Silicic Acid and Oxidizable Carbon Movement in a Walla Walla Silt Loam as Related to Long‐Term Management

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Abstract
Agricultural ecosystems can alter the natural soil‐forming process, but little information is available to project the effects. Such alterations can be recognized early in soils that have little natural horizon differentiation, such as the Walla Walla silt loam (coarsesilty, mixed, mesic Typic Haploxeroll). The potential for movement of silicic acid (H4SiO4) and soluble organic carbon (C) was evaluated in selected plots subjected to residue and nitrogen (N) addition treatments for a period of 50 y. Soil samples from the four upper 15‐cm layers of a plot, assembled as a column, were leached and simultaneously monitored for solubilization and movement of H4SiO4 and oxidizable organic C. Leachate concentrations and net transfers of H4SiO4 from the 0‐ to 15‐cm layer decreased as long‐term C additions increased, soil pH increased, or both. High concentrations of H4SiO4 were noted throughout the upper 60 cm, which confirms earlier observations using perfusion and batch equilibration techniques. The large concentrations of soluble organic C and their failure to correlate with H4SiO4 movement indicated that soluble organic C was not controlling solubilization of sorbed silica. Liming decreased the release of soluble H4SiO4 from the 0‐ to 15‐cm layer. All soluble H4SiO4 observations were consistent with silicic acid release from surfaces that could be partially covered (or otherwise deactivated) by coatings of organic materials. The decrease in H4SiO4 solubilization by either liming or a high organic matter level should be examined further as a possible mechanism involved in the beneficial effects of these two managements on soil structure and physical properties.

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