Assessment of soluble phosphorus load in surface runoff by soil analyses

A series of laboratory experiments was carried out to quantify physical and chemical factors that control the level of soluble P in soil suspensions. The objective was to identify the variables needed in models simulating the load of dissolved reactive P (DRP) in the surface runoff from cultivated land. Release of soil P as a function of water-to-soil ratio and ionic strength was investigated with soil samples collected from two experimental fields. The dynamic relationship between soil and solution P was studied by sorption-desorption isotherms. The results of the laboratory tests were compared with the mean flow-weighted concentration of DRP in the runoff water from the fields. The DRP concentration was too high to have originated solely from the eroded soil material transported in the runoff. It was concluded that DRP arose mainly by the desorption of P from the surface soil during a rain or snow-melt period. The mean DRP concentration seems to be primarily controlled by the P status of the surface soil. The DRP concentration of soil extracts obtained at wate-to-soil ratios of 250–1001 kg⁻¹ (soil concentration range of 4–10 g l⁻¹) corresponded to the flow-weighted mean DRP concentrations of surface runoff water and may thus be used to quantify the removal of DRP by surface runoff. The variations in the ionic strength and water-to-soil ratio in the surface soil may control the temporal variation of the DRP in the runoff water. In the present soils, a series of water extractions at different water-to-soil ratios was found to be a more reliable procedure in assessing the potential P load than the parameters derived from sorption-desorption isotherms.