EXCHANGEABLE AND NONEXCHANGEABLE PHOSPHATE SORPTION IN PORTUGUESE SOILS

Total amounts of phosphate (P) sorbed were measured for 6 Portuguese soils of widely varying properties. P-32 was used to assess the isotopically exchangeable and non-exchangeable sorbed P. Total sorbed and exchangeable P were described by modified Freundlich equations and non-exchangeable P by a Temkin equation. The Langmuir equation also proved to fit the data for non-exchangeable P well. The amount of total sorbed P required to attain 0.2 mg P l-1 in solution ranged from 5.3 to 819 mg P kg-1. At this concentration exchangeable and non-exchangeable P values varied from 62.4 to 536.6 and from 0.4 to 322.1 mg P kg-1 respectively. There were highly significant (p < 0.01) correlations between soil organic matter and all forms of sorbed P (total sorbed, exchangeable and non-exchangeable). The P sorption parameters with correlation coefficients greater than 0.967 were parameters a and b of the modified Freundlich equation b(n) of Temkin and parameters a of the Langmuir equation. Aluminium extracted by acid oxalate (Al(OX)) and dithionite (Al(di)) showed highly significant correlation coefficients (r = 0.972) with the same sorption parameters. But P sorption was not closely related to the clay content Fe(OX) and Fe(di). It was concluded that extractable aluminium (Al(OX) and Al(di)) had the most important effects on P sorption in these soils.