Fjölrit RALA - 15.06.2004, Page 152

Soil P status and eutrophication on volcanic watershads of the Azores
J. Madruga, J. Pinheiro, and L. Matos
Departamento de Ciencas Agrarias, Universdade dos Agores Angra do Heroismo, Azores, Portugal
Soil P status of Fumas watershed of S. Miguel island (the Azores) was studied, based on P
Olsen and P saturation index (PSI) referred as the saturation degree of the sorption-
precipitation capacity of a soil to P, estimated from an acid-ammonium oxalate (pH 3)
extraction of Fe, A1 and P associated with non-crystalline Fe and A1 constituents. Both
methods were used in Typic Udivitrands under an intensive pasture management, which as
involved a P overfertilization process and consequent eutrophication of the adjacent lake.
Soil test P Olsen was in general above the agronomic sufficiency, and in some cases above
150 mg kg"1, 5 times above the considered agronomic requirements as referred to the Olsen
method.
ISP was also very high in some areas of the watershed, above the threshold value of 25%,
mentioned in the Uterature as the environmental critical hmit.
Soil P Olsen was highly correlated with PSI (R2= 0.82). In accordance to the regression
equation to the ISP critical value of 25% corresponds a P Olsen of 78 mg kg'1.
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