LIMITATIONS IN ISOTOPIC MEASUREMENTS OF LABILE PHOSPHATE IN SOILS

Methods used for measuring labile P and assessing the phosphate-fertility status of soils by isotopic dilution as well as assumptions involved in applying the isotope dilution technique were critically evaluated. In the presence of high phosphate-fixing compounds, e.g. reactive iron oxide, labile P may be erroneously overestimated; especially when carrier-free ³²P is used. At the end of the isotopic-exchange reaction period, high phosphate-fixing soils may have a higher specific activity in the solid phase than in the solution phase. Methods involving the use of a 0·2 ppm P carrier solution or the addition of an unlabelled carrier to a labelled soil may give satisfactory labile P measurements in low and in medium phosphate-fixing soil. All the methods tried (carrier-free, carrier, and inverse dilution), however, failed to give satisfactory results for the high phosphate-fixing soils.