The release of silicon from amorphous silica and rice straw in Sri Lankan soils

Silicon release from rice straw and amorphous silica when shaken in solution with five Sri Lankan soils was studied indirectly using sorption isotherms and changes in concentration and directly using straw in dialysis bags examined using electron microscopy. The aim was to further our understanding of the processes and factors affecting the release of straw-Si in soils and its availability to rice. The soils (alfisols and ultisols) shaken with 0.1 M NaCl (5 g per 125 mL for 250 days) produced concentrations of 1–4 mg L⁻¹ of monosilicic acid-Si. Amorphous silica added to these suspensions (36.5 mg, containing 17 mg Si) raised the concentrations to 20–40 mg L⁻¹, and added rice straw (0.5 g, containing 17 mg Si) gave 10–25 mg L⁻¹. Sorption isotherms (7 days equilibrations) were used to calculate from the concentrations the amounts of Si released (24–38% and 8–21%, respectively). Both materials gave about 40 mg L⁻¹ of monosilicic acid-Si plus 30 mg L⁻¹ of disilicic acid-Si when shaken in solution alone (5 g per 125 mL). Straw in dialysis bags (0.5 g per 25 mL in 0.1 M NaCl) was shaken in soil suspension (5 g per 100 mL) for 60 days. Similar concentrations and releases were measured to those obtained above. About one fifth of the mass of straw was lost by decomposition in the first 15 days. A chloroform treatment prevented decomposition, but Si release was unaffected. Disintegration continued throughout the experiments, with phytoliths being exposed and dissolved. Compared to the rate of release from straw into solution without soil, the release of Si into soil suspensions was increased during the first 20 days by adsorption on the soil, but was then reduced probably through the effect of Fe and Al on the phytolith surfaces. The extent of this blocking effect varied between soils and was not simply related to soil pH.