SOIL ISOTOPICALLY EXCHANGEABLE PHOSPHORUS - A COMPARISON BETWEEN E AND L-VALUES

This study was designed to explain the apparent discrepancies often re ported in the literature between E and L values, two parameters obtain ed from isotopic exchange experiments and commonly used to quantify av ailable soil phosphate. The E and L values of the surface horizons of 10 soils from tropical, mediterranean, and temperate regions were dete rmined. The L value was measured from a 13-wk pot experiment with comm on bentgrass (Agrostis capillaris L.) where the available soil P was l abeled with (PO4)-P-32 ions in the presence of a carrier (25-50 mg P-3 1 kg-1 soil). To determine the E value, the isotopic exchange kinetic experiment was carried out on each soil. Carrier-free (PO4)-P-32 was a dded to the soil-solution system at a steady state and the quantity of isotopically exchangeable soil phosphate at time t, E(t), was calcula ted from the kinetic equation describing the decrease of radioactivity in solution with time. Results showed that L values determined after 13 wk were not significantly different from E(t) values extrapolated t o the same period (t = 131 040 min). It was concluded that (i) the L v alue is a particular point of the kinetic equation and (ii) isotopical ly exchangeable phosphate is the available P for common bentgrass. A s trict equality between E(13wk) and L values was not, however, reached for all samples. Possible causes for the differences were: (i) an over estimation of the water-soluble phosphate due to the presence of silic a and (ii) disturbance of the steady state following a too large uptak e of phosphate by the crop or the application of too large quantities of carrier compared with the initial quantity of exchangeable soil pho sphate.