Studies on soluble silica in soils. II. The release of monosilicic acid from soils

Abstract

The release of monosilicic acid into solution from soils has been studied under near-neutral and acid conditions. For all soils examined the final concentration of silica decreased with decreasing soil : solution ratio but larger total amounts were released into the larger volumes of extractant. The amount of silica extracted from any one soil varied with pH in a manner similar to that reported previously for residual amounts of monosilicic acid added to soil suspensions, viz. release was minimal at pH 7-9 and increased continuously with acidity. Citrate ions promoted release of native silica from soils as well as partially preventing sorption of added monosilicic acid. The inference of Part I, that much readily soluble silica may be derived from sorption sites in soils, has thus received additional confirmation. Studies have also been made of the release of silica both from some clay minerals and from synthetic sesquioxides on which monosilicic acid was previously sorbed. The release of sorbed monosilicic acid from sesquioxides resembles release from soils in its dependence on pH; iron and aluminium oxides are considered to be responsible for most of the retention of monosilicic acid by soils. Much of the silica rapidly dissolved from soils by N hydrochloric acid is also believed to be derived from sorption sites, and the use of an acid extractant for assessing the 'reactive silica' status of soils is briefly discussed. Suspensions of fine quartz or amorphous silica were partly converted to sorbed silicic acid in the presence of sesquioxides, and it appears that finely divided silica, even quartz, is not stable at near-neutral pH values in the presence of excess sesquioxides. The results of the investigations have been discussed briefly in relation to the availability of soil phosphate and the amounts of sorbed silicic acids occurring in various soils.