EFFECTS OF AUTOCLAVING ON SURFACE PROPERTIES AND SORPTION OF PHOSPHATE AND ZINC IN PHOSPHATE-TREATED SOILS

Autoclaving, commonly used in soil microbiological studies, may alter soil surface properties and influence subsequent reactions. Adsorption and desorption of added P or Zn and changes in Zn fractions were studied using autoclaved and nonautoclaved soil samples. Changes in surface tension with autoclaving were estimated by fitting the Gibb's surface excess isotherm to P adsorption data. Autoclaving reduced P adsorption but increased P and Zn desorption. Reduced P adsorption or increased P desorption was associated with the decreases in extractable Fe and Al and with increases in cation exchange capacity and surface tension. Autoclaving reduced the rate of gain of surface tension with adsorbed P. Increased surface tension in terms of each mol of adsorbed or desorbed P was greater for coarse- than for finetextured soils, suggesting that energy released for every mol of adsorbed P was greater in coarse- than in fine-textured soils. Increased Zn desorption was a result of reduced specific Zn adsorption, which was associated with autoclaving. Autoclaving was less effective in changing Zn adsorption or Zn fractions in fine than in coarse textured soil. Decreased Zn adsorption by autoclaving may have been due to a balance between increased nonspecific and reduced specific Zn adsorption in that both Zn desorption and KNO3-extractable Zn were increased. In terms of charge, the increased negative charge was consistent with decreases in P adsorption and increases in KNO3-extractable Zn.