Applications of ammonium-based N and P fertilizers have been shown to increase silica concentrations in soil solutions. Recent evidence also suggests that silica compounds are leached and deposited in lower profile zones as potential cementing and blocking agents. Mollisols in northern Idaho are suspected to be conducive to such phenomena because of surface acidification and the presence of a plow pan in the vicinity of the pH transition zone.
The objectives of this study were to determine (1) the relationship between ex-tractable silica and the presence of well-developed plow pans in Mollisols of the Palouse of northern Idaho and (2) profile conditions that may cause or enhance silica deposition. Soil samples collected by horizons and depth within horizons were allowed to equilibrate with H2O or Na-acetate (buffered at pH 4.8) before extraction with immiscible displacement. Silica concentrations in H2O extracts were significantly higher in the plow pan zones relative to amounts found above and below in soils containing well-developed pans. Soil profiles with poorly developed pans were shown not to contain increased levels of silica. Data suggest that silica is being deposited or precipitated in well-developed plow pan zones probably as a result of physical and chemical changes that occur in that zone. At lower depths in the soil profile silica is apparently being sorbed at soil particle surfaces, which appears to be correlated to increasing pH with soil depth. Consequently, it is postulated that as pH increases the silica compounds become more reactive and surface sorption is increased.
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