SURFACE CHEMISTRY OF THE HYDRANDEPTS AND ITS RELATION TO NITRATE ADSORPTION AS AFFECTED BY PROFILE DEPTH AND DEHYDRATION1

The pH₀ and ZPC of two Hydiandepts were determined by potentiomettic titiation in NaCl and by ion adsorption measurements from NH₄NO₃, respectively. The decrease of pH, pH₀, and ZPC due to drying was explained by the hydrolysis reaction. Partial drying, high organic matter content, and low Al₂ O₃ concentration in the surface soil were responsible for its low pH, pH₀ and ZPC. The high pH₀ of subsoils was attributed to the presence of amorphous aluminosilicates and mixed polymeric gels of Fe(III) and A1 with high water content. The significant decrease of nitrate adsorption due to dehydration is discussed in relation to the change in pH, pH₀, ZPC, and surface area on drying.