To determine the silica content of rice straw, 250 mg of strawwas burned at 500◦C for 2 h in a furnace. Then, 2.5 ml hydrogen per-oxide and 5 ml sodium hydroxide were added to the burned strawin a polyethylene tube and gently vortexed. To eliminate any con-taminations, the tube was then autoclaved at 138 kPa for one hourand diluted with distilled water up to 250 ml. Next, 1 ml of preparedsolution was transferred to another polyethylene tube and mixedwith 0.1 ml ammonium molybdate solution. After 10 min, 0.4 mltartaric acid solution was added by swirling. Finally, the reducedsolution was added to a tube and diluted to 10 ml with distilledwater. To determine silica content, the absorbance of solution at650 nm was read using a spectrophotometer (AOAC, 1999; Elliottand Snyder, 1991).