PHOSPHATE ADSORPTION CHARACTERISTICS OF 2 SOILS RESPONDING DIFFERENTLY TO P-FERTILIZATION

The soil samples of the present study originated from two field experi ments in which five rates of P (annually 0, 13 or 16, 26 or 32, 47 or 56, 60 or 72 kg P/ha) had been applied for 11 or 12 years. The fields were silty clay soils (Cryochrepts) not differing markedly in pH, cont ents of clay, organic C, or poorly crystalline Al and Fe oxides. Befor e the field experiment, the quantities of P extracted with an ammonium acetate solution (pH 4.65) were approximately 6 mg/dm3 in both fields . However, the fields differed considerably in the response of the cro p to P fertilization. Phosphorus adsorption by the soil samples was st udied by shaking the samples in solutions of different P concentration s (0-0.5 mg/l). Soil I, showing greater response to P fertilization in the field, adsorbed P considerably more effectively than did soil II. Also the quantities of reversibly adsorbed P were smaller in the subs amples of soil I as compared to those of soil II receiving the same fe rtilization. Fertilizer P applied during the field experiments had bee n adsorbed and converted to forms unavailable to plants to a larger ex tent in soil I, resulting in greater response to P fertilization in th is soil. The difference in response to applied P or in residual effect of P fertilization could not be predicted from soil characteristics o ther than P sorption.