Soils, representing a variety of great soil groups, were shaken with solutions containing monosilicic acid at concentrations not exceeding 135 p.p.m. The residual concentrations of monosilicic acid under near-equilibrium conditions depended on the nature of the soil, the level of added monosilicic acid, the solution : soil ratio and the pH of the soil suspension. The experimental results indicate that the residual concentration of monosilicic acid is controlled by an adsorption equilibrium which is dependent on pH; with acidification below pH 8-9 the residual concentration in soil suspensions steadily increases.Several samples of oxides and hydroxides of iron and aluminium were also shown to sorb monosilicic acid from solution. This sorption resembled that recorded for soils in its strong dependence on pH in the range 4-9. Sesquioxides appear to be responsible for much of the capacity of soils to sorb monosilicic acid. Wyoming bentonite and two kaolinite samples also sorbed monosilicic acid, at least from suspensions of pH above 7, and some natural and synthetic carbonates sorbed monosilicic acid at their natural pH values. Such minerals may contribute to the ability of some alkaline soils to sorb monosilicic acid.The avidity with which some soils sorb added monosilicic acid suggests that some native silica occurs in soils in the sorbed state. Some implications of this suggestion and of the other results are discussed in relation to some previous work on soils and natural waters, possible adsorption mechanisms, and some pedogenetic processes.Considering the abundance of silicon in the earth's crust surprisingly little attention has been given to its reactions in soils. The less abundant element aluminium, for example, has received much more investigation in recent years, presumably because of its importance to the chemistry of phosphorus in soils. However, there have been several indications of important effects of reactive silica in soils. Thus, among others, Dutt (1947) and Laws (1950) have reported beneficial effects of alkali metal silicates on soil physical properties and claims of decreased phosphate requirements of plants as a result of applications of silicates to soils have been made during several decades (e.g. Toth 1939). From such experiments it seems that reactive silica, resulting from hydrolysis of the metal silicates, can affect the properties of soils, for neither the pH change nor the addition of alkali metal ions would produce the observed results. Nevertheless the influence of native silica on the properties of field soils has apparently been considered unimportant by most workers (see, for example, Mehta et al. 1960).

Phosphate sorption capacity of soils has meaning only if the equilibrium supernatant solution concentration is specified. Measurements have been made, on a variety of Queensland soils, at an equilibrium concentration of 0.2 p.p.m. P ; reasons for this choice of cencentration are discussed. Phosphate sorption values measured in this way appear to parallel the phosphate needs of legumes growing on a number of the soils examined in the laboratory. The approach is put forward for testing by others on present and future phosphate rate trials. Present phosphate sorption measurements are interpreted as indicating (1) that even where native phosphate is inadequate, or has been depleted by cropping, heavy-textured grey and brown soils of the brigalow lands will only require small field applications of superphosphate. (This statement may not apply to soils containing free carbonate in the surface). (2) that phosphate requirements of krasnozems vary considerably but may exceed 1 ton of superphosphate an acre in some areas. Loss of the surface horizon by erosion, or mixing the subsoil With surface soil, could increase the phosphate requirement of some of these soils. (3) that the phosphate status of soils formed from phyllite in the Gympie district is intermediate between these extremes. Here also the subsoils must be expected to have larger phosphate requirements than the surface soils.

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 ses~uioxides 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.Evidence was presented in Part I of this series (Beckwith and Reeve 1963) that the residual concentration of monosilicic acid added to soil suspensions is controlled by an adsorption equilibrium. It was shown that the amount of monosilicic acid remaining in solution depended on the concentration added, the soil : solution ratio, the nature of the soil and constituent minerals, and the pH of the soil suspension. Sesquioxides were shown to sorb added monosilicic acid by a reaction resembling that with soils in its dependence on pH and were suggested as the most important contributors to the sorptive capacity of soils. The experiments described in the present paper were, for the most part, designed to characterize the release into solution of native silica from soils. Comparison of the results with those found previously provides additional evidence that untreated soils contain sorbed monosilicic acid.

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