Previous work showed that at soil P concentrations below 57 mg 0.5 M NaHCO3‐extractable P (Olsen P) kg−1 soil, little P was found in drainage waters collected from tile drains set 65 cm below the soil surface in soils from the Broadbalk Continuous Wheat Experiment. Above this soil P concentration (termed the Change‐Point) both total P and molybdate‐reactive P (MRP) in drainage waters were linearly related to soil Olsen P concentrations. We now need to know if the Change‐Point measured on Broadbalk occurs on other soils, and if so, whether a common value applies or if it varies depending upon soil type, management, and site hydrology. We investigated the possibility of 0.01 M CaCl2‐extractable P being an indicator of the Change‐Point. In all the soils studied, we found that the dynamics of P solubility in CaCl2 closely resembled the dynamics of P solubility in drainage waters of Broadbalk, since very distinct Change‐Points occurred under both conditions. However, Change‐Points measured following extraction with CaC2 varied widely between soils, from 10 to 119 mg Olsen P kg−1 soil. Lysimeter studies showed, with some exceptions, good agreement between Change‐Points measured in drainage water and in 0.01 M CaCl2. We therefore suggest that this approach may provide a valid indicator of the soil Olsen P concentration at which significant amounts of P begin to leach from soil to water, provided preferential pathways exist in the subsoil to permit P leaching down the soil profile in drainage water.
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