6Schuman G E. Nitrogen losses in surface runoff from agricultur alwatersheds in Missouri Valley loess[J]. Journal of Environ mental Quality, 1973(2) : 299-302.
7RomkensM J,and D W Nelson. Phosphorus relationships in run offfrom fertilized soils[J]. Journal of Environmental Quality 1974(3) :10-13.
8Sharpley A N, et al. An improved soil sampling procedure for theprediction of dissolved inorganic phosphate Concentrations in surfacerunoff from pasture[J]. Journal of Environment Quality, 1978(7) :455-456.