ACIDIFICATION AND LIMING INFLUENCE ON SURFACE-CHARGE BEHAVIOR OF KENTUCKY SUBSOILS

Specific adsorption of ions onto soil clay surfaces influences the magnitude and behavior of the surface charge. This study evaluated the influence of acidification and liming on surface charge magnitude and behavior of three Kentucky subsoils (Eden and Lowell, fine, mixed, mesic Typic Halpludalfs; and Nicholson, a fine-silty, mixed, mesic Typic Fragiudalf) that have consistently shown poor crop response to added fertilizer. Soil samples were adjusted to three pH values (4.0-4.3, 5.5-5.8 and 6.8-7.3) with either HCl or Ca(OH)2 and were titrated with either NH4OH or KOH as titrants and NH4Cl or KCl as background electrolytes. The titration procedure was a rapid stop-and-go technique. The soil samples initially adjusted with HCl to a pH range of 4.0 to 4.3 exhibited titration behavior similar to a clay surface loaded with trivalent Al. The KOH titration plots exhibited a common cross-over point, a titration behavior commonly associated with variable-charge surfaces, for two of the soils (Eden and Lowell). The same titration behavior was not apparent when titrated with NH4OH. For the samples initially adjusted with Ca(OH)2 to pH ranges of 5.5 to 5.8 or 6.8 to 7.3, titration behavior was influenced by the clay permanent negative charge. With increasing background electrolyte concentration, the titration plots were displaced to the right without an apparent cross-over point. These samples also exhibited a pH-buffering region in the range of 6.0 to 8.5. On most soils, this is often attributed to the CaCO3-HCO3 buffer system. This buffer system cannot be implicated in this study, since CO2 and CaCO3 were excluded. It appears that in these soils, K and NH4 do not behave as analogs of each other; rather, each reacts with adsorption sites exhibiting certain ion specificity. The findings suggest that, when soils with vermiculite-Fe oxide mineralogy change their pH, they also change their surface chemistry with respect to adsorption of K and NH4, but not in a predictable manner.