ASSESSMENT OF SOLUBLE PHOSPHORUS LOAD IN SURFACE RUNOFF BY SOIL ANALYSES

A series of laboratory experiments was carried out to quantify physica l and chemical factors that control the level of soluble P in soil sus pensions. The objective was to identify the variables needed in models simulating the load of dissolved reactive P (DRP) in the surface runo ff from cultivated land, Release of soil P as a function of water-to-s oil ratio and ionic strength was investigated with soil samples collec ted from two experimental fields, The dynamic relationship between soi l and solution P was studied by sorption-desorption isotherms. The res ults of the laboratory tests were compared with the mean flow-weighted concentration of DRP in the runoff water from the fields, The DRP con centration was too high to have originated solely from the eroded soil material transported in the runoff. It was concluded that DRP arose m ainly by the desorption of P from the surface soil during a rain or sn ow-melt period. The mean DRP concentration seems to be primarily contr olled by the P status of the surface soil. The DRP concentration of so il extracts obtained at water-to-soil ratios of 250-1001 kg(-1) (soil concentration range of 4-10 gl(-1)) corresponded to the flow-weighted mean DRP concentrations of surface runoff water and may thus be used t o quantify the removal of DRP by surface runoff. The variations in the ionic strength and water-to-soil ratio in the surface soil may contro l the temporal variation of the DRP in the runoff water. In the presen t soils, a series of water extractions at different water-to-soil rati os was found to be a more reliable procedure in assessing the potentia l P load than the parameters derived from sorption-desorption isotherm s.