1. Brief history of silicon research in Japan. 2. Silicon sources for agriculture. Silicon supply for paddy rice from natural sources. Irrigation water. Soils. Silicon supply from organic and inorganic fertilizers. Compost. Application rate in the past and the present. Short-term availability of Si in rice straw for rice plants. Long-term availability of Si in compost for rice plants. Rice husk. Silicate fertilizers. Calcium silicate slags. Fused magnesium phosphate. Potassium silicate fertilizer. Porous hydrate calcium silicates. Silica gel. Estimation of available silicon in silicate fertilizers. 3. Silicon in soil. Behavior of silicon in paddy soil. Estimating the silicon-supplying capacity of paddy soils. Measuring acetate-buffer soluble silicon (Acetate buffer method). Measuring silicon dissolved under submerged condition (Incubation method). Measuring silicon in supernatant (Supernatant method). Measuring easily soluble silicon (Easily soluble Si method). Measuring silicon dissolved in surface water (Surface water dissolution method). Measuring Si dissolved in phosphate buffer (Phosphate buffer method). Environmental factors controlling the availability of silicon for rice plants in paddy soils. Balance sheet of silicon in paddy soil-past and present. 4. Effect of silicate fertilizer application on paddy rice. Criteria for predicating silicate fertilizer requirement for paddy rice. Field experiments on the effects of silicate fertilizer application. Slag-calcium silicate. Porous hydrate calcium silicate. Silica gel and potassium silicate. Effect of calcium in slags on silicon uptake by rice. 5. Silicon-accumulating plants in the plant kingdom. Criteria for discriminating Si-accumulating plants from non-accumulating plants. Characteristics of silicon accumulators and their distribution in plant kingdom. Variety difference in silicon content in the Si-accumulating and intermediate-type species.