PHOSPHATE FIXING CAPACITY OF SOME LOW ACTIVITY CLAY SOILS

This study was aimed to clarify the influence of secondary mineral constituents (iron and aluminium oxides, kaotinite) and organic matter on the determination of the phosphate-fixing capacity (P(FIX)) of well-drained low-activity clay soils. This study was done on soil samples (0-2 mm) and on their particle size fractions. The phosphate-fixing capacity estimated by an isotopic exchange method, was highly related to the total iron content of the soils. The phosphate-fixing capacity of the particle size fractions was related to their mineralogical composition, and to the organisation of their constituents. H2O2 strongly increased the phosphate-fixing capacity of the soils and of their particle size fractions. Two hypotheses were proposed to explain this. Either H2O2 caused the formation in vitro of amorphous constituents with a high phosphate fixing capacity; or the destruction of organic matter by H2O2 caused the apparition of new surfaces previously masked by organo-mineral aggregation and/or caused the apparition on surfaces of fixation sites previously occupied by organic compounds.