THE significance of the role of colloidal solutions of silicic acid in soilforming processes, particularly with respect to the Mediterranean Terra Rossa soils, has been expounded by Reifenberg (1927Reifenberg ( , 1929Reifenberg ( , 1947.Preliminary studies in this laboratory indicated that sodium salts of chloride, nitrate, sulphate, borate, hypobromite, and acetate anions do not effect a significant release of silica from soils or from kaolinite, whereas the phosphate anion does. The presence of orthophosphates in solution in contact with soils or clay minerals resulted in appreciable amounts of silica entering the solution phase, while at the same time (except for alkaline reactions) some phosphate was being attached to the solid phase-in other words, a displacement of silica accompanied phosphate fixation. In view of its importance in soil fertilit as well as the influence of certain factors affecting the quantity of silica displaced. Discussion of the mechanism involved is left for a forthcoming paper. It may be mentioned that later studies by the authors have revealed that a number of other anions, including certain products of biological activity in soil, also exhibit this property of releasing silica from the solid phase of soils and clay minerals. MethodsSamples of air-dry soil (< 2 mm. diameter) in stoppered 500-ml.Pyrex flasks were shaken with sodium-phosphate solution and allowed to stand at room temperature for 72 hours with occasional shaking. The amount of soil used was equivalent to IOO g. on an oven-dry basis, and the sodium-phosphate solution was such as to give a total solution volume of 250 ml. and a concentration of one-third molar ( I N) with respect to phosphate. The phosphate solutions were prepared by mixing M/3 solution of NaH,PO, and of Na,HPO, to obtain the desired H.to the factor being investigated, in amount of soil, volume and concentration of solution, and time of standing, are indicated in the text.The pH of the clear filtrate was electrometrically determined by the glass electrode. The silica content was gravimetrically determined by the acid-dehydration method followed by hydrofluorization in the presence of sulphuric acid (Kolthoff and Sandelf, 1943). The tedious classical gravimetric method was followed since preliminary trials had shown that the colorimetric method of silica determination as ammonium silicomolybdate (Snell and Snell, 1936) was unreliable for these experiments. The removal of interfering phosphate ions prior to the * This paper includes part of a Ph.D. thesis by the junior author.in weathering and soil-forming rocesses a study of this p r l enomenon was undertaken. This paper dea P s with one phase of that study, namely,The solution also exerted some buffer effect. Any variations, accor 8. ing