Leaching of Phosphate in Structured Soil – a Two Step Process

This paper summarizes a series of experiments on P-leaching in structured clayey soil, mostly conducted on intact soil columns with diameter 0.5 m, and length 0.5, 0.73 or 1.0 m. Breakthrough curves of pulse-applied 32P-. and 3H2O-tracers demonstrated that preferential flow was the dominant process controlling solute leaching. In the upper 0.5 m of the soil profile, dye tracing revealed the active preferential pathways to consist mainly of tubular biopores, while at increasing depths fracture planes became increasingly important. Tracing of surface applied phosphate by use of 32P-autoradiography pointed to larger diameter biopores (diam. > 3 mm) as sole contributors to long-distance phosphate transport. The impacts of ionic strength, reducing soil conditions, faeces application, and soil depth, on P-leaching were investigated, as were the P-sorption characteristics of different macropore wall materials and bulk top- and subsoil. The results suggested P leaching to proceed through two continuously repeated steps: A loading step in which P is loaded from its solid phase sources into mobile solution, and a translocation step in which P is translocated through the soil profile with the mobile solution. Both steps depend on a range of parameters, and P-leaching will occur only if both steps are completed successfully.