Leaching of phosphate in structured soil - A two step process

This paper summarizes a series of experiments on P-leaching in structured c layey soil, mostly conducted on intact soil columns with diameter 0.5 m, an d length 0.5, 0.73 or 1.0 m. Breakthrough curves of pulse-applied P-32- and (H2O)-H-3-tracers demonstrated that preferential flow was the dominant pro cess controlling solute leaching. In the upper 0.5 m of the soil profile, d ye tracing revealed the active preferential pathways to consist mainly of t ubular biopores, while at increasing depths fracture planes became increasi ngly important. Tracing of surface applied phosphate by use of P-32-autorad iography pointed to larger diameter biopores (diam. > 3 mm) as sole contrib utors 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 mac ropore wall materials and bulk top- and subsoil. The results suggested P le aching to proceed through two continuously repeated steps: A loading step i n 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 wi th the mobile solution. Both steps depend on a range of parameters, and P-l eaching will occur only if both steps are completed successfully.