In many EU countries fertilisation of grassland with nitrogen (N) and phosphorus (P) is limited by legislation. In earlier research in grassland experiments usually one nutrient was varied and a non-limiting amount of the other was applied. The interaction of the effects of N and P has been less addressed. Additionally, grass is sometimes presumed to have a constant N:P ratio. In the Netherlands, in the period 1994 - 2003, four separate field experiments were undertaken on four soil types. In these field experiments permanent grassland was fertilised with different N and P levels over a period of five and six years. Phosphorus fertilisation level had a negative effect and N fertilisation level had a positive effect on N:P ratio. At higher N levels, the influence of P fertilisation was stronger. Over time the (negative) influence of P fertilisation level also became stronger. The influence of N did not change systematically over time. Both N and P fertilisation levels should be taken into account to estimate N:P ratio.
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