Mineralogical aspets of reactivity to phosphorus among highly weathered soils

SAMPLES FROM 23 HIGHLY WEATHERED SOILS FROM PUERTO RICO, BRAZIL, GHANA, VENEZUELA, AND MALAWI WERE STUDIED. EXTRACTIONS OF POORLY ORDERED MINERAL FRACTIONS OF THE SOILS WERE CARRIED OUT IN ORDER TO ESTABLISH POSSIBLE RELATIONS WITH REACTIVITY OF THE SOILS TO PHOSPHORUS. A GIVEN ISOTHERM WAS PREPARED BY REACTING SUBSAMPLES OF A SOIL WITH KH2PO4 SOLUTIONS COVERING A RANGE OF INITIAL CONCENTRATIONS, WITH 0.01M CACL2 USED AS A CARRIER SALT. RELATIONS BETWEEN FINAL CONCENTRATIONS OF P AND P REMOVED FROM SOLUTION BY SOIL WERE FOUND TO CONFORM TO LANMUIR AND FREUNDLICH ISOTHERM TREATMENT OF THE DATA FOR ALL INCUBATION PERIODS. HOWEVER, EVIDENCE WAS FOUND THAT EQUILIBRIUM CONDITIONS, ASSUMED IN THE LANGMUIR ADSORPTION MODEL, WERE NOT ATTAINED IN SHORT INCUBATION PERIODS. ESTIMATION OF POORLY ORDERED MINERAL FRACTIONS IN SOILS WAS ATTEMPTED BY EXTRACTION WITH 0.5M CACL2, DILUTE HC HCL; COLD, 5% NA2CO3; OR ACID AMMONIUM OXALATE IN DARKNESS. VALUES OF ALUMINUM EXTRACTED BY DILUTE HCL OR 5% NA2CO3 WERE FOUND TO CORRELATE STRONGLY WITH PHOSPHORUS SORPTION INDICES DERIVED FROM ISOTHERM DATA FOR ALL INCUBATION PERIODS. VALUES FOR IRON EXTRACTED BY DILUTE HC1 OR ACID OXALATE WERE FOUND TO RELATE POORLY TO REACTIVITY INDICES, THOUGH CORRELATIONS IMPROVED SLIGHTLY FOR INDICES DERIVED FROM LONGER TERM ISOTHERMS. SIGNIFICANT CORRELATIONS WERE FOUND BETWEEN PHOSPHORUS REACTIVITY INDICES AND CLAY OR ORGANIC MATTER CONTENT. THE LATTER TWO FACTORS WERE FOUND TO CORRELATE STRONGLY WITH AL EXTRACTED BY DILUTE HC1 OR 5% NA2CO3.