The present work reports the studies and application of the colorimetrics methods for determination of total and soluble silica in soils. Two aspects were studied: the interference of several ions and silica solubilization. The problem of solubilization was solved through adaptation of a technique devised by CASE (1944), based on the heat solubilization of the samples by hydrofluoric acid in mineral acid, with subsequent addition of boric acid. The precision and accuracy of this technique was evaluated by application on samples of cleaned san, and the accuracy obtained was good. The technique of solubilization associated to the colorimetrics methods of silicon determination, was applied to soils. In order to compare the colorimetrics gravimetric methods, the samples were analysed also by the latter. The data obtained allowed to conclude that the precision and accuracy of the refered techniques are satisfactories. The contents of soluble silica were determined by the molybdenum blue, from extraction by acqueous solution of CaCl2 0,0025M, H3C-COONa 0,01 M, H2SO4 0,025M, and by distilled water.