EXCHANGEABLE AND NONEXCHANGEABLE PHOSPHATE SORPTION IN PORTUGUESE SOILS

Total amounts of phosphate (P) sorbed were measured for 6 Portuguese s oils of widely varying properties. P-32 was used to assess the isotopi cally exchangeable and non-exchangeable sorbed P. Total sorbed and exc hangeable P were described by modified Freundlich equations and non-ex changeable P by a Temkin equation. The Langmuir equation also proved t o 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 v alues varied from 62.4 to 536.6 and from 0.4 to 322.1 mg P kg-1 respec tively. There were highly significant (p < 0.01) correlations between soil organic matter and all forms of sorbed P (total sorbed, exchangea ble and non-exchangeable). The P sorption parameters with correlation coefficients greater than 0.967 were parameters a and b of the modifie d 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 re lated to the clay content Fe(OX) and Fe(di). It was concluded that ext ractable aluminium (Al(OX) and Al(di)) had the most important effects on P sorption in these soils.