Mg:Al indices observed under extremely acid soil conditions (maize site) along with increasing kieserite (Mg) rates suggest that Mg:Al cannot be treated as a direct index describing the Mgex – Alex  interaction, especially at 25 and 50 kg Mg ha–1 rates. The values of S:Al and Mg:Al indiceswere lower at the maize site than at the oilseed rape site because of large amounts of Alex concentrated in soil solution (amounts of Alex on the maize site were ca 3-fold higher than those determined at the oilseed rape site).