Effect of pH and added slag on the extractability of Si in two Si-deficient sugarcane soils

The effects of increasing pH on the adsorption and extractability of Si in two Si-deficient Australian sugarcane soils was investigated and the effects of increasing rates of fertilizer Si (as blast furnace slag) on pH and extractable Si were also examined. Equilibrium studies showed that maximum adsorption of Si by the two soils occurred in the pH range 9-10. When soil pH was increased from 5.0 to 6.5, subsequent adsorption of Si by the two soils, as measured by adsorption isotherms, increased. After incubation with progressive lime additions there was a decline in CaCl₂- extractable Si due to its increased adsorption and an increase in acid (H₂SO₄^- and acetic acid)-extractable (mainly adsorbed) Si. The increase in acid extractable Si was greater than the decrease in CaCl₂^- extractable Si suggesting a supply from an additional source. Alkali (Na₂CO₃ and Tiron)-extractable Si decreased greatly with increasing pH suggesting dissolution of the amorphous (mainly biogenic) pool of silica was occurring with increasing pH. When increasing rates of slag were incubated with the soils, pH, CaCl₂^- and acid- extractable Si were all increased because upon dissolution slags release both silicic acid and OH^- ions. There was, therefore, a positive relationship between extractable Si and soil pH. However, Na₂CO₃^-and Tiron-extractable Si decreased with increasing slag rates (and increasing soil pH) suggesting dissolution of the biogenic pool of soil Si. It was concluded that future research needs to examine the desorption potential of adsorbed Si and the effects of liming on dissolution of the biogenic pool of soil silica under field conditions.