The effect of doses and time of application of siliceous amendments (rice husk-ashes and sodium metasilicate) on the phosphorus availability in two acid soils (Oxisol and Andept) was quantified. Alaboratory essay was conducted in order to determine both the nearly appropiate doses and the time of application of the amendments against phosphate application. By using this information a green house experiment was conducted to evaluate the amendrnents effect on yield of radish Raphanus sativus) and on phosphorus and silica content in soils and plant. The application of ashes twenty davs after phosphorus application showed the best results on the Oxisol, but no significant difference was shown on the Andept. Sodium metasilicate shown negative effects on both soils. The best amoun of rice husk-ashes seems to be close to three tons/ha applied twenty days after phosphate application on the Oxisol.