Silicon in Soil

Forms of Silicon in Soil

Soils generally contain from 50 to 400 g Si kg^-1 of soil. Soil-silicon compounds usually are present as SiO₂ and various aluminosilicates. Quartz, together with crystalline forms of silicates (plagioclase, orthoclase, and feldspars), secondary or clay- and silicon-rich minerals (kaolin, vermiculite, and smectite), and amorphous silica are major constituents of most soils (75). These silicon forms are only sparingly soluble and usually biogeochemically inert. Monosilicic and polysilicic acids are the principal soluble forms of silicon in soil (76).

For the most part, monosilicic acid occurs in a weakly adsorbed state in the soil (13,37). Monosilicic acid has a low capacity for migration down the soil profile (77). The chemical similarity between the silicate anion and the phosphate anion results in a competitive reaction between the various phosphates and monosilicic acid in the soil. Increasing monosilicic acid concentration in the soil solution causes transformation of the plant-unavailable phosphates into the plant-available ones (12). Monosilicic acid can interact with aluminum, iron, manganese, and heavy metals to form slightly soluble silicates (29,30).

Polysilicic acids are an integral component of the soil solution. They mainly affect soil physical properties. The mechanism of polysilicic acid formation is not clearly understood. Unlike monosilicic acid, polysilicic acid is chemically inert and basically acts as an adsorbent, forming colloidal particles (34). Polysilicic acids are readily sorbed by minerals and form siloxane bridges (78). Since polysilicic acids are highly water saturated, they may have an effect on the soil water-holding capacity. Polysilicic acids have been found to be important for the formation of soil structure (79). There is a pressing need to obtain additional information about biogeochemically active silicon-rich substances involved in soil-formation processes.

Soil Tests

Silicon forms may be defined as total, extractable, and soluble. Total silicon comprises all existing forms of soil silicon that can be dissolved by strong alkali-fusion or acid-digestion methods (80). This parameter does not provide information about plant-available and chemically active silicon because silicon in soil is in the form of relatively inert minerals (62).

Usually for determination of soil plant-available silicon, different extracts are used. Extracts remove silicon of intermediate stability that is often associated with crystalline or amorphous soil components. The most common chemical extracts used are 0.5 M ammonium acetate (pH 4.8), 0.1 or 0.2 M HCl, water, sodium acetate buffer (pH 4.0), and ammonium oxalate (pH 3.0) among others (71,81–83). Unfortunately, soil drying is a component of all these extraction methods. During drying, all monosilicic acid (plant-available form of Si) is dehydrated and transformed into amorphous silica (21). Concern has been expressed that data obtained on dried soil may not adequately describe plant-available soil silicon and may be unsatisfactory for evaluating soil previously amended with silicon fertilizer (71). Nevertheless, extractable silicon has been correlated with the plant yield (84).

To overcome problems associated with soil drying, soluble monosilicic acid can be determined in water extracted from field-moist soil samples. After 1 h of shaking and filtration, the clean extract is analyzed for soluble monosilicic acid. This method also facilitates the testing for polysilicic acid in the soil (13). It should be noted that a change in the soil-water concentration from 5 to 50% of the field capacity had no effect on the sensitivity of the method (12,13).

To fully characterize soil plant-available silicon, it appears that more than one parameter of measurement is required. The combination of data on soluble monosilicic acid, polysilicic acid, and silicon in some extracts could give more complete information about the soil-silicon status.