SULPHATE-BASED ALUMINUM PHYTOTOXICITY MITIGATION UNDER STRONG SOIL ACIDIFICATION

Field trials were undertaken in order to evaluate sulphate-based aluminum phytotoxicity mitigation with maize as a test plant. Sulphate treatments corresponded to 0, 33.0, 66.0 and 132.0 kg S-SO(4) ha(-1) applied as MgSO(4) H(2)O (23.2% S-SO(4)). Exchangeable aluminum (Al(ex)), readily exchangeable Al (REAl) - difference between Al(ex) and aqueous aluminum (Al(aq)) were assayed. Chemical tests involved also the aqueous sulphate [S-SO(4(aq))] forms extracted by using deionized water. Gibbs-free energy was calculated for Al(ex) and REAl fractions in investigated soils. Data obtained in the current study have indicated that the experimental site was characterised by pH 3.73 and exchangeable (Al(ex)) amounting to 135.2 mg kg(-1). The low level of organic matter implies, that the soil is typically mineral, i.e., conditions favorable for high monomeric activity (aqueous Al = 51.6 mg kg(-1)). Furthermore, soil incorporation of sulphates decidedly induced a net formation of insoluble Al(x)(SO(4))(y), and the efficiency of this process, as compared to the control treatment, varied as follows: 11.6, 45.7, 63.2%, accordingly to S-SO(4): 33, 66, 132 kg ha(-1). At S-SO(4) rate of 66 kg ha(-1), Gibbs-free energy change (Delta G) values amounted to -5.11 and -5.79 kJ mol(-1) for readily exchangeable aluminum pool (REAl) and exchangeable aluminum (Al(ex)), respectively. This implies the predominance of Al-mineral controlling processes, hence a natural stability strengthened by anthropogenically incorporated S-SO(4). Aluminum phytotoxicity weakened definitely to concentration levels practically not exhibiting further harmful effects. This resulted in a significant increase of maize biomass (at anthesis) of ca 46, 70 and 58%, as compared to the control treatment, for S-SO(4) rates of 33, 66 and 132 kg ha(-1), respectively. The application of sulphate may be a targeted practice for counteracting acidification-based aluminum constrains.