Limitations in Isotopic Measurements of Labile Phosphate in Soils

Summary 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 32 P 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.