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.
Peat and aquatic humic and fulvic acids have been isolated and characterized from a site at Whitray Fell in the area of the Forest of Bowland, UK. The substances cover a range of molecular masses from approximately 2800 (aquatic fulvic acid) to 40500 (peat humic acid). The enthalpies of interaction of these humic substances with a range of metal ions, including Cu2`, Cd2`, La3`and Al3`, have been measured by microcalorimetry in aqueous solutions of ionic strengths ranging from ca. 50 mmol dm~3 to 5 mol dm~3 at 25 ¡C. Apart from the interactions of La3`ions at low ionic strength, the enthalpies of interaction were endothermic, in marked contrast to what would be expected for the complexing of metal ions by chemical groupings known to be present in humic substances such as carboxylate, hydroxy and phenolic groups. By representing the humic molecules by a solvent and ion penetrable sphere surrounded by a di †use electrical double layer, the thermodynamic parameters for the discharge of the double layer were estimated. The enthalpy change on discharge of the humic-associated double layer is endothermic and ionic strength dependent and of the same magnitude as the experimental enthalpies of ion interaction with the humic substances. In contrast the enthalpy change on discharge of the humic molecules is exothermic. These observations suggest that the discharge of the electrical double layers makes a signiÐcant contribution to the enthalpy changes on ion interaction with humic substances. However, the major contribution to the Gibbs energy change on discharge of the electrical double layer arises from a large increase in entropy.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.