ELECTROKINETIC BEHAVIOR OF A PALEUDULT PROFILE IN RELATION TO MINERALOGICAL COMPOSITION

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Abstract
By means of electrokinetic measurements, we investigated the pH-dependent surface charge characteristics of soils from eight horizons of a Dothan soil profile from the Atlantic Coastal Plain. The zero point of chrge (ZPC) varied from a low value (∼2) in the Ap horizon, increased abruptly with the transition from the A2 to the Bit horizon, and attained a value in the range of 7 to 8 in the B2 horizons. This increase in ZPC with depth is primarily a manifestation of a corresponding elevation in iron content with depth, inasmuch as iron removal from the B25tpl horizon reduced the ZPC from 7.9 to 2.3. The ability of 1 to 3 percent Fe to impart to the soil electrokinetic properties characteristic of pure hydrous iron oxides supports the belief that iron occurs principally as coatings, rather than discrete mineral entities, in the soil matrix. Specific adsorption of Ca2+ at the soil-solution interface made it difficult to establish a unique, well-defined ZPC for some of the soils. The findings suggest that, at typical field pH, the B2 horizon soils have a pH-dependent anion exchange capacity that will influence movement of certain plant nutrients through the soil and may mitigate the extent of NO3- leaching to groundwater from land application of wastewaters, sludges, and nitrogenous agricultural residues. By means of electrokinetic measurements, we investigated the pH-dependent surface charge characteristics of soils from eight horizons of a Dothan soil profile from the Atlantic Coastal Plain. The zero point of chrge (ZPC) varied from a low value (∼2) in the Ap horizon, increased abruptly with the transition from the A2 to the Bit horizon, and attained a value in the range of 7 to 8 in the B2 horizons. This increase in ZPC with depth is primarily a manifestation of a corresponding elevation in iron content with depth, inasmuch as iron removal from the B25tpl horizon reduced the ZPC from 7.9 to 2.3. The ability of 1 to 3 percent Fe to impart to the soil electrokinetic properties characteristic of pure hydrous iron oxides supports the belief that iron occurs principally as coatings, rather than discrete mineral entities, in the soil matrix. Specific adsorption of Ca2+ at the soil-solution interface made it difficult to establish a unique, well-defined ZPC for some of the soils. The findings suggest that, at typical field pH, the B2 horizon soils have a pH-dependent anion exchange capacity that will influence movement of certain plant nutrients through the soil and may mitigate the extent of NO3- leaching to groundwater from land application of wastewaters, sludges, and nitrogenous agricultural residues. © Williams & Wilkins 1981. All Rights Reserved.