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Soluble Silica in Soil¹

ABSTRACT

Silica, as dissolved Na₂SiO₃, was added to soil samples or to soil cores; the silica was measured as the molybdenum blue complex of mono- and disilicic acid. The concentration of silica in a soil solution is determined by the soil's properties. Solubility is both temperature and pH sensitive. When large quantities are added to soil as a solution of NaSiO₃ and then dried, only a smaller concentration of soluble silica characteristic of the soil will remain in simple soluble forms. The fate of the bulk of the added silica is believed to be some reaction with disorganized or amorphous surfaces of silicate particles in the soil. This alteration of soluble silica is rapid and results in a lower silica concentration in the soil extract than would be expected if no reaction occurred. The change in concentration of soluble silica can occur in the time required for water to percolate through the soil profile. The change can also occur either during drying or while the soil is maintained in a moist condition.

Addition of CaCO₃ to soil reduced the "equilibrium" silica concentration of a soil extract. This is believed to result mostly because of a change in pH of the soil. Higher temperatures increased the concentration of soluble silica obtained.

The presence of soil pans containing cementation by silica might be at least partly explained by changes in pH of profile layers, temperature alterations with depth, and loss of water by soil dehydration. Aging and drying would aid the development of deposits of a nonreversible nature.

¹ Technical Paper No. 579, Utah Agr. Exp. Sta.

² Associate Professor of Soils, Utah State University, Logan.

Received for publication June 1, 1966. Accepted for publication July 25, 1966.