Development of an indicator for risk of phosphorus leaching

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 dr ainage waters were linearly related to soil Olsen P concentrations. We now need to know if the Change-Point measured on Broadbalk occurs on other soil s, 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 a ll the soils studied, we found that the dynamics of P solubility in CaCl2 c losely resembled the dynamics of P solubility in drainage waters of Broadba lk, since very distinct Change-Points occurred under both conditions. Howev er, Change-Points measured following extraction with CaCl2 varied widely be tween soils, from 10 to 119 mg Olsen P kg(-1) soil. Lysimeter studies showe d, with some exceptions, good agreement between Change-Points measured in d rainage 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 si gnificant amounts of P begin to leach from soil to water, provided preferen tial pathways exist in the subsoil to permit P leaching down the soil profi le in drainage water.