Factor analysis investigated the relationship between different compounds of nitrogen, phosphorus, potassium and suspended dry matter in runoff from catchments in agricultural areas in southern and central Norway. The plant nutrients were grouped in three factors: phosphorus factor, nitrogen factor and erosion factor. In south-eastern Norway there was a significant correlation between runoff and concentration levels of phosphorus and suspended dry matter in winter and spring erosion periods. Climate, agricultural practices and soil conditions caused seasonal variation of the different factors. In the catchments in south-western Norway there was a decreasing trend in concentration level of nitrogen due to changed practice for slurry application. In the catchments in eastern and central Norway there was a decreasing concentration level of phosphorus. The reduction in applied phosphorus was 15-50% but the decrease in concentration level of phosphorus in runoff accounted for 5 to 15% of the variation.

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