Soil Isotopically Exchangeable Phosphorus: A Comparison between E and L Values

This study was designed to explain the apparent discrepancies often reported in the literature between E and L values, two parameters obtained from isotopic exchange experiments and commonly used to quantify available soil phosphate. The E and L values of the surface horizons of 10 soils from tropical, mediterranean, and temperate regions were determined. The L value was measured from a 13‐wk pot experiment with common bentgrass ( Agrostis capillaris L.) where the available soil P was labeled with 32 PO 4 ions in the presence of a carrier (25–50 mg 31 P kg −1 soil). To determine the E value, the isotopic exchange kinetic experiment was carried out on each soil. Carrier‐free 32 PO 4 was added to the soil‐solution system at a steady state and the quantity of isotopically exchangeable soil phosphate at time t, E(t) , was calculated 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 to the same period ( t = 131040 min). It was concluded that (i) the L value is a particular point of the kinetic equation and (ii) isotopically exchangeable phosphate is the available P for common bentgrass. A strict equality between E (13wk) and L values was not, however, reached for all samples. Possible causes for the differences were: (i) an overestimation of the water‐soluble phosphate due to the presence of silica and (ii) disturbance of the steady state following a too large uptake of phosphate by the crop or the application of too large quantities of carrier compared with the initial quantity of exchangeable soil phosphate.