Potential phosphorus losses from Organic and Podzol Soils: Prediction and the influence of soil physio-chemical properties and management

Organic and Podzols Soils commonly have properties that exacerbate phosphorus (P) loss following development for pastoral agriculture, including high porosity and hydraulic conductivity, and limited P retention. Soil samples (0-7.5cm and 30-37.5cm) were taken from 80 sites within a catchment to quantify the impacts of land-use change and P inputs on the potential for P loss. Water and dilute CaCl2 extractable P represented surrogates for surface and sub-surface runoff potential, and were 2 and 66 times higher for Organic Soils, compared with Podzols respectively. Step-wise regression identified the most important variables for P loss as degree of P saturation, plant-available P (Olsen P) and anion storage capacity. Dilute CaCl2-P concentrations were highly variable and did not correlate to any of the measured parameters. A mass balance estimated that mean P losses with time since development for Organic Soils were nearly double those of Podzol Soils.