Effect of liming with different sized limestone on the forms of aluminium in a Galician soil (NW Spain)

The aim of this study was to evaluate the effects of the particle size (2–4 mm, 0.5–2 mm, 0.25–0.5 mm and less than 0.25 mm) of magnesium limestone and the method of application (in a single dose or split in three yearly doses), on the forms of Al in the solid and liquid phases of an acid Galician soil destined for pasture production. Monitoring the experimental plots over a period of 3 years showed that the plots treated with a single application of the finest limestone (< 0.25 mm) exhibited the highest pH and the lowest concentrations of exchangeable Al, with less than 10% Al saturation throughout the period of study. The average values of pH in water along the period of study were 5.30 in the most effective treatment and 5.06 in control plots. The average values of aluminium saturation were 6.5% in the most effective treatment and 16.9% in control plots. One year after liming, the plots treated with a single application of the finest limestone presented also the highest concentrations of weakly stable organo-aluminium complexes in the solid phase and of organically bound Al in the soil solution, as well as the lowest concentrations of labile Al, while no Al3+ (considered the most toxic Al species) was detected. Over time non-crystalline inorganic Al tended to precipitate, apparently at the expense of exchangeable Al and/or Al bound to the organic matter. The application of the finest limestone split in three doses (until completion of 3 t ha− 1 in the third year of the study) was less effective than the application of a single dose. The plots treated with the coarsest limestone (between 2 and 4 mm) and the control plots provided similar results for all parameters throughout the three years of the study. In the former the exchangeable Al was higher than 20% and the concentration of labile Al was the highest of all plots.